Modes of occurrence of elements:- Depending upon the reactivity of the elements, they generally occur in two states:- (1) Native State (2) Combined state.

1. Native State:- The elements which are low metals occur in the free state called their native state e.g. nitrogen gas, noble gases, metals like gold, sliver, platinum etc.
2. Combined State:- The elements which are reactive and have tendency to combine with oxygen or air, moisture, carbon dioxide, nitrogen, phosphorous etc. occur in combined state. These elements occur in the earth crust in the form of their compounds. Metals are generally found in the oxidized form and the non metals in the reduced form. e.g.  Iron, Zinc etc.

Oxygen is the most abundant element in the  earth curst (49.5%). Aluminium is third most abundant element and the most abundant metal, than comes iron.