Periodicity: Atomic Radius and Electronegativity

Periodicity is the recurring pattern of physical and chemical properties as you move across a period of the periodic table. These patterns arise directly from electronic structure. In this lesson, we examine two key periodic properties: atomic radius and electronegativity.

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Atomic Radius

The atomic radius is a measure of the size of an atom. Because electron clouds do not have sharp edges, we define atomic radius in practical terms:

• Covalent radius: Half the distance between the nuclei of two bonded atoms of the same element.
• Metallic radius: Half the distance between the nuclei of adjacent atoms in a metallic crystal.
• Van der Waals radius: Half the distance between the nuclei of two non-bonded atoms at their closest approach (used for noble gases and comparing non-bonded atoms).

Trend Across a Period

Atomic radius decreases across a period (left to right). For example, across Period 3:

Element	Na	Mg	Al	Si	P	S	Cl	Ar
Atomic radius (pm)	186	160	143	117	110	104	99	—