How will you find the valency of chlorine, sulphur and magnesium?

To find the valency of chlorine:

The atomic number of chlorine is 17

Number of electrons is equal to the number of protons in chlorine i.e., 17

The distribution of electrons in chlorine atom is K-2, L-8, M-7

As the atom always wants to follow the octet rule for its stability, so it will try to make its last orbit filled with 8 electrons. There arises two conditions:
Either it can donate its 7 electrons and make its L shell as the last shell having 8 electrons or it will accept 1 electron in its M shell and will fulfill the octet rule. But it will choose to accept 1 electron which consumes less energy as compared to donating 7 electrons which will consume more energy and can result in the instability of the atom.

Hence, from the distribution of chlorine, it will fill the M shell by accepting 1 electron. Therefore its valency is -1.

The atomic number of sulphur is 16

Number of electrons is equal to the number of protons in sulphur i.e., 16

The distribution of electrons in sulphur atom is K-2, L-8, M-6

As the atom always wants to follow the octet rule for its stability, so it will try to make its last orbit filled with 8 electrons. There arises two conditions:
Either it can donate its 6 electrons and make its L shell as the last shell having 8 electrons or it will accept 2 electrons in its M shell and will fulfill the octet rule. But it will choose to accept 2 electrons which consumes less energy as compared to donating 6 electrons which will consume more energy and can result in the instability of the atom.

Hence, from the distribution of sulphur, it will fill the M shell by accepting 2 electrons. Therefore its valency is -2.

The atomic number of magnesium is 12

Number of electrons is equal to the number of protons in magnesium i.e., 12

The distribution of electrons in magnesium atom is K-2, L-8, M-2

As the atom always wants to follow the octet rule for its stability, so it will try to make its last orbit filled with 8 electrons. There arises two conditions:
Either it can donate its 2 electrons and make its L shell as the last shell having 8 electrons or it will accept 6 electrons in its M shell and will fulfill the octet rule. But it will choose to donate 2 electrons which consumes less energy as compared to accepting 6 electrons which will consume more energy and can result in the instability of the atom.

Hence, from the distribution of magnesium, it will lose or donate the 2 electrons from the M shell Therefore, its valency is +2.