Elements with same mass number and different atomic numbers are called isobars.

Complete the Table 4.1 on the basis of information available in the symbols given below

(a) ³⁵₁₇CI

(b) ¹²₆C

(c) ⁸¹₃₅Br

Helium atom has 2 electrons in its outermost shell and its duplet is complete. Hence the valency is zero.

(a) Rutherford’s α-particle scattering experiment led to the discovery of the __________.
Ans. atomic nucleus
(b) Isotopes have same __________ but different __________.
Ans. atomic number, mass number
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will
be __________ and __________ respectively.
Ans. 0 and 1.
(d) The electronic configuration of silicon is __________ and that of Sulphur is __________.
Ans. Silicon—2, 8, 4, Sulphur— 2, 8, 6 

Valency is zero as K shell is completely filled.

Helium has two electrons in its only energy shell, while Argon and Neon have 8 electrons in their valence shells. As these have maximum number of electrons in their valence shells, they do not have any tendency to combine with other elements. Hence, they have a valency equal to zero.

(ii) If the atom is represented by the planet earth (R_e= 6.4 �10⁶ m) then the radius of the nucleus would be r_n = R_e /10⁵

r_n = 6.4 x 10⁶ m / 10⁵
= 6.5 x 10 m
= 64 m

Rutherford concluded from the α-particle scattering experiment that–
(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space. 
(iii) A very small fraction of α-particles were deflected by 180^o, indicating that all the positive charges and mass of the gold atom were concentrated in a very small volume within the atom.
From the data he also calculated that the radius of the nucleus is about 10⁵ times less than the radius of the atom.