Atomic Structure and Periodicity - SS2 Chemistry Past Questions and Answers - page 5

The atomic radius refers to the size of an atom, typically defined as half the distance between the nuclei of two adjacent atoms in a molecule or crystal lattice.

Across a period (from left to right) in the periodic table, the atomic radius generally decreases. This is primarily due to the increasing number of protons in the nucleus as you move across the period. The increased positive charge exerts a stronger pull on the electrons, bringing them closer to the nucleus and reducing the atomic radius.

Down a group (top to bottom) in the periodic table, the atomic radius generally increases. This is because each successive energy level or electron shell is farther from the nucleus. The addition of more energy levels increases the distance between the nucleus and the outermost electrons, resulting in a larger atomic radius.

These variations in atomic radius are influenced by the balance between the attractive force of the protons in the nucleus and the repulsive force between electrons. As you move across a period, the increasing positive charge of the nucleus pulls the electrons closer, decreasing the atomic radius. Down a group, the increasing number of energy levels increases the shielding effect, reducing the attractive force between the nucleus and the outermost electrons, leading to an increase in atomic radius.