Explain the occurrence of elements in nature.

(N/A) Elements are found in nature in two forms: $(i)$ Native state and $(ii)$ Combined state.
$1$. Native state: Elements that are chemically inert or less reactive,such as carbon,sulphur,gold,and noble gases,occur in their free or native state.
$2$. Combined state: Most elements are highly reactive and occur in the earth's crust in the form of compounds like oxides,sulphides,carbonates,and halides.
Abundance of elements:
- Aluminium is the most abundant metal and the third most abundant element in the earth's crust ($8.3\%$ by weight). It is a major component of minerals like mica and clays. Gemstones like "Ruby" and "Sapphire" are impure forms of $Al_{2}O_{3}$ containing traces of chromium and cobalt,respectively.
- Iron is the second most abundant metal in the earth's crust and is crucial for biological systems.
Principal Ores of Important Metals:

Metal	Ores and Composition
Aluminium	Bauxite $(AlO_{x}(OH)_{3-2x})$,Kaolinite
Iron	Haematite $(Fe_{2}O_{3})$,Magnetite $(Fe_{3}O_{4})$,Siderite $(FeCO_{3})$,Iron pyrites $(FeS_{2})$
Copper	Copper pyrites $(CuFeS_{2})$,Malachite $(CuCO_{3}\cdot Cu(OH)_{2})$,Cuprite $(Cu_{2}O)$,Copper glance $(Cu_{2}S)$
Zinc	Zinc blende $(ZnS)$,Calamine $(ZnCO_{3})$,Zincite $(ZnO)$