Group 2 Trends in first ionization energy

The first ionization energy is the energy needed to remove the most loosely held electron from each of one mole of gaseous atoms to make one mole of singly charged gaseous ions. In other words, for 1 mole of reactant in the process shown below:

Notice that first ionization energy decreases down the group. Ionization energy is governed by three factors:

• the charge on the nucleus,
• the amount of screening by the inner electrons,
• the distance between the outer electrons and the nucleus.

The increase in nuclear charge down the group is exactly offset by the increase in the number of inner electrons. As mentioned before, the outer electrons feel a net charge of +2 from the center. However, down the group, the distance between the nucleus and the outer electrons increases and so they become easier to remove—the ionization energy decreases.