the mass of an atom of a chemical element expressed in atomic mass units. It is approximately equivalent to the number of protons and neutrons in the atom (the mass number).
Electron cloud is an informal way to describe an atomic orbital. Bohr talked about electrons orbiting the nucleus. Atomic orbitals also explain the patterns in the periodic table.
Chemical stability when used in the technical sense in chemistry, means thermodynamic stability of a chemical system. Thermodynamic stability occurs when a system is in its lowest energy state, or chemical equilibrium with its environment.
a solid substance that has a conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects.