How many protons are present in one copper(ii) ion?

Copper has 29 protons, when dealing with Cu^2+ all that means is it lost two electrons. so now the element has 29 protons and 27 electrons. Protons are positive and electrons are negative and neutrons are neutral. So say you had an element X^2- then you have gained two more electrons so the element has an overall negative charge. hope that helps

The mass has escaped into a gas which you can not see so the law stays true.