CBSE Class 10 Answered

The maximum number of electrons present in the shell is given by theformula (2n²), where n is the orbit number or shell number.

 First orbit or K shell will be = 2 X 1² = 2, Second orbit or L shell will be = 2 X 2² = 8,   Third orbit or M shell will be = 2 X 3² = 18, Fourth orbit or shell will be = 2 X 4² = 32 and so on.

The maximum number of electrons that can be accommodated in the outermost orbit is 8.

Further,

Starting with the energy level/shell closest to the nucleus electrons fill the different shells in order of increasing energy.

Energy levels or shells can be divided into sub-shells, which can hold different numbers of electrons.

Within each sub-shell there are one or more orbitals. Each orbital canhold a maximum of two electrons (of opposite spin).

Atomic number of magnesium is 12; therefore electronic configuration will be 1s²2s² 2p⁶3s². 

There are three shells, out of that third shell consist of two electrons.

 Hence 3s is the valance shell and electrons in this shell are valence electrons of Mg.

Similarly there are seven valence electrons in bromine (Br = 1s²2s²2p⁶3s² 3p⁶ 3d¹⁰4s² 4p⁵).