Which of the following has small size?

Explanation

The correct answer is sulfur (S) because it has the smallest atomic radius among the given options. This is due to its position in Period 3, where it experiences a higher effective nuclear charge compared to magnesium (Mg), phosphorus (P), and sodium (Na), all of which have larger atomic sizes. Magnesium has a larger radius because it has an additional electron shell compared to sulfur, while phosphorus is larger than sulfur as it is to the left in the periodic table and has fewer protons, resulting in less nuclear charge. Sodium also has a larger radius for similar reasons, being further left in the periodic table and having more electron shells than sulfur.