Lakhmir Singh Manjit Kaur Chemistry 2020 2021 for Class 9 Science Chapter 4 - Structure Of Atom

Answer:

The subatomic particle that is not present in an ordinary hydrogen atom is neutron.

Answer:

Neils Bohr was the scientist who described the arrangement of electrons in an atom.

Answer:

2 electrons can be accommodated in the K shell of an atom.

Answer:

The maximum number of electrons that can be accommodated in the L shell of an atom given by
2(n)², where n is the number of shell.
Therefore, number of electrons in L (2^nd) shell

2 × 2² = 2 × 4 = 8

Answer:

For 3^rd (M) energy level, n = 3.

The maximum number of electrons that can be accommodated in the M (3^rd) shell of an atom is
2 × 3² = 2 × 9 = 18

Answer:

N is the 4^th shell in an atom
For 4^th energy level, n = 4.

The maximum number of electrons that can be accommodated in N shell of an atom is
2 × 4² = 2 × 16 = 32

Answer:

(a) The innermost shell of an atom  can accommodate 2 electrons.
(b) The outermost shell of an atom can accommodate 8 electrons.

Answer:

The three subatomic particles present in an atom are: electron, proton and neutron.

Answer:

Electron (symbol: e⁻) is the negatively charged particle present in the atoms of all elements.

Answer:

J. J. Thomson discovered electron (e^-). 

Answer:

(a) The symbol of an electron is e^–.
(b) The symbol of a proton is p⁺.
(c) The symbol of a neutron is n.

Answer:

(a) False - Rutherford proposed that nucleus consist of neutrons and protons.
(b) True
(c) False - anode ray is a beam of positive ions or cations.

Answer:

The nucleus is the central part of an atom where protons and neutrons are held together.

Answer:

K, L, M, and N are the letters used by Niels Bohr to represent electron shells in an atom.

Answer:

The particles which actually determine the mass of an atom are neutrons and protons.

Answer:

The proton is a positively charged particle present in the atoms of all elements.

Answer:

Electronic configuration of a hydrogen atom is: K = 1

Answer:

We know,
Mass of an electron = 9.1 × 10^–31 kg
Mass of a proton = 1.6 × 10⁻²⁷ kg

Thus, a proton is 1840 times heavier than an electron.

Answer:

Hydrogen gas produces anode rays consisting of protons in the discharge-tube experiment.

Answer:

The nucleus was discovered in Rutherford’s alpha particle-scattering experiment.

Answer:

The positive charge in the nucleus of an atom is due to the presence of protons.

Answer:

(a) 8 electrons are present in the outermost shell of Neon 
(b) 7 electrons are present in the outermost shell of Chlorine

Answer:

(a) L shell of an atom can accommodate a maximum of 8 electrons.
(b) N shell of an atom can accommodate a maximum of 32 electrons.

Answer:

(a) K shell of an atom can accommodate a maximum of 2 electrons.
(b) M shell of an atom can accommodate a maximum of 18 electrons.

Answer:

(i) Neutron was discovered by James Chadwick.
(ii) Electron was discovered by J.J Thomson.
(iii) Proton was discovered by Goldstein.

Answer:

(a) A Proton has a relative charge of +1.
(b) An electron has a relative charge of –1.
(c) The neutron has a relative charge of 0.

Answer:

(a)  The number of protons in the nucleus of an atom is called its atomic number.
(b)  The total number of protons and neutrons in the nucleus of an atom is called its mass number.
(c)  In general, an atom has an atomic mass number 23 atomic number 11 and 11 electrons.
(d)  An atom of an element has 11 protons, 11 electrons and 12 neutrons. The atomic mass of an atom is 23.
(e)  If the nucleus of an atom has atomic number 17, mass number 37 and there are 17 electrons outside the nucleus, the number of neutrons in it is 20.
(f)  All the mass of an atom is concentrated in a small region called the nucleus.
(g) Cathode rays are a beam of fast moving electrons.
(h) The anode rays obtained from hydrogen gas consist of protons.
(i) The maximum number of electrons that can be accommodated in the L shell are 8.
(j) The maximum number of electrons that can be accommodated in the M shell is 18.
(k) The subatomic particle not present in a hydrogen atom is neutron.
(l) An electron has negative charge,  a proton has positive charge, and a neutron has no charge.

Answer:

An electron (e⁻) is a fundamental subatomic particle with a negative elementary electric charge. Its mass is 1/1840^th of that of a hydrogen atom.

