Name or state the following.

1. The three isotopes of hydrogen.
2. Two elements having same number of protons and electrons but different number of neutrons.
3. The valency of an element whose electronic configuration is 2, 8, 3.
4. The shell closest to the nucleus of an atom.
5. An element having valency zero.

1. Protium [$^1$1\text{H}], Deuterium [$^21\text{H}$], Tritium [$^3_1\text{H}$]
2. Sodium [e = p = 11, n = 12] and Chlorine [e = p = 17, n = 18]
3. The valency is 3 positive. Electronic configuration = 2, 8, 3. Therefore, the element tends to lose 3 electrons and gain stable octet configuration.
4. K shell.
5. Noble gases [He, Ne, Ar etc.]