## Topic outline

• ### Chapter 4 - Structure Of The Atom – 5

• Structure Of The Atom

Q71. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Ans. The following rules are followed for writing the number of electrons in different energy levels or shells:

(i) 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 different shells are as follows: first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be = 2 × 42= 32, and so on.

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Q72. Match the names of the Scientists given in column A with their contributions towards the understanding of the atomic structure as given in column B.

 Column (A) Column (B) (a) Ernest Rutherford (i) Indivisibility of atoms (b) JJ.Thomson (ii) Stationary orbits (c) Dalton (iii) Concept of nucleus (d) Neils Bohr (iv) Discovery of electrons (e) James Chadwick (v) Atomic number (f) E. Goldstein (vi) Neutron (g) Mosley (vii) Canal rays

Ans.

 Column (A) Column (B) (a) Ernest Rutherford (iii) Concept of nucleus (b) JJ.Thomson (iv) Discovery of electrons (c) Dalton (i) Indivisibility of atoms (d) Neils Bohr (ii) Stationary orbits (e) James Chadwick (vi) Neutron (f) E. Goldstein (v) Atomic number (g) Mosley (vii) Canal rays

Q73. How will you find the valency of chlorine, sulphur and magnesium?

Ans. Valency is the combining capacity of an atom. It is the number of electrons gained, lost or shared so as to complete the octet of electrons in the valence shell.

Valency of chlorine:

Atomic number of chlorine = 17

Protons = Electrons = 17

Electronic configuration = 2, 8, 7

Thus, one electron is gained to complete its octet and so its valency is 1. Valency of sulphur

Atomic number of chlorine = 16

Protons = Electrons = 16

Electronic configuration = 2, 8, 6

Thus two electrons are gained to complete its octet and hence its valency = 2

Valency of magnesium

Atomic number of chlorine = 12

Protons = Electrons = 12

Electronic configuration = 2, 8, 2

Thus, it can lose two electrons to attain octet and hence its valency = 2.

Q74. Compare all the proposed models of an atom given in this chapter.

Ans.

Thomson’s Model of an Atom

1. Sphere is positively charged.

2. Electrons are negatively charged and scattered all over in the sphere.

3.

Image from NCERT

4. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Rutherford’s Model of an Atom

1. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

2. The electrons revolve around the nucleus in well-defined orbits.

3.

Image from NCERT

3. The size of the nucleus is very small as compared to the size of the atom.

Bohr’s Model of Atom

1. Positive Charge in the nucleus.

2. Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom. While revolving in discrete orbits the electrons do not radiate energy.

3.

Image from NCERT

4. These orbits or shells are represented by the letters K, L, M, N,… or the numbers, n=1,2,3,4,….

Q75. Explain with examples (i) Atomic number, (ii) Mass number,

(iii) Isotopes and iv) Isobars. Give any two uses of isotopes.

Ans. (i) Atomic number

We know that protons are present in the nucleus of an atom. It is the number of protons of an atom, which determines its atomic number. It is denoted by ‘Z’. All atoms of an element have the same atomic number, Z. In fact, elements are defined by the number of protons they possess. For hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus. Similarly, for carbon, Z = 6. Therefore, the atomic number is defined as the total number of protons present in the nucleus of an atom.

(ii) Mass number

Mass of an atom is practically due to protons and neutrons alone. These are present in the nucleus of an atom. Hence protons and neutrons are also called nucleons. Therefore, the mass of an atom resides in its nucleus. For example, mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u. Similarly, the mass of aluminium is 27 u (13 protons+14 neutrons).

(iii) Isotopes

Isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers. For example, take the case of

hydrogen atom, it has three atomic species, namely protium, deuterium and tritium. The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.

(iv) Isobars

Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. Let us consider two elements — calcium, atomic number 20, and argon, atomic number 18. The number of electrons in these atoms is different, but the mass number of both these elements is 40.

Uses of isotopes

Some isotopes have special properties which find them useful in various fields. Some of them are:

(i) An isotope of uranium is used as a fuel in nuclear reactors.

(ii) An isotope of cobalt is used in the treatment of cancer.

(iii) An isotope of iodine is used in the treatment of goitre.