Answer:

The absolute mass of an electron is approximately 9.108 × 10^–28.
Its absolute charge is 1.6 × 10^–19 C.

Answer:

The evidence for the existence of a nucleus in an atom comes from these observations in Rutherford's experiment:  
(1) Few of the rays get deviated from their path.
(2) One out of 12000 rays return on their path

From these two observations, the following conclusions can be drawn that support the existence of a nucleus in an atom: -
(1) Deviation shows the presence of some positive particle.
(2) The returning back of the ray on its own path shows, there is something which is solid in nature and its number (i.e 1/12000^th ray) shows the particle is very small in size.

Answer:

Some important facts about the nucleus of an atom gathered after Rutherford's alpha- particle scattering experiment are –
1. The scattering of alpha particles was due to the repulsion from a positively charged centre in the gold foil.
2. The heavy central part of the atom was called nucleus. The nucleus of an atom is positively charged and is extremely hard and dense.
3. The nucleus of an atom is very small as compared to the size of an atom as a whole and is concentrated in the centre of an atom.

Answer:

Rutherford's alpha- particle scattering experiment demonstrated that there is a lot of empty space in an atom. Almost all the alpha particles passed through the gold foil without any deflection. This is possible only when a higher fraction of space in an atom is empty.

Answer:

An atom consists of protons which are positively charged, neutrons which are neutral and electrons which are negatively charged. The number of electrons is same as the number of protons. So, the two opposite charges balance each other and the atom remains neutral.

Answer:

(a) The number of protons present in the atom of any particular element i.e., the atomic number is fixed.
(b) This number which is the characteristic of a particular element is called its atomic number.

Answer:

Protons are the fundamental particles in an atom that carry one unit positive charge and have a mass, which is almost equal to that of a hydrogen atom. The relative mass of a proton is 1 U. The relative charge of a proton is +1.

Answer:

Absolute mass of a  proton is 1.6 × 10^–24 gram.
Absolute charge of a proton is 1.602 × 10^–19 coulomb .

Answer:

Difference between protons and neutrons:
Based on charge: Protons are positively charged subatomic particles, whereas electrons are negatively charged particles.
Based on mass: Mass of a proton is approximately 1840 times more than the mass of an electron.

Answer:

Two observations, which show that an atom is divisible are-

(1) Discharge-tube experiment:

In J. J. Thomson's experiment, the stream of cathode rays in the gas-discharge tube shows the presence of negatively charged subatomic particles called electrons.

(2) In Goldstein's experiment, the faint red glow in the gas discharge tube shows the presence of positively charged subatomic particles called protons.

These experiments show that atoms are divisible and made up of subatomic particles, electrons, protons and neutrons.

Answer:

(i) A cathode ray is a stream of high velocity electrons that are produced in a vacuum tube between a negatively charged cathode and a positively charged anode. Formation of cathode rays tells about the presence of electrons in all the atoms.
(ii) When a perforated cathode is used in a discharge tube, another set of rays travelling in the direction opposite to that of the cathode rays is seen. These rays are called anode rays. Formation of anode rays tells about the presence of protons in all the atoms.

Answer:

Electron configuration is the distribution of electrons of an atom in various shells or orbits of the atom (K, L, M etc.).
Electronic configuration of oxygen (atomic no = 8) is

$\underset{2}{\mathrm{K}} \underset{6}{\mathrm{L}}$

Answer:

Electronic configuration of element with atomic number 12 is:   
      
$\underset{2}{\mathrm{K}} \underset{8}{ \mathrm{L}} \underset{2}{\mathrm{M}}$     

There will be 2 electrons in the K shell, 8 electrons in the L shell and 2 electrons in the M shell of its atom.

Answer:

(a) The nucleus is a very dense region at the centre of an atom, consisting of protons and neutrons. The charge of the nucleus is positive as it contains protons which are positively charged and neutrons which are neutral.

(b) Ernest Rutherford discovered the nucleus of an atom.

Answer:

Rutherford used Alpha particles in his experiment.
These particles are positively charged.

Answer:

(a) The atomic number of given element is 13 which is defined by the number of protons in an atom. Thus, there are 13 e^-(s) in each atom of the given element  because in a neutral atom, number of protons is equal to the number of electrons.

(b) Electronic configuration of given element (2,8,3)  is

     K = 2; L = 8; M = 3

Answer:

Atomic number is 18
Electron configuration or the distribution of electrons in this atom is  :

(2, 8, 8), i.e.,

K = 2; L = 8; M = 8.

