Periodic table trends explained

Atomic radius trend

Atomic radius decreases from left to right across a period and increases down a group. Moving right, each element adds a proton, but its new electron goes into the same outer shell, where it shields the others only weakly. The effective nuclear charge felt by the outer electrons rises and pulls them closer. Moving down, each period adds a new shell farther from the nucleus, so the atoms get bigger.

The radii shown are van der Waals radii from PubChem, in picometers. They follow the trend clearly down a group, from lithium (182 pm) to sodium (227 pm) to potassium (275 pm), but are less regular across a period.

Ionization energy trend

Ionization energy, the energy needed to remove an atom's outermost electron, increases across a period and decreases down a group. Smaller atoms hold their outer electrons more tightly, so it takes more energy to pull one away. In periods 2 to 6, the alkali metal has the lowest value and the noble gas the highest, with small dips at group 13 and, in periods 2 to 4, at group 16.

Electronegativity trend

Electronegativity, an atom's pull on the shared electrons in a bond, also increases across a period and decreases down a group, for the same reason: a stronger effective nuclear charge acting on electrons that are closer to the nucleus. Fluorine, at the top right, is the most electronegative element. The noble gases are usually left out, because helium, neon and argon form no compounds.

Metallic character trend

Metallic character describes how easily an atom loses electrons and behaves like a metal. It follows atomic radius: it increases down a group and decreases across a period, because large atoms with loosely held outer electrons give them up easily. The most metallic elements are at the bottom left, the most nonmetallic at the top right, and the metalloids along the staircase line mark the changeover. The metals, nonmetals and metalloids view shows this split.

Example: sodium and chlorine

Sodium and chlorine sit at opposite ends of period 3. Sodium has a radius of 227 pm, a first ionization energy of 5.139 eV and an electronegativity of 0.93. Chlorine has a radius of 175 pm, an ionization energy of 12.968 eV and an electronegativity of 3.16. Sodium gives up its single outer electron easily and chlorine pulls electrons strongly, so when they react the electron moves from sodium to chlorine, forming the ionic compound sodium chloride.