When sodium is obtained, great care is taken to prevent air
coming into contact with sodium. Give a reason for this.

When sodium is obtained, great care is taken to prevent air coming into contact with sodium atom because sodium is highly reactive in atomic state and make bonds with the gases present in the air such as oxygen.

Explanation:

Sodium is highly reactive because its outermost shell is incomplete so in order to attain stability, Sodium react with other atoms and become stable. Sodium make ionic bond in which sodium losses electron which is present in the outermost shell.

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.