Valency vs Atomic Size?

📌 CONCEPT: Valency refers to the number of electrons an atom loses, gains, or shares to form a chemical bond, whereas atomic size is the distance between the nucleus and the outermost electron in an atom.

📐 RULE / FORMULA: Generally, as we move across a period in the periodic table from left to right, the valency increases due to an increase in the effective nuclear charge, which pulls the electrons closer to the nucleus, resulting in a decrease in atomic size.

💡 WORKED EXAMPLE: Consider the elements sodium (Na) and magnesium (Mg) in period 3 of the periodic table. Sodium has a valency of 1 and atomic size of 186 pm, whereas magnesium has a valency of 2 and atomic size of 145 pm. This decrease in atomic size is due to the increase in the effective nuclear charge as we move from sodium to magnesium.

⚠️ COMMON MISTAKE: Students often confuse valency with atomic number, which is the total number of protons in an atom's nucleus.