Why silver articles become black after sometime when exposed to air?

Air contains traces of hydrogen sulphide (H₂S) gas. Silver has a tendency to combine with H₂S to form silver sulphide (Ag₂S) which is black in colour. On account of this, silver articles become black after some time when kept exposed to air.

$2Ag\left(s\right)$ + $H_{2}S\left(g\right)$ $\to$ ${Ag}_{2}S\left(s\right)$ + $H_2\left(g\right)$