(a) Correct distribution is 2, 8, 8, 2. two shell (K, L) are completely filled.
(b) The teacher told Shyam that a principle should be applied in totality to solve a problem.

\(_6^{12}C\) and  \(_6^{14}C\) are isotopes, have the same electronic configuration as (2, 6) \(_{10}^{22}Ne\) and \(_{11}^{22}Na\)are isobars. They have different electronic configurations as given below:

The atomic masses of  \(_{17}^{37}Y\) and \(_{17}^{35}Y\)  are 37 and 35 respectively. Let percentage of \(_{17}^{37}Y\) be A, then we have
\(\left(37\times{A\over 100}\right)+\left(35+{100-A\over 100}\right)=35.5\)
or A = 25
\(_{17}^{37}Y\) is 25% and  \(_{17}^{35}Y\) is 75%

(i) A
(ii) D
(iii) B
(iv) C and E.

Ordinary elements are composed of a mixture of isotopes. For example, chlorine is a mixture of 75% \(_{17}^{35}Cl\) (35-17=18 neutrons) and 25% \(_{17}^{37}Cl\) (37-17=20 neutrons).The atomic mass of chlorine is the average of these two isotopes and is, therefore, equal to \(\left(35\times{75\over 100}\right)+\left(37\times{25\over100}\right)=35.5\) This accounts for the fractional atomic masses of elements.

Atoms of the element having the same atomic number but different atomic masses are called isotopes. This means isotopes of an element have the same number of protons but different numbers of neutrons in their nuclei. For example, carbon has two isotopes
(i) \(_6^{12}C\) - Atomic number of 6 and mass no. 12 and thus has 6 neutrons
(ii) \(_6^{14}C\) - Atomic number of 6 and mass no. 14 and thus has 8 neutrons.

(i) The atomic number (Z) is the number of protons present in the nucleus of an atom
(ii) Mass number (A) is sum of the number of protons (p) and the number of neutrons (n) in the nucleus of an atom.
Thus, A = n + p
or = n + Z.

(a) Electronic configuration of the atom is K (2), L(5).
(b) It has a valency of - 3 as it can accommodate three more electrons to complete its octet.

Cathode rays are made up of negatively charged particles called electrons.The nature of cathode rays does not depend on the nature of the gas from which theses are produced.The mass of a cathode ray particle is very small compared to the mass of the atom from which it is formed.These particles were found to be about 2000 times smaller in mass than the hydrogen atom.
Anode rays are a stream of positively charged particles.The mass of an anode ray particle is equal to the mass of the atom from which it is formed.The nature of the anode rays depends on the gas from which these are produced.

Properties of anode rays:
(i) Anode rays travel straight lines. This is revealed by the fact they cast shadows of the solid objects placed in their path.
(ii) Anode rays material particles. They are capable of producing mechanical effects, e.g., they rotate a light paddle wheel placed in their path.
(iii) Anode rays are positively charged. This is shown by the fact that anode rays are deflected towards the negative plate of an electric field.

The statement is true because:
(i) Whatever be the nature of the gas or the material of the cathode, the electrons have the same charge to mass(e/m) ratio i.e., 1.7589x10¹¹ coulomb per kg
(ii) The electrons obtained by different methods are identical.

(i) The mass of an electron is about 1/1840 that of a hydrogen atom, i.e., about 9.0x-10^-31kg.
(ii) An electron is a negatively charged particle and has a charge of 1.6x10^-19 coulomb. This is one unit of charge.