The special thing about the outermost shell of this atom is that it is completely filled with electrons. The element will be inert in nature.

Answer:

A neutron is a subatomic particle denoted by symbol 'n'. It has no net electric charge and has a mass slightly more than that of a proton. The neutron is a neutral particle found in the nucleus of an atom. Its relative mass is 1u.

Answer:

Relative masses: relative mass of a neutron is 1u, that of a proton  is also 1u and that of an electron  is (1/1840) u.

Charge: Charge of a proton is +1, that of an electron is –1 and that  of a neutron is zero.

Answer:

A proton is a positively charged particle found in the atoms of all elements.
Protons carry a positive charge, whereas neutrons are neutral.

Answer:

The relative mass of a proton is 1u, as the mass of the proton is almost same as that of a hydrogen atom. The relative mass of an electron is (1/1840) u, as the mass of an electron is 1/1840^th of the mass of a hydrogen atom.
A proton is positively charged, whereas an electron is negatively charged.

Answer:

Proton: The relative mass of a proton is 1u and its absolute mass is approximately 1.602 × 10^–24g. Charge on a proton is +1.

Neutron: Relative mass of a neutron is 1u and its absolute mass is approximately                 1.602 × 10^–24g . Charge on a neutron is zero.

Answer:

Answer:

In an atom, the position of subatomic particles is as follows:

Protons: They are positively charged particles present at the centre of the nucleus of an atom.
Neutrons: They are neutral subatomic particles also present in the nucleus of an atom.
Electrons: They are negatively charged particles which revolve in particular orbits around the nucleus of an atom.

Answer:

Answer:

Answer:

(a) In an experiment, Thomson passed electricity at high voltage through a gas, in a discharge tube. Stream of minute particles were emitted by the cathode. This stream of particles was called cathode ray as it was emitted from the cathode.
Nature of cathode rays:  Cathode rays consist of small negatively charged particles called electrons.

(b) Formation of cathode rays: When electricity at high voltage is passed through a gas taken in a discharge tube, streams of minute particles are given out by the cathode. This happens because the gas taken in the discharge tube consists of atoms, and all the atoms contain electrons. When high electrical voltage is passed, the electrical energy excites and pushes out some of the electrons from the atoms of the gas. These fast moving electrons form the cathode rays.

(c) Formation of cathode rays show that one of the subatomic particles present in all atoms is the negatively-charged electrons.

Answer:

(a) Thomson proposed a model of an atom that is similar to a Christmas pudding.
The electrons, in a sphere of positive charge, are like currants (dry fruits) in a spherical Christmas pudding. We can also compare an atom to a watermelon. The positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere like the seeds in the watermelon.

 Thomson proposed that:
  (i) An atom consists of a positively charged sphere and the electrons are embedded in it.
  (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

This model did not tell us about the presence of neutrons in the atom.

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model, as we will see below.
            


Neutron was not considered in Thomson's model of the atom.

(b) When mass number is 18 and number of electrons is 7, then:
   (i)  No. of protons = No. of electrons = 7
   (ii) No. of neutrons = Mass no. – No. of protons = 18 – 7 = 11
   (iii)Atomic no. = 7

Answer:

(a) Rutherford's model of atom-
    
Ernest Rutherford was interested in knowing how the electrons are arranged within an    atom. He conducted an experiment in which  a thin gold foil was bombarded with fast    moving alpha (α) particles.

• He selected a gold foil because he wanted a layer as thin as possible. The gold foil was about 1000 atoms thick.
• The particles used for bombarding the gold foil were doubly-charged helium ions. Since, they have a mass of 4 u, the fast-moving α particles have a considerable amount of energy.
• It was expected that α particles would be deflected by the sub-atomic particles in the gold atoms. Since, the α particles were much heavier than the protons, he did not expect to see large deflections.
But the α particle scattering experiment produced totally unexpected results.

The following observations were made:

(i) Most of the fast moving α-particles passed straight through the gold foil.
(ii) Some of the α-particles were deflected by the foil by small angles.
(iii) Surprisingly, one out of every 12000 particles appeared to rebound.

Conclusions:

(i) The first observation led to the conclusion that most of the atom is hollow.
(ii) The small angle deviation of the α particles confirms the presence of a positively charged centre called nucleus.
(iii) The third observation confirms that the nucleus of an atom is solid as the ray returns on its path and is very small in size because 1 out of 12000 rays returns.

Drawbacks of Rutherford’s model of the atom
The major drawback of Rutherford’s model is that it does not explain the stability of an atom. The orbital revolution of an electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable. Therefore, matter would not exist in the form that we know. However, we know that atoms are stable. So, this model could not explain this reasoning.                             

(b) Given:
     Mass no. = 23
     No. of electrons = 11,
     No of protons = No. of electrons = 11
     No. of neutrons = 23 – 11 = 12
Atomic number of the element = 11

Answer:

(a) Bohr's model of atom 

An atom is made up of three particles: electrons, protons, and neutrons.

(i) The protons and neutrons are located in a small nucleus at the centre of the atom.
(ii) The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells.
(iii)  There is a limit to the number of electrons that each energy level can hold.
(iv)  Each energy level is associated with a fixed amount of energy.
(v)  There is no change in the energy of electrons as long as they keep revolving in the same energy level, and the atom remains stable.

Niels Bohr explained the stability of the atoms through the concept of revolution of electrons in different energy levels. The change in the energy of an electron occurs when it jumps from a lower energy level to a higher energy level or vice versa. When it gains energy, it jumps from a lower energy level to a higher energy level, and when it loses energy it comes down from a higher energy level to a lower energy level. In this way, energy is not lost and the atom remains stable.


A few energy levels in an atom


(b).Given: Atomic no. = 11
Mass no. = 23,
Electronic configuration – (2, 8, 1)

Nuclear composition of an atom of the element is – 11 protons and 12 neutrons

Answer:

(a)
(i) Atomic number is the number of protons in the nucleus of an atom. In other words, each element has a unique number that identifies how many protons or electrons are present in one atom of that element. For example, all hydrogen atoms contain one proton and have an atomic number of 1. All carbon atoms contain six protons or electrons and have an atomic number of 6.

(ii) The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. The mass number is different for each  isotope of a chemical element. This is not the same as the atomic number (Z), which denotes the number of protons in a nucleus. Thus, it uniquely identifies an element. Therefore, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N = A − Z.
Example- One atom of Na contains 11 protons and 12 neutrons; so its mass no. is 23.

(b) Relation between mass number and atomic number:

Mass No. = Atomic no. + No. of neutrons

(c) No. of neutrons = Mass No. - Atomic no.
=  24 – 12 = 12

Answer:

(a) Nucleus

Rutherford conducted an experiment using alpha particles which led to the discovery of  a nucleus  situated at the centre of an atom.

Answer:

(a) 2, 8, 1

2, 8, 1 is the correct electronic configuration as the first shell can accommodate 2 electrons and second shell can accommodate 8 electrons. When the inner shell is full, electrons are filled in the outer shell.

Answer:

(b) neutron
A hydrogen atom ${}_1{}^1\mathrm{H}$ has 1 proton and 1 electron, but it does not have any neutron.

Answer:

(a) J.J. Thompson
J.J. Thompson discovered electrons by conducting a cathode-ray experiment.

Answer:

(a) 2,8
Magnesium ion, Mg²⁺ has 10 electrons; so, its electronic configuration is 2, 8.

Answer:

(c) 2, 8, 8
Chloride ion, Cl^– has 18 electrons; so, its electronic configuration is 2, 8, 8.

Answer:

(b) protons
Goldstein’s experiment, which involved passing high-voltage electricity through gases at very low pressure, resulted in the discovery of protons.

Answer:

(a)  ${}_{15}{}^{31}\mathrm{X}$

In an atom, the number of electrons is equal to number of protons ,which in turn is equal to the atomic number of the element. Also, the sum of number of protons and neutrons represents the mass number of the element. Therefore, the correct option is (a).

Answer:

(b) 10
Mass number of the element = 27
Number of neutrons = 14
Number of protons = mass number – number of  neutrons
= 27 – 14 = 13
As the ion of this element has 3 positive charges,  it means that it has lost 3 electrons. Therefore, there are now 10 electrons in the ion.

Answer:

(c) J.J. Thompson

J.J. Thompson gave the first model of an atom.

Answer:

(a) An atom has equal number of protons and electrons

An atom always has equal number of protons and electrons in order to maintain electrical neutrality.

Answer:

(i) Mass no. = 31
(ii) Atomic no. = 15
(iii) Electronic configuration: (2, 8, 5)

That is,  

$\underset{2}{\mathrm{K}} \underset{ 8}{ \mathrm{L}} \underset{5}{\mathrm{M}}$

Answer:

(a) Electronic configuration of the element X is
     Element X:         $\underset{2}{\mathrm{K}} \underset{ 8}{ \mathrm{L}} \underset{7}{\mathrm{M}}$           
(b) Atomic number of the element X will be 17.

(c) Element X is a non-metal because there are 7 electrons in its outermost orbit. In order to complete its octet (to get 8 electrons), it will gain one electron. An element which gains electrons in order to complete its octet is a non-metal.  

(d) The element will form an anion by gaining an extra electron in order to complete its octet.
   Symbol of an ion of X will be X^–.

(e) X is Chlorine.

Answer:

(a) Its atomic number is 3.

(b) Mass no. = 3 + 4 = 7

(c) Electronic configuration is

 $\underset{2}{\mathrm{K}} \underset{1}{\mathrm{L}}$   

(d) Element E is a metal because in order to complete its octet, the element loses 1 electron from its outermost orbit and attains a stable configuration with 2 electrons in its first shell. Elements which lose electrons to attain stability are called metals.

(e) A Cation will be formed by an atom of element A because its outermost single electron can be easily donated.

(f) E⁺

(g) Element X is lithium.

Answer:

(a) Figure 9 indicates the mass number.
(b) Figure 4 indicates the atomic number.
(c) No. of protons in atom X = 4
(d) No. of neutrons in atom X = 9 – 4 = 5
(e) No. of electrons in atom X = 4
(f) Electrons in the outermost shell of an atom of element X = 2
(g) X²⁺

Answer:

(a) Atomic no. of the element = 18.
(b) Element Z is a non-metal
(c) Element Z will not form any ion because its outermost shell is complete.
(d) Outermost electronic shell of the atom of this element is completely filled with electrons.
(e) Name of element Z is Argon Symbol is Ar.
(f) Z belongs to the noble gases group or the zero group of the periodic table.

Answer:

Nitrogen contains 7 electrons. So, the electronic configuration is 2, 5. As the outermost orbit has 5 electrons,  there are 5 valence electrons in it.

Answer:

The general name given to the elements with 8 electrons in their outermost orbit is noble gases.

Answer:

Helium is the noble gas which has less than 8 electrons in the valence shell of its atom. Its atomic no. is 2.

Answer:

Radioactive isotopes are used as tracers in medicine to detect the presence of tumours and blood clots.

Answer:

Uranium-235 is used as a fuel in the reactors of nuclear power plants.

Answer:

Cobalt-60 is a radioactive isotope used in the treatment of cancer.

Answer:

Iodine-131 is a radioactive isotope of iodine used to determine the activity of the thyroid gland.

Answer:

Radioactive isotopes are used in industries to detect the leakage in underground oil pipelines, gas pipelines and water pipes.

Answer:

The given statement is false.

The true statement is, iodine-131, the radioactive isotope of iodine is used as a tracer to determine the manner and rate of iodine intake by the thyroid gland in our body.

Answer:

Atoms which contain the same number of protons and electrons, but different number of neutrons are known as isotopes of an element.

Answer:

As the number of electrons and protons are same but the number of neutrons are different,  these atoms are isotopes of each other.

Answer:

The given elements are isotopes of nitrogen because they contain the same number of protons and electrons but different number of neutrons.

Answer:

Radioactive isotopes are the isotopes that have unstable nuclei and emit various types of radiation.

Answer:

Answer:

(a) Magnesium has 2 valence electrons in the M shell.
(b) The valency of nitrogen in N₂ molecule is 3.
(c) Isotopes have different mass numbers because their nuclei contain different number of neutrons.
(d) Some boron atoms have mass number 10 and some have mass number 11. These boron atoms with different mass numbers are called isotopes.

Answer:

(a) Atomic number = 5
(b) Here, mass no. = 6 + 5 = 11
(c) No. of electrons = 5
(d) Here no. of valence electrons per atom of the element is 3.

Answer:

Atomic number is 17
So, the no. of protons and no. of electrons is 17.

Electronic configuration is

$\begin{array}{ccc}\underset{2}{\mathrm{K}}& \underset{8}{\mathrm{L}}& \underset{7}{\mathrm{M}}\end{array}$

Here, the outermost orbit has 7 electrons. In order to make it 8, the element will have to gain 1 electron. So, the valency of the element is 1.

Answer:

Atomic No. of X = 16
Electronic configuration of X is

$\begin{array}{ccc}\underset{2}{\mathrm{K}}& \underset{8}{\mathrm{L}}& \underset{6}{\mathrm{M}}\end{array}$

Here, the outermost orbit has 6 electrons. In order to make it 8, X will have to gain 2 electrons. So, the valency of X is 2.

