How did the group number of s block element ?
Number of electronic shells is represented by period number and number of electrons present in the valence shell is represented by the group number.
For example:—
Consider chlorine having atomic number 17.
It’s electronic configuration is
1s²2s²2p⁶3s²3p⁵
it requires three electronic shells ,so it belongs to third period. It contains 7 electrons in the valence shell,hence it is placed in 7th A group of the periodic table.