In a particular calcium atom, number of protons are 20, number of neutrons are 21 and that of electrons are 19. Here, protons are positively charged particles, neutrons are neutral and electrons are negatively charged particles. For a neutral atom, number of protons should be equal to number of electrons. If number of protons are more than number of electrons cation will form (positive charge) and if number of protons are less than number of electrons anion will form (negative charge).
Here, number of protons are 20 and that of electrons are 19, protons is 1 more than electron thus, Ca will get one positive charge and exist as Ca^{+}. Thus, the electrical charge on the atom will be +1.