CBSE - Chemistry - Structure of the Atom

1. Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) 1, 8, 3
(d) 8, 2, 2
Ans. (b) 2, 8, 2
Explanation: The first shell can have at the most two electrons in it. Hence option �b� is correct.

2. Rutherford�s �alpha (α) particles scattering experiment� resulted in to discovery of
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass
Ans. (c) Nucleus in the atom
Explanation: The observation that some alpha particles returned on their original path showed the presence of nucleus in the centre of an atom.

3. 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. (a)

Explanation: Atomic number (no. of electrons) is written in subscript, while mass number is written in superscript; before the symbol of element.

4. Dalton�s atomic theory successfully explained
(i) Law of conservation of mass
(ii) Law of constant composition
(iii) Law of radioactivity
(iv) Law of multiple proportion
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)
Ans. (d) (i), (ii) and (iv)

Explanation: Dalton�s theory explained the law of conservation of mass, law of constant composition and law of multiple proportions. But the theory did not talk about law of ratio activity.

5. Which of the following statements about Rutherford�s model of atom are correct?
(i) considered the nucleus as positively charged
(ii) established that the α�particles are four times as heavy as a hydrogen atom
(iii) can be compared to solar system
(iv) was in agreement with Thomson�s model
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) only (i)
Ans. (a) (i) and (iii)
Explanation: Alpha particles are positively charged and hence were deflected by the nucleus. This showed that nucleus is positively charged. Rutherford also postulated that electrons are arranged around the nucleus; the way planets are arranged around the sun.

6. Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)
Explanation: Atomic number = number of protons = number of electrons. Since electron has negligible mass; so, masses of protons and neutrons are taken into account for mass number.

7. 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 stabilize 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 negatively charge electrons are stabilized by positively charged nucleus. Hence, option (iv) is not correct. Rest of the options are correct.

8. 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)