Periodic Trends

Atomic Radius

How big is an atom, really?

⏱ — min read

Atomic radius is a measure of atomic size. There are three different ways in which atomic radius can be calculated: covalent radius, metallic radius, and Van der Waals radius. The atomic radius is typically calculated using the X-ray diffraction method.

Across a period, the nuclear charge increases while electrons fill the same shell — the cloud is pulled in tighter, so atomic radius decreases. Down a group, a new shell is added each time, so the radius grows substantially even though nuclear charge also climbs.

Three types are used depending on the bonding context:

Covalent radius diagram — Covalent radius — non-metals (e.g. Cl₂)

Metallic radius diagram — Metallic radius — metals (e.g. Cu, Fe)

Van der Waals radius diagram — Van der Waals radius — noble gases

Covalent radius

Half the distance between the nuclei of two identical atoms bonded covalently. Used for non-metals. For diatomic molecules like Cl₂, the covalent radius is half the bond length.

Metallic radius

Half the distance between the nuclei of two adjacent atoms in a metallic crystal lattice. Used for metals — copper, iron, silver, gold.

Van der Waals radius

Half the distance between the nuclei of two non-bