## Topic outline

• ### Chapter 4 - Structure Of The Atom – 3

• Structure Of The Atom

Q46. In the Thomson’s model of atom, which of the following statements are correct?

(i) The mass of the atom is assumed to be uniformly distributed over the atom.

(ii) The positive charge is assumed to be uniformly distributed over the atom.

(iii) The electrons are uniformly distributed in the positively charged sphere.

(iv) The electrons attract each other to stabilise the atom.

(a) (i), (ii) and (iii)

(b) (i) and (iii)

(c) (i) and (iv)

(d) (i), (iii) and (iv)

Ans. (a) (i), (ii) and (iii)

Explanation

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.

Q47. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Ans. The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,….

Hence the maximum number of electrons in first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8.

Thus, total no. of electrons = 2 + 8 = 10

Q48. Write the electronic configuration of any one pair of isotopes and isobars.

Ans. Isotopes are atoms of the same element, which have different mass numbers.

Example:

Isobars are atoms having the same mass number but different atomic numbers.

Example:

Q49. If number of electrons in an atom is 8 and number of protons is also

8, then (i) what is the atomic number of the atom? and (ii) what is the charge on the atom?

Ans. (i) No. of protons = 8

No. of electrons = 8

We know that,

Atomic Number = No. of protons = No. of electrons

Therefore, Atomic number = 8

(ii) No. of protons = No. of electrons = 8

Thus, the number of protons is equal to the number of electrons. So the atom will be neutral.

Q50. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Ans. Atomic Mass of helium atom = 4

Number of protons = 2

Number of neutrons = (n)

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

4 = 2 + n

n = 4 – 2

n = 2

Thus, no. of neutrons = 2

Q51. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

Ans.

Q52. What information do you get from the figure about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form.

Ans.

 Element Atomic No. Mass No. No. of electrons Electronic Configuration Valency X 5 5+6=11 5 2,3 3 Y 8 8+10=18 8 2,6 2 Z 15 15+16=31 15 2,8,5 3,5

Q53. Which of the following in Fig. do not represent Bohr’s model of an atom correctly?

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (ii) and (iv)

(d) (i) and (iv)

Ans. (c) (ii) and (iv)

In Fig. (ii) contains 4 electrons in K shell and Fig. (iv) contains 9 electrons in L shell which are not in accordance with Bohr’s model. First orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8.

Q54. Rutherford’s α-particle scattering experiment showed that

(i) Electrons have negative charge.

(ii) The mass and positive charge of the atom is concentrated in the nucleus.

(iii) Neutron exists in the nucleus.

(iv) Most of the space in atom is empty.

Which of the above statements are correct?

(a) (i) and (iii)

(b) (ii) and (iv)

(c) (i) and (iv)

(d) (iii) and (iv)

Ans. (b) (ii) and (iv)

Two important observations of Rutherford’s α-particle scattering experiment were:

1. An atoms consists of a positively charged, dense and very small nucleus containing all the protons (positive charge) and neutrons (no charge). Almost the entire mass is concentrated in the nucleus.

2. Most of the atom is empty space.

Q55. Find out the mass number of oxygen and sulphur atom.

Ans. (i) No. of protons = 8

No. of neutrons = 8

Mass number of oxygen atom

= No. of protons + No. of neutrons = 8 + 8 = 16

(ii) No. of protons = 16

No. of neutrons = 16

Mass number of sulphur atom

= No. of protons + No. of neutrons = 16 + 16 = 32

Q56. What are the limitations of Rutherford’s model of the atom?

Or

What were the drawbacks of Rutherford’s model of an atom?

Ans. Drawbacks of Rutherford’s model of the atom

The orbital revolution of the 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 and hence matter would not exist in the form that we know. We know that atoms are quite stable.

Q57. Define valency by taking examples of silicon and oxygen.

Ans. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element, that is, the valency.

Valency of silicon: It has electronic configuration  2, 8, 4. Thus, 4 electrons are shared with other atoms to complete the octet and so its valency = 4

Valency of oxygen: It has electronic configuration  2, 6. Thus; it will gain 2 electrons to complete its octet. So its valency = 2

Q58. Write any two observations which support the fact that atoms are divisible.

Ans. Dalton’s atomic theory suggested that atom was indivisible – which could not be broken down into smaller particles. But the discovery of subatomic particles such as electrons and protons suggested that atoms are divisible and they do have an inner structure.

‘Atoms are divisible’ is supported by the following facts

(i)   Formation of ionic compounds involves the transfer of electrons.

(ii) Presence of isotopes for the same element is possible due to the difference in the number of neutrons.

Q59. Complete the following table.

 Atomic Number Mass Number Number  of Neutrons Number  of Protons Number  of Electrons Name of Atomic Species 9 - 10 - - - 16 32 - - - Sulphur - 24 - 12 - - - 2 - 1 - - - 1 0 1 0 -

Ans.

 Atomic Number Mass Number Number  of Neutrons Number  of Protons Number  of Electrons Name of Atomic Species 9 19 10 9 9 Fluorine 16 32 16 16 16 Sulphur 12 24 12 12 12 Magnesium 1 2 1 1 1 Deuterium 1 1 0 1 0 Hydrogen