Answer:

Answer:

(a) Uranium-235:
It is used as a fuel in nuclear reactors of nuclear power plants for generating electricity.

(b) Cobalt-60:
It is used in the treatment of cancer cells.

Answer:

Isotopes are the atoms of an element with the same atomic number but different mass numbers. There are three isotopes of hydrogen atom, namely protium $\left({}_1{}^1\mathrm{H}\right)$, deuterium $\left({}_1{}^2\mathrm{H}\right)$ and tritium $\left({}_1{}^3\mathrm{H}\right)$. ${}_1{}^3\mathrm{H}$ and ${}_2{}^3\mathrm{H}$ are not considered as isotopes because they do not have the same atomic number. Such atoms that have same mass number but different atomic numbers are called isobars. 

Answer:

It has been found that, in nature, atoms of certain elements might have the same number of protons i.e., same atomic number but different mass numbers. This difference in the masses of isotopes of an element is due to difference in the number of neutrons in their nuclei.

Answer:

Many elements consist of a mixture of isotopes.The chemical properties of isotopes are similar. This is due to the fact that isotopes of an element have the same atomic number, which means that they have the same number of valence electrons.
Their physical properties are slightly different because the mass numbers of the isotopes are different.
For example- ${}_{17}{}^{35}\mathrm{Cl}$ and ${}_{17}{}^{37}\mathrm{Cl}$ show identical chemical properties as they have the same number of  valence electrons (7).

Answer:

The mass numbers of the isotopes of an element are slightly different. So, the physical properties of isotopes are slightly different from each other.

Answer:

The mass of an atom of any natural element is taken as the average mass of all the naturally-occurring atoms of that element. If an element has no isotopes, then the mass of its atom would be the same as the sum of the protons and neutrons in it. But if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then calculate the average mass.

$$\begin{gathered} \mathrm{average} \mathrm{atomic} \mathrm{mass} \mathrm{of} \mathrm{chlorine}=35\times \left[\frac{75}{100}\right]+37\left[\frac{25}{100}\right] \\ =\frac{2625}{100}+\frac{925}{100} \\ =26.25+9.25 \\ =35.5 \mathrm{u} \end{gathered}$$

Therefore, the masses of certain elements are in fractions and not whole numbers.

Answer:

Deuterium $\left({}_1{}^2\mathrm{H}\right)$, protium $\left({}_1{}^1\mathrm{H}\right)$ and tritium $\left({}_1{}^3\mathrm{H}\right)$ are isotopes of hydrogen.

${}_{18}{}^{40}\mathrm{Ar} \mathrm{and} {}_{20}{}^{40}\mathrm{Ca}$ (Argon and Calcium, respectively) are isobars with same mass number but different atomic numbers.

Answer:

(i) The chemical properties of isotopes of hydrogen are similar because they have the same number of protons, which means that they have the same number of electrons that are responsible for the chemical nature of the isotopes.
(ii) All of them have 1 electron and 1 proton. So, they are electrically neutral.

Answer:

Isotopes of Neon are ${}_{10}{}^{20}\mathrm{Ne} \mathrm{and} {}_{10}{}^{22}\mathrm{Ne}$ with 90% and 10% abundance, respectively.

$$\begin{gathered} \mathrm{average} \mathrm{atomic} \mathrm{mass} \mathrm{of} \mathrm{Neon}=20\times \left[\frac{90}{100}\right]+22\left[\frac{10}{100}\right] \\ =\frac{1800}{100}+\frac{220}{100} \\ =18.00+2.2 \\ =20.2 u \end{gathered}$$

Answer:

Isobars are atoms of different chemical elements that have the same mass number but different atomic number i.e., same number of protons but different number of neutrons.

For example, ${}_{18}{}^{40}\mathrm{Ar} \mathrm{and} {}_{20}{}^{40}\mathrm{Ca}$ are isobars with same mass number but different atomic numbers.

Answer:

H-   1 proton, 1 electron and no neutron.
D-  1 proton, 1 electron and 1 neutron.
T-  1 proton, 1 electron and 2 neutrons.

Answer:

We know that protons are present in the nucleus of an atom. It is the number of protons of an atom that determines its atomic number. It is denoted by ‘Z’. Here,
Atomic number = 7
Number of electrons = 7
Electronic configuration is

$\begin{array}{cc}\underset{2}{\mathrm{K}}& \underset{5}{\mathrm{L}}\end{array}$

There are 5 electrons in the outermost orbit. So, in order to complete its octet the atom will have to gain 3 electrons. Therefore, the valency of the atom is 3.
Given element is nitrogen.

Answer:

(a) In an atom, the electrons are arranged in different shells. The electrons found in the outermost orbit of an atom are called valence electrons.
So, valence electrons are situated in the outermost orbit of an atom.

(b) The electronic configuration of the element with atomic number 13 is (2, 8, 3).  There are 3 valence electrons present in the outermost shell in the given element.
Valence shell of this atom is M.

Answer:

(a) Isotopes are the atoms of the same element with the same atomic number but different mass numbers. It means that the isotopes have same number of electrons and protons but different number of neutrons. For example, hydrogen has three isotopes- protium, deuterium  and tritium .
Here, all three of them have same atomic number i.e. 1 but different mass numbers.

(b) Similarity –A pair of isotopes have the same atomic number.
Difference – A pair of isotopes have different mass numbers.

(c) In  ${}_{17}{}^{35}\mathrm{Cl}$, there are 17 protons, 17 electrons and 18 neutrons.
In  ${}_{17}{}^{37}\mathrm{Cl}$, there are 17 protons, 17 electrons and 20 neutrons.

Answer:

(a) Those isotopes which are unstable due to the presence of extra neutrons in their nuclei and emit various types of radiations are called radioactive isotopes. The radiations emitted are in the form of alpha particles, beta particles and gamma rays.
Common examples of radioactive isotopes are Arsenic−74, Iodine−131 and Cobalt−60.

(b) Uses of isotopes−
(i) Cobalt−60 is used in the treatment of cancer cells. When the high−energy gamma radiations emitted by cobalt−60 isotopes are directed at the cancerous tumours , the cells are burnt.
(ii) Iodine−131  is used as a tracer to determine the rate at which the thyroid gland takes up iodine.

(c) Average atomic mass of Z = 35.5u
Let the % of isotope with mass number 35 be x
Then the % of isotope with mass number 37 will be 100−x.

$$\begin{gathered} \mathrm{Average} \mathrm{atomic} \mathrm{mass} \mathrm{of} \mathrm{chlorine} = 35\left[\frac{x}{100}\right]+37\left[\frac{100-x}{100}\right] \\ 35=\frac{35x}{100}+\frac{37\left(100-x\right)}{100} \\ x=75 \end{gathered}$$

It means that the % of the isotope ${}_{17}{}^{35}\mathrm{Cl}$ is 75 and the % of the isotope  ${}_{17}{}^{35}\mathrm{Cl}$ is 25.

Answer:

(a) Valency of an element is the number of electrons which an atom of the element loses or gains or shares during a chemical reaction, in order to complete its octet.
By sharing electrons an element with atomic number 14 will show a valency of 4, as it has 4 electrons in its outermost shell. Electronic configuration (2,8,4).

(b) The electrons present in the outermost shell of an atom are known as the valence electrons. The valency of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence shell.
When an atom has to lose an electron to completely fill its outermost orbit ,  the valency of the atom is equal to the number of the valence electrons. But when an atom has to gain electrons in order to complete its octet,  valency of the atom is equal to the number of electrons gained and not the number of valence electrons.
For example, in chlorine (atomic number = 17), the number of valence electrons is 7, but here the atom will not lose these electrons. Here, the atom will gain 1 electron to complete its octet. Therefore, here the valency will be 1.
On the other hand, if we consider the example of magnesium (atomic number = 12), the number of valence electrons is 2. Here, the atom will lose 2 electrons to fill its outermost orbit. Therefore, the valency is also 2.

Answer:

(d) Isobars

Isobars are the atoms of different elements with same mass number but different atomic numbers.

Answer:

(c) an atom can have only electron and proton but no neutron

An atom must have a proton and an electron, but it may not have a neutron. For example, in hydrogen atom, there is 1 proton, 1 electron but no neutron.

Answer:

(c) they have same number of protons

As the mass numbers are different, the correct option is (c). They have the same number of protons.

Answer:

(c) cobalt-60

Cobalt-60 is used in the treatment of cancer. High-energy gamma rays emitted by cobalt-60 isotopes destroy cancerous tumours.

Answer:

(d) either metals or non metals

Elements with valency 1 can be either metals or non-metals.
When an atom loses one electron to gain stability, it is a metal. When an atom gains an electron to complete its octet  it is a non-metal.

Answer:

(c) the two oxygen atoms are isotopes

As the number of protons is same but the number of neutrons is different,  the mass number of the two oxygen atoms is different. So, these two are isotopes of each other.

Answer:

(c) carbon

Carbon has 4 electrons in its outermost orbit. It completes its octet by sharing 4 electrons.

Answer:

(b) 4
Graphite is an allotrope of carbon. It is made up of all C atoms. The atomic number of carbon is 6; so, the number of valence electrons is 4.

Answer:

(c) C
Element C has 15 electrons; so, the electronic configuration is 2 , 8 , 5. This element will gain 3 electrons in order to complete its octet; so, it cannot form cations.

Answer:

(d) 8

Atomic number of a sulphur atom S is 16. It has 6 electrons in its outermost orbit. After gaining 2 electrons it becomes S²⁻. So, now the outermost orbit will have 8 electrons.

Answer:

(c) 1
Atomic number is 9, which means that number of electrons is also 9. Electronic configuration is 2 , 7. So, the atom will gain 1 electron to complete its octet. Therefore, the valency is 1.

Answer:

(b) X

Element X can’t form anions as the outermost orbit contains only 1 electron. The atom will rather lose this electron to complete its octet and  will become a cation.

Answer:

(b) (ii) and (iii)

${}_{61}{}^{200}\mathrm{X} \mathrm{and} {}_{58}{}^{200}\mathrm{X}$ are isobars because they have the same mass number but different atomic numbers.

Answer:

(d) S
Element S has electronic configuration 2, 8, 6. It can exhibit covalency as well as electrovalency.

Answer:

(b) 2

Atomic number of element X is 8.
Electronic configuration is 2, 6. It will gain 2 electrons to complete its octet; so, its valency is 2.
Atomic number of Y is 4.
Electronic configuration is 2, 2. It will lose 2 electrons in order to completely fill its outermost orbit . So, the valency of Y is also 2.

Answer:

(d) different number of neutrons but same number of protons

Isotopes are the atoms of the same element with the same atomic number but different mass numbers. It means that they have the same number of protons and electrons but different number of neutrons.

Answer:

(a) Atomic number of sodium atom = 11
In Na⁺ ion, there are only 10 electrons. The electronic configuration is 2 , 8. Therefore, the number of valence electrons in sodium ion is 8.

(b) Atomic number of oxygen atom = 8
In O^2- ion, there will be 10 electrons, as it has gained two electrons. The electronic configuration is 2, 8. Therefore, the number of valence electrons in O^2- is 8.

Answer:

Atom A – ${}^{209}\mathrm{A}_{82}$
Atom B – ${}^{209}\mathrm{A}_{83}$

(i) A has 82 protons as the atomic number is 82.
(ii) B has 83 protons as the atomic number is 83.
(iii) No, A and B are not isotopes of each other because their atomic numbers are not same.

Answer:

(i) Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. ${}^{58}\mathrm{A}_{26}$ and ${}^{58}\mathrm{B}_{28}$ are not isotopes because they have different atomic numbers.
(ii) ${}^{79}\mathrm{X}_{35}$and ${}^{80}\mathrm{Y}_{35}$ are isotopes as they have same atomic number.

Answer:

(i) Subscript represents atomic number, whereas superscript represents atomic mass.
(ii) Number of neutrons is responsible for the change in superscripts.
(iii) Isotope is the usual name for such atoms of an element.
(iv) Nuclear composition of ${}^{18}\mathrm{O}_8$ is-
No. of protons = 8
No. of neutrons = 18 − 8 = 10

Answer:

Atoms of different elements with different atomic numbers but same mass number are known as isobars.
Here, A and B are the examples of isobars, as they have same number of nucleons.

Answer:

Mass number is the sum of the number of neutrons and the number of protons. So, mass number of A and B is 40. As the mass number of A and B is same,  the two are isobars.
Therefore, A represents Argon $\left({}_{18}{}^{40}\mathrm{Ar}\right)$ while B represents $\left({}_{20}{}^{40}\mathrm{Ca}\right)$ Calcium.

Answer:

${}_{26}{}^{58}\mathrm{A}$ and ${}_{28}{}^{58}\mathrm{B}$ are isobars because they have same number of nucleons i.e. their mass numbers are same but atomic number is different.

Answer:

Here, A and D are isotopes as they have the same number of protons but different number of neutrons.

Answer:

(i) Mass number of X = 8+8 = 16
(ii) Mass number of Y = 8+9 = 17
(iii) X and Y are isotopes, as they have the same number of protons but different number of neutrons.
(iv) X and Y represent the isotopes of oxygen.