Neon
Explanation
Valency is defined as the combining capacity of an element, which is determined by the number of electrons present in the outermost shell (valence electrons). Elements react to achieve a stable electronic configuration, typically an octet (8 electrons) in the outermost shell.
The valency of the given elements is analyzed as follows:
- Hydrogen (Atomic No. 1): It has 1 electron in its outermost shell ($K$ shell). It needs 1 more electron to complete its duplet (stable configuration like Helium). Hence, its valency is 1.
- Nitrogen (Atomic No. 7): Its electronic configuration is 2, 5. It has 5 valence electrons and requires 3 more to complete its octet. Hence, its valency is 3.
- Neon (Atomic No. 10): Its electronic configuration is 2, 8. The outermost shell is completely filled with 8 electrons (stable octet). Consequently, it has no tendency to lose, gain, or share electrons under normal conditions. Hence, its valency is 0.
- Silicon (Atomic No. 14): Its electronic configuration is 2, 8, 4. It has 4 valence electrons and typically shares them to achieve stability. Hence, its valency is 4.
Key Takeaway: Noble gases (Group 18 elements like Helium, Neon, Argon) possess a stable, fully filled outermost shell, resulting in a zero valency as they are chemically inert.