What makes up most of a calcium atom

Nuclear protons, and neutrons.

Explanation:

The standard calcium isotope is ⁴⁰Ca. Because it is a calcium nucleus, WE KNOW that the nucleus contains 20 protons, 20 massive, positively-charged nuclear particles. The calcium nucleus also contains approx. 20 neutrons, 20 massive, neutrally-charged nuclear particles.

Because the calcium atom is neutral, also associated with the atom are 20 electrons, 20 negatively charged particles, of negligible mass, that are conceived to whizz about the massive nuclear core.

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If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom

Ithink it's the third answer