Many atomic properties change periodically with atomic number. Ionization energy and electronegativity are the two cited most often, and they tell us whether an element tends to lose or gain electrons.

Ionization energy

Ionization energy is the energy required for a neutral atom to release an electron. When a lithium atom loses an electron and becomes Li⁺, the energy absorbed is its ionization energy. The reverse process, in which a neutral atom accepts an electron to form an anion, releases energy that is called the electron affinity.

The periodic pattern

Listing ionization energies by atomic number shows that, within a period, the value climbs in a sawtooth pattern as the nuclear charge grows, and reaches its maximum at the noble gases, whose closed electron shells are very stable. Group 1 elements, with a single valence electron, give it up most easily. This regular rise and fall with atomic number is the periodicity of ionization energy.

Electronegativity

Combining ionization energy and electron affinity gives a relative number, electronegativity, which describes how strongly an atom attracts electrons. It is roughly proportional to ionization energy: the larger the value, the more readily the atom forms an anion.

Picture an electron lying between two atoms; the atom with the higher electronegativity pulls it closer. Electronegativity strongly influences chemical reactions, and on the periodic table it increases toward the upper right, where elements are more electronegative.