Final answer:
The first ionization energy is the energy required to remove the outermost electron from a neutral gaseous atom to form a cation, measured in kJ/mol or eV. It generally increases across a period and decreases down a group, with some exceptions due to electron subshell configurations.
Step-by-step explanation:
The first ionization energy (IE) of an element is defined as the amount of energy needed to remove the most loosely bound electron from a gaseous atom in its ground state. This is the energy required for the reaction:
X(g) → X+(g) + e¯ IE1
The energy is measured in kilojoules per mole (kJ/mol) or electron volts (eV) because the ionization energy is always positive. In a periodic trend, the first ionization energy generally increases across a period due to increasing nuclear charge and decreases down a group due to electron shielding. Notably, there are exceptions to this trend based on electron configurations, such as the lower first ionization energy of boron compared to beryllium, which is due to boron having an electron in the higher energy p subshell, making it easier to remove compared to the s electron in beryllium's full s subshell.