NCERT Standard 8 : Materials Metals and Nonmetals Class 8 Notes

The property of metals by which they can be beaten into thin sheets is called malleability. The property of metal by which it can be drawn into wires is called ductility.  Since metals produce ringing sounds, they are said to be sonorous. The materials other than metals are not sonorous.

Some materials are hard, lustrous, malleable, ductile, sonorous and good conductors of heat and electricity. The materials which generally posses these properties are called metals. The examples of metals are iron, copper, aluminium, calcium, magnesium, etc. In contrast, materials like coal and sulphur are soft and dull in appearance. They break down into  powdery mass on tapping with hammer. They are not sonorous and are poor conductors of heat and electricity. These materials are called non-metals. The examples of non-metals are sulphur, carbon, oxygen etc. 

In a chemical reaction, new substances are formed. These substances are different from those which underwent the reaction. Now, if a substance cannot be broken down further by chemical reactions, by cooling, heating,or by electrolysis, it is called ‘element’. Sulphur is an element. So is iron. Carbon, too, is an element. 

The smallest unit of an element is atom. A sample of an element contains only one kind of atoms. The atom of an element remains unaffected by physical changes in the element. For  ample, an atom of liquid sulphur would be exactly the same as the atom of solid or vapour sulphur. Although we have an infinite variety of substances in the universe, the number of elements forming these substances is limited. 

There are no more than 92 naturally occurring elements. An important classification of elements is in terms of metals and non-metals. Most of the elements are metals. Less than 20 are non-metals. A few are metalloids which possess characters of both metals and non-metals.