energy to ionize a mole of atoms or molecules, usually as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). Comparison of ionization energies In physics and chemistry, ionization energy (IE) is the minimum energy required to remove the most loosely bound electron(s) (the valence electron(s)) of an isolated gaseous atom, positive ion, or molecule. The first ionization energy is quantitatively expressed as X(g) + energy ⟶ X+(g) + e− where X is any atom or molecule, X+ is the resultant ion when the original atom was stripped of a single e There are exceptions to the general trend of rising ionization energies within a period. For example, the value decreases from beryllium ( 4Be: 9.3 eV) to boron ( 5B: 8.3 eV), and from nitrogen ( 7N: 14.5 eV) to oxygen ( 8O: 13.6 eV). These dips can be explained in terms of electron configurations. Boron has its last electron in a 2p orbital, which has its electron density farther away from the nucleus on average than the 2s electrons in the same shell. The 2s electrons then shield the 2p electron from the nucleus to some extent, and it is easier to remove the 2p electron from boron than to remove a 2s electron from beryllium, resulting in a lower ionization energy for B. In oxygen, the last electron shares a doubly occupied p-orbital with an electron of opposing spin. The two electrons in the same orbital are closer together on average than two electrons in different orbitals, so that they shield each other from the nucleus more effectively and it is easier to remove one electron, resulting in a lower ionization energy. Furthermore, after every noble gas element, the ionization energy drastically drops. This occurs because the outer electron in the alkali metals requires a much lower amount of energy to be removed from the atom than the inner shells. This also gives rise to low electronegativity values for the alkali metals.