atoms are made of 3 types of subatomic particles; electrons, protons and neutrons
atomic number is the number of protons. atomic number is characteristic for the element. In ground state atoms, the number of electrons and protons are the same.
the electronic configuration of Ca in the ground state is
Ca - 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
when Ca loses its 2 valence electrons, it becomes positively charged and the electronic configuration becomes
Ca - 1s² 2s² 2p⁶ 3s² 3p⁶
number of electrons in Ca²⁺ is 18
the atom in the ground state would have the same number of electrons and protons. Therefore number of protons are 18. then the atomic number of the element is 18
the atom having an atomic number of 18 is Ar.
the answer is 1) Ar