Question

How many valence electrons does calcium have?

Answer 1

Hint The atomic number of Ca is 20. The valence electrons are those electrons which are present in the valence shell or outermost shell of an atom.

Complete answer:
In the question it is asked how many valence electrons are present in a Calcium (Ca) atom. From the lower classes we are dealing with different elements and its position in the periodic table. We know that the elements in the periodic table are arranged according to the increasing order of the atomic number. So as the atomic number increases the number of electrons in the atom is also increasing.
The atomic number of an element is equal to the number of electrons present in the atom. So now let’s discuss the position of Ca atom in the periodic table, the atomic number and about the valency shown by Calcium.
The calcium is present in the left side of the periodic table, with the alkaline earth metal groups i.e. the group-2 elements and it is present in the fourth period of the group.
While studying about the periodic table we have studied about the shells and orbitals and we know that in a group as we move down a new shell is introduced. The period number will be equal to the total number of shells present in the atom.
As the Ca is present in the fourth period, there are four shells present in the Ca atom. And the atomic number of Ca is 20, hence there are 20 electrons in the atom.
Now let’s write the electronic configuration of Ca-$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$
We know that the valence electrons are those which are present in the outermost shell of the atom. From the electronic configuration we know that the outermost shell of Ca is the fourth shell and the number of electrons in that shell is the valence electrons. Here there are two valence electrons in the outermost shell.

So the number of valence electrons in Ca is 2.

Note: The valence electrons accounts for the valency of the atom i.e. the valency of the atom is the number of electrons that can be shared or removed in the chemical reaction to form the chemical bonds. These electrons in the outermost shells are the electrons which actively participate in the bond formation. The valency of Ca is +2, i.e. two electrons can be donated by the atom for the chemical reaction.