Chemistry

9. Exceptions if classification of metals and non-metals is done on the basis of physical properties :
(i) All metals are hard except sodium, potassium, lithium, They can be cut even with knife. Osmium (Os) is hardest metal. Lithium is lightest metal.
(ii) All metals are solids except mercury. Cesium, francium, germanium and gallium are low melting solids. Gallium becomes liquid if kept on palm. But gallium has very high boiling point which makes it useful for high temperature thermometers.
(iii) Iodine is a non-metals but has metallic lustre. Diamond (an allotrope of carbon) is highly lustrous.
(iv) Non-metals have low melting and boiling points but diamond, graphite, boron and silicon have high melting and boiling points.
(v) Metals have high melting and boiling points. Tungsten has highest melting point whereas sodium, potassium have low melting and boiling points.
(vi) Non-metals are bad conductors of heat and electricity, but graphite is a non-metals which is good conductor of heat and electricity.
(vii) Metals generally have high density e.g., Cu, Cr, Ag, Au, Pt, Os, Ir, Ti, W, etc. Sodium, potassium, and lithium have density less than 1 g cm^–3. Diamond is non-metals but still has high density.
(viii) Generally, metals have 1 to 3 valence electrons. Hydrogen has one valence electron but still it is non-metal. Helium has 2 valence electrons but still it is non-metals.
(ix) Metals usually have a bright metallic lustre and are silvery white in appearance, but gold is yellow in colour, copper is reddish brown.
(x) Non-metals generally do not from alloys but carbon is alloyed with iron to form steel.
Thus metals and non-metals cannot be classified on the basis of physical properties only. Chemical properties are more suitable to classify them.