1) Across the period as one moves from left to right, atomic size decreases.
2) Consider 3rd period elements containing non-metal i.e Si, P, S and Cl.
3) Here, atomic radius of these elements are in following order Si>P>S>Cl.
4) However, electronegative of these elements increases as we move from left to right
5) Thus, the corresponding hydrides have higher electro-negativity difference,as one moves from left to right.
6) Due to this, bond polarity increases from left to right,
7) Increase in bond polarity, results in increasing proton donating capability.
8) Therefore, acidity increases as one moves along the period.
