Acidity of an acid depends upon the nature of the compound, its structure or its position in the periodic table. The acids are normally classified into three categories :

(i)    Hydro acids: e.g. HCI, H₂S, PH₃, H₂ Te etc.
(ii)    Oxo acids : e.g. HCIO₄, H₃PO₄, HSO₄^-etc.
(iii)   solvated captions: e.g. [AI( H₂O)₆]³⁺ NH₄⁺ etc.

(a)    Strength of Hydro Acids. There is a general increase in hydro acid strength in going form left to right along a period. For example the following trend of acid strength is observed along the second period.
                                                         HF > H₂O > NH₃ > CH₄
This trend is explained on the basis of increase in electronegative as we more form left to right along a period
                                                         C < N <O < F
With the increase in electronegative of the central atom, the tendency, to pull the electron pair form H increases, and dissociation of H becomes easier and acidic character increases.

The above tend based on electro negativity holds true along a period. It does not hold good as we move down a group. In fact, acid in a group increases with decrease in electronegative as we go down the group. For example in the hydro acids of group VII, HI is the strongest acid and HF is the weakest acid which is opposite to the trend along a period.
                                                           HI > HBr > HCI > HF
Similarly n group VI the acid strength increases as we move down the group with H₂O as the weakest acid
                                                     H₂O < H₂S < H₂Se < H₂ Te.
and H₂Te as the strongest acid in spite of H₂O being the most polar on the basis of electronegative.

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