Why should you never touch an alkali metal with your fingers?

Hint: Alkali metals, due to their low ionisation energies, are some of the more reactive metals in the periodic table. Our skin contains some moisture on it. Alkali metals react with the moisture on our skin to form alkali metal hydroxides.

Complete Step by Step Solution:
Alkali metals are present in Group 1 of the periodic table just below hydrogen. They are Lithium (\[Li\]), Sodium (\[Na\]), Potassium (\[K\]), Rubidium (\[Rb\]), Caesium (\[Cs\]) and Francium (\[Fr\]). Their valence orbital consists of a single s electron which, when lost, results in the alkali metals attaining the electronic configuration of their nearest noble gas thereby, becoming stable. Thus, alkali metals easily lose their single valence electron which is why they have low ionisation energies.

Due to low ionisation energies, alkali metals are highly reactive to other substances, including air and water. Alkali metals on exposure to air, burn vigorously and form alkali metal oxides.
\[4Li(s) + {O_2}(g) \to 2L{i_2}O(s)\]
\[2Na(s) + {O_2}(g) \to N{a_2}{O_2}(s)\]

Alkali metals react with water to form alkali metal hydroxides with the liberation of hydrogen gas. The alkali metal hydroxides are caustic, which means they can corrode or burn organic tissue by reacting with it. Also, this reaction is highly exothermic which can inflame the hydrogen gas causing an explosion. Elemental potassium and caesium are particularly dangerous in this regard. The general reaction of alkali metals with water is given below (M represents alkali metals):
\[2M(s) + 2{H_2}O(l) \to 2MOH(aq) + {H_2}(g)\]
The above reaction can happen when our skin comes into contact with alkali metals since our skin contains some moisture which can react with the alkali metal. The resulting hydroxide will cause chemical burns to the skin.

Alkali metals should not be touched with our fingers because they can react with the moisture in our skin to form caustic alkali metal hydroxides which can cause chemical burns and injuries.

Note: All alkali metals are hazardous to human skin in their elemental forms. Certain alkali metal salts such as Sodium chloride (\[NaCl\]) are safe to handle with our fingers although it is not advised to handle any chemical with our fingers without wearing proper safety gear. Even if certain chemicals are handled with bare hands, it is necessary to wash our hands with soap after handling them.