Which one of the following is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
ANS (d) 2, 8, 1

Complete the following table.

 

Atomic number	Mass number	Number of Neutrons	Number of protons	Number of electrons	Name of the Atomic species
9	−	10	−	−	−
16	32	−	−	−	Sulphur
−	24	−	12	−	−
−	2	−	1	−	−
−	1	0	1	0	−

Answer
 

Atomic number	Mass number	Number of Neutrons	Number of protons	Number of electrons	Name of the 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 ion

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)

9. The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
(a) 13
(b) 10
(c) 14
(d) 16
Ans. (b) 10
Explanation: Mass number = 27 and number of neutrons = 14
Hence, number of electrons in atom = 27 � 14 = 13
Now, ion has 3 positively charges, so number of electrons in the ion = 13 � 3 = 10

10. Identify the Mg2+ ion from the Fig.4.1 where, n and p represent the number of neutrons and protons respectively
(a) (b)(c)(d)

(d)
Explanation: Electronic configuration of Mg atom is: 2, 8, 2
So, electronic configuration of Mg2+ ion is; 2, 8.

11. In a sample of ethyl ethanoate (CH₃COOC₂H₅) the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?
(a) One of the oxygen atoms has gained electrons
(b) One of the oxygen atoms has gained two neutrons
(c) The two oxygen atoms are isotopes
(d) The two oxygen atoms are isobars.
Ans. (c) The two oxygen atoms are isotopes

Explanation: Isotopes are elements with same atomic number but different mass numbers. The different in mass number is because of different number of neutrons in them.

12. Elements with valency 1 are
(a) always metals
(b) always metalloids
(c) either metals or non-metals
(d) always non-metals
Ans. (c) either metals or non-metals
Explanation: If an element show positive valency then it is a metal; otherwise it is a nonmetal.

13. The first model of an atom was given by
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson
Ans. (d) J.J. Thomson

14. An atom with 3 protons and 4 neutrons will have a valency of
(a) 3
(b) 7
(c) 1
(d) 4
Ans. (c) 1
Explanation: Electronic configuration of this element: 2, 1
Since number of electron in outermost shell is 1, hence valency is 1.

15. The electron distribution in an aluminium atom is
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8
Ans. (a) 2, 8, 3
Explanation: Atomic number of aluminium is 13 and the first shell can have at the most two electrons in it only. Hence option (a) is correct.

16. Which of the following in Fig. 4.2 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)
Explanation: The second shell cannot have more than 88 electrons and first shell cannot have more than 2 electrons.

17. Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
Ans. (a) An atom has equal number of electrons and protons.

18. Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order
(i) Rutherford�s atomic model
(ii) Thomson�s atomic model
(iii) Bohr�s atomic model
(a) (i), (ii) and (iii)
(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)
Ans. (c) (ii), (i) and (iii)

Yes, it is true for hydrogen atom which is represented as ¹₁H

Discovery of electrons and protons

They are isotopes. Isotopes have same number of electrons in them. Hence, their valencies do not differ.

Gold is highly malleable. Hence, it can be made into very thin sheet. Rutherford wanted a metal sheet which could be as thin as possible. So, he selected gold foil for his alpha-ray scattering experiment.