Explain why the quantum number set (2, 2, -1, -½) is not possible for an electron in an atom.

This quantum set is not possible because electrons cannot exist half way between two energy levels they needs discrete amount of energy to move from one energy level to the next and do not move until the exact amount is received so they can't be found at varying energy levels.

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