Question

Why do alkali metals have low ionization energy?

Answer 1

We need to remember that an ionization energy is defined as the minimum energy that is the minimum energy required to remove the outermost electron in the valence shell or the loosely bound electron of an isolated neutral gaseous atom or molecule.

Complete answer:
As we know, in the modern periodic table elements are arranged in increasing atomic number (number of protons) of the elements. As the number of protons is almost equal to the number of electrons, we can also say that the periodic table is arranged according to the increasing number of electrons. As the number of electrons increases the effective size of the atom also increases.
As we have studied earlier, the size of the atom decreases as we move from left to right in a periodic table, except for Noble Gases which has the largest size of the atom in the period. Also, the size of atoms decreases as we move down the group. With the decrease in the size of the atom, the electrons come closer to the nucleus hence the effective charge increases and the electrons are tightly bound to the nucleus.
Alkali Metals belong to the first group of the periodic table and consists of the elements: lithium $\left( {Li} \right)$ , sodium $\left( {Na} \right)$ , potassium $\left( K \right)$ , rubidium $\left( {Rb} \right)$ , caesium $\left( {Cs} \right)$ , and francium $\left( {Fr} \right)$
Alkali Metals have low ionization energy because:
Size of the atom: The size of alkali metals is the largest in a particular period. As the size of the atom increases, the outermost electron moves far off from the nucleus and the nuclear attraction becomes weak. Because of this the outermost electron can be easily removed because it's not tightly bound to the nucleus.
Stable electronic configuration: Every atom loses or gains electrons to attain the stable noble gas configuration. In case of alkali metal losing the outermost electron from the s orbital can make it attain this Noble gas configuration. Hence alkali metal easily loses this electron and forms mono cations.
Shielding effect: Due to the large size of the alkali metals, and increase in shielding effect on increase in size of the alkali metals, it becomes easy to remove the outermost electron hence resulting in low ionization energy. Shielding effect is the effect that results in the decrease in the nuclear attraction because of an increase in the number of electrons in between the nucleus and the outermost electron.
The above-mentioned reasons are hence responsible for the low ionization energy of alkali metals.

Note:
We must have to remember that the noble gases have the highest ionization energy in spite of their large size because Noble gases have stable electronic configuration and removal of electrons results in the atom being unstable. Hence a high amount of energy is required to remove an electron from Noble Gases.