Define ionisation energy and explain how it var... - SS2 Chemistry Atomic Structure and Periodicity Question

Ionisation energy refers to the energy required to remove an electron from an atom or ion in the gaseous state.

Across a period in the periodic table, ionisation energy generally increases. This is because, as you move from left to right across a period, the atomic radius decreases, and the effective nuclear charge (the net positive charge experienced by valence electrons) increases. The increased attraction between the positively charged nucleus and the electrons requires more energy to remove an electron, leading to higher ionisation energy.

Down a group in the periodic table, ionisation energy generally decreases. This is due to the increasing atomic radius and the shielding effect from inner electron shells. The larger atomic radius and shielding reduce the effective nuclear charge, making it easier to remove an electron and resulting in lower ionisation energy.