Very small quantity of carbon in iron as in steels forms interstitial solid solution mainly because atomic size(s) of

Concept:

 Impurity atoms are of two types either substitutional or interstitial atoms:

1. Interstitial impurity atom refers to that atom which occupies an interstitial position i.e. between the atoms of the ideal crystal. For example- the presence of carbon in iron where carbon occupies an interstitial position in the iron lattice structure without displacing any of the parent atoms of iron lattice from their positions.
2. Substitutional impurity atom refers to that atom which substitutes for or replaces a parent atom in the crystal. For example in brass, zinc is a substitutional atom in the copper lattice.