Calcium (Ca) has an atomic number of 20, which means it has 20 electrons. The electronic configuration of a calcium atom is:

2, 8, 8, 2

In the outermost energy level (n = 4), there are two valence electrons. These valence electrons are the electrons that participate in chemical bonding.

To achieve a stable electron configuration, calcium atoms tend to lose the two valence electrons to form a positively charged ion, Ca²⁺. These two valence electrons are the ones involved in chemical bonding when calcium atoms participate in the formation of compounds.

In summary, two electrons in a calcium atom are involved in chemical bonding.