Robert Boyle was the first scientist to use the term element in 1661. Antoine L. Lavosier defined an element as basic form of matter that can not be broken down into simpler substances by chemical reactions. Elements contain atoms of only one type. At present,118 elements are known to us.All these have different properties.
Elements can be classified as metals , non metals and metalloids on the basis of their properties.
Metals combine with oxygen to form metal oxide. Metal + O2 --> Metal oxide
Examples: (i) 2Cu + O2 --> 2CuO
(ii) 4Al +3O2 --> 2Al2O3
Different metals show different reactivities towards O2.
a) Na and K react so vigorously that they catch fire if kept in open so they are kept immersed in kerosene oil.
b) Mg, Al, Zn and Pb react slowly with air. These elements are covered with a thin layer of oxide.
c) Fe does not burn on heating but iron filings burn vigorously.
d) Cu does not burn but is coated with black copper oxide.
e) Ag(Silver) and Au(Gold) do not react with oxygen.
2. Reaction of metals with water :-
Metal + Water --> Metal oxide + Hydrogen
Metal oxide + Water --> Metal hydroxide
Some Examples -
1. 2Na + 2H2O --> 2NaOH + H2 + Heat
2. Mg + 2H2O --> Mg(OH)2 + H2 Examples
3. 2Al + 3H2O --> Al2O3 + 3H2
Metal oxides which react with both acids as well as bases to produce salt and water are called Amphoteric Oxides.
Examples : Zinc and Aluminium oxides.