The law of conservation of energy states that energy can
neither be created nor destroyed. What does
"conservation" mean in this law?

The word “conservation” refers to an amount that is unchanged. The total amount of energy in the universe remains the same.

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