The simplest molecules that could be imagined would have two atomic nuclei surrounded by one to four electrons derived originally from the two 1s atomic orbitals. We shall look at such molecules first, and what we learn about combining 1s orbitals will carry over into the combining of other orbitals for more complicated molecules. The most familiar example of a molecule built from 1s atomic orbitals is hydrogen, H, which has two electrons. The hydrogen molecule-ion, , also can exist with only one electron to hold the nuclei together. The bond is weak, and the molecule-ion is hyperreactive, seizing an electron where it can and turning into a neutral hydrogen molecule again. Another short-lived but observable molecule-ion is He, with two nuclei and three electrons. As we shall see shortly, two of these electrons help to bond the nuclei together, and the third electron weakens the bond. The molecule with four electrons, He, is only imaginary, because with two electrons holding the nuclei and two repelling them, He would fall apart into two helium atoms. These four molecules, one stable, two less stable, and one nonexistent, offer a simple test of the MO theory because they all use the same orbitals and energy levels.