What is the trend for atomic radius?

Periodic Trends: The Atomic Radius

Atomic radius is a model-based measure of atomic size, often described as how far the outer electrons are from the nucleus on average rather than a hard physical boundary.

There are two things you should know about the periodic trend for atomic radius:

Atomic radius trends are explained by two competing ideas: (1) adding occupied energy levels increases the average distance of valence electrons from the nucleus and increases shielding, so atoms get larger down a group; (2) across a period, electrons are added to the same general energy level while the number of protons increases, so the greater effective nuclear charge pulls the electron cloud closer to the nucleus and atomic radius decreases.

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First, atomic radius increases as you go down the periodic table.

• The higher the period, the higher the energy level. Energy levels are represented by the coefficients when you write electron configurations (ex. the 1 in 1s², the 2 in 2p⁶, etc.).
• Electrons in the highest energy level are farthest away from the nucleus, so the size of the electron cloud increases as you move down a group on the periodic table.

Second, atomic radius decreases from left to right across a period.

• This is because the effective nuclear charge increases with the addition of more protons.
• Effective nuclear charge is the net positive attraction felt by valence electrons after accounting for shielding by inner electrons. A simple estimate is Zeff ≈ (number of protons) − (core electrons), but students should treat this as an approximation, not an exact rule in every case.
• In carbon, for example, a simple estimate of the attraction on the valence electrons is +4 because carbon has 6 protons and 2 core electrons.
• In short, because you're adding protons while electrons are still being added to the same general energy level, electrons feel a stronger attraction to the nucleus and the size of the atom decreases.

Check out these reviews for more help on periodic trends and the table of elements:

• Watch this 🎥 video on all of the periodic table trends
• For a quick review read this study guide on Unit 1: Atomic Structure and Properties