The correct option is (c) 1 and 3 only.
Explanation
Valency is the combining capacity of an element, determined by the electronic configuration of its atoms. It represents the number of electrons an atom must lose, gain, or share to achieve a stable noble gas configuration (octet or duplet).
Statement 1 is Correct:
For elements having 1, 2, 3, or 4 electrons in their outermost shell, the valency is typically equal to the number of valence electrons.
- Hydrogen: Has 1 valence electron; its valency is 1.
- Sodium (Atomic No. 11): Configuration is 2, 8, 1. It has 1 valence electron; its valency is 1.
Statement 2 is Incorrect:
For elements with 5, 6, or 7 electrons in the outermost shell (typically non-metals), valency is determined by subtracting the number of valence electrons from 8 (Valency = 8 – Valence Electrons).
- Oxygen (Atomic No. 8): Configuration is 2, 6. Valency is \(8 - 6 = 2\).
- Fluorine (Atomic No. 9): Configuration is 2, 7. Valency is \(8 - 7 = 1\).
Statement 3 is Correct:
Magnesium (Atomic No. 12) has an electronic configuration of 2, 8, 2. Being a metal, it achieves stability by losing the 2 electrons in its outermost shell to attain the stable configuration of Neon (2, 8). Consequently, its valency is 2.
Key Takeaway:
Valency equals the number of valence electrons for atoms with 1 to 4 valence electrons, whereas for atoms with 5 to 7 valence electrons, it is calculated as 8 minus the number of valence electrons.