Question

For main-group elements using the older numbering system the group number on the periodic table corresponds to which characteristic of the elements of that group

Answer 1

The Group number and period number corresponds to the outer most shell and number of electrons present in outer most shell.

For Example:

Oxygen has atomic number 8, and its electronic configurations is,

                                    1s², 2s², 2pₓ², 2py¹, 2pz¹

The outermost shell is 2, means oxygen is present in period 2, and the number of electrons in n = 2 are 6, so the group number is 6 in periodic table.

Lets take another example of Sodium, it has atomic number 11 with electronic configuration;

                                         1s², 2s², 2p⁶, 3s¹

The outermost shell is 3, means sodium is present in period 3, and the number of electrons in n = 3 are 1, so the group number is 1 in periodic table.