What trends in ionisation energy are observed in alkali metals?

Alkali metals show a trend of decreasing ionisation energy as you move down the group.

In the periodic table, alkali metals are found in Group 1. They include elements such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). One of the key characteristics of alkali metals is their ionisation energy, which is the energy required to remove an electron from an atom to form a positive ion.

As you move down Group 1 from lithium to francium, the ionisation energy decreases. This is because the outermost electron (the one that is removed during ionisation) is further from the nucleus and is therefore less strongly attracted to it. The outermost electron is also shielded from the positive charge of the nucleus by the inner shells of electrons. This shielding effect increases as you go down the group because there are more inner shells of electrons.

For example, lithium has an ionisation energy of 520 kJ/mol, while sodium has an ionisation energy of 496 kJ/mol. This trend continues down the group, with francium having the lowest ionisation energy of all the alkali metals.

This trend in ionisation energy is important as it affects the reactivity of the alkali metals. The lower the ionisation energy, the more easily an atom can lose an electron to form a positive ion, and therefore the more reactive the element is. This is why alkali metals become more reactive as you go down the group.

In summary, the trend in ionisation energy in alkali metals is a decrease as you move down the group. This is due to the increasing distance of the outermost electron from the nucleus and the increasing shielding effect of the inner shells of electrons.