Question

what do an ion of ca and an atom of ar have in common?
an ion of ca²⁺ and an atom of ar have the same number of electrons, but a different number of protons and neutrons in their atoms.
an ion of ca²⁺ and an atom of ar have the same total number of protons and neutrons in their atoms, but differ in the number of electrons.
there are no similarities between the two.
an ion of ca²⁺ and an atom of ar have the same number of electrons and the same total number of protons plus neutrons in their atoms.

Explanation:

Calcium (Ca) has an atomic number of 20. A Ca²⁺ ion has lost 2 electrons, so it has 18 electrons. Argon (Ar) has an atomic number of 18 and 18 electrons. The number of protons in Ca is 20 and in Ar is 18, and they have different neutron - proton combinations. So, a Ca²⁺ ion and an Ar atom have the same number of electrons but different numbers of protons and neutrons.

Answer:

A. An ion of Ca²⁺ and an atom of Ar have the same number of electrons, but a different number of protons and neutrons in their atoms.