Have a region surrounding the central area called

The nucleus of the atom is the region which is surrounded by orbits of electrons around it. The central area of the atom is called as its nucleus which usually contains protons and neutrons in it. These are called subatomic particles of the atom, which includes proton, electron and neutron.

Protons are positively charged species, neutrons are neutrally charged they do not have any charge on it whereas electrons are negatively charged species. The number of protons and electrons is same in a particular specific elemental atom.

2 F atoms

Explanation:

A Ca atom has a valence number of +2, which means that when ionized, it will give up two electrons. In turn, a F atom has a valence number of -1, which means it can only accept 1 electron when ionized. So to accommodate the two electrons from Ca atom, we need 2 F atoms.