Oxidation state

element	usual oxidation state
Group 1 metals	always +1
Group 2 metals	always +2
Oxygen	usually -2
Hydrogen	usually +1
Fluorine	always -1
Chlorine	usually -1

Solids

Q. 1.  (a)       If the radius of the Br ion is 0.182 nm, how large a cation can fit in each of the tetrahedral hole.

            (b)       AgI crystallizes in a cubic closed packed ZnS structure. What fraction of tetrahedral site is occupied by Ag ion ?

            (c)       At what temp. range, most of the metals becomes super conductors ?

Ans.    (a)       For a tetrahedron the limiting ratio is 0.225 – 0.414

                        For largest cation highest value 0.414 would be considered.

                        r+ / r– = 0.414

                        r+ = 0.414 × 0.182 = 0.075 nm.

            (b)       In FCC there are 8 tetrahedral voids. Out of this ½ is occupied by Ag cation.

            (c)        2 k – 5 k.

Elements

Phosphorus

Phosphorus contains P₄ molecules. To melt phosphorus you don't have to break any covalent bonds - just the much weaker van der Waals forces between the molecules.

Sulphur

Sulphur consists of S₈ rings of atoms. The molecules are bigger than phosphorus molecules, and so the van der Waals attractions will be stronger, leading to a higher melting and boiling point.

Chlorine

Chlorine, Cl₂, is a much smaller molecule with comparatively weak van der Waals attractions, and so chlorine will have a lower melting and boiling point than sulphur or phosphorus.

Argon

Argon molecules are just single argon atoms, Ar. The scope for van der Waals attractions between these is very limited and so the melting and boiling points of argon are lower again.