Why Does First Ionization Energy Decrease Down Group 1 (Alkali Metals)?

The first ionization energy is the energy required to remove the outermost electron from an atom in its gaseous state. Down Group 1 (also known as the alkali metals), the first ionization energy decreases even though the atomic number increases due to two main factors: increasing atomic radius and increasing shielding effect.

1. Atomic radius: As you go down Group 1, the atomic radius increases. This is because each successive element has an additional electron shell, resulting in an increase in the distance between the nucleus and the outermost electron. The larger the atomic radius, the weaker the attraction between the nucleus and the outermost electron. Therefore, less energy is required to remove the outermost electron, resulting in a decrease in the first ionization energy.

2. Shielding effect: Down Group 1, each successive element has an additional electron shell, which acts as a shield between the nucleus and the outermost electron. The inner electrons repel the outermost electron, reducing the effective nuclear charge felt by the outermost electron. As a result, the attraction between the nucleus and the outermost electron weakens. With a weaker attraction, less energy is required to remove the outermost electron, leading to a decrease in the first ionization energy.

Overall, the combination of increasing atomic radius and the shielding effect causes the first ionization energy to decrease down Group 1, despite the increase in atomic number.