What is the number of elements in the first four periods of the long form of the periodic table?

Rewrite the following statement after corrections (if necessary) : 

Groups have elements with same number of electrons.

Rewrite the following statement after corrections (if necessary):

Isotopes are the elements of same group.

Why do the elements in a same group have generally similar properties?

The atomic radii of elements decrease across a period from left to right. Justify.

What was Dobereiner's basis of classification of elements?

How many periods and groups are there in the long form of the periodic table?