Alkali metals are a group of chemical elements in the periodic table that have one valence electron, making them highly reactive. They include lithium, sodium, potassium, rubidium, cesium, and francium.
Ionization energy is the amount of energy required to remove an electron from an atom or molecule in its gaseous state. It is usually measured in units of kilojoules per mole (kJ/mol) or electron volts (eV).
Alkali metals have low ionization energy because they have only one valence electron, which is loosely held and easily removed. This is due to their large atomic size and low effective nuclear charge, which decreases the attraction between the nucleus and the outermost electron.
The trend in ionization energy for alkali metals is that it decreases down the group. This is because the atomic radius increases down the group, resulting in a weaker attraction between the nucleus and the outermost electron, making it easier to remove.
The lower the ionization energy, the more reactive the alkali metal is. This is because a lower ionization energy means it is easier to remove an electron, making it more likely to form compounds and react with other elements.