difference between long period and short period in modern periodic table

The difference between short period and long period is based upon the number of elements in each period. There are two short periods in the periodic table- the 2^nd period and the 3^rd period. Both these contain eight elements only. The long periods are the 4^th period and the 5^th period. Both these contain 18 elements each. 

The shortest period is the first period which contains two elements, while the longest period is the 6^th period which contains 32 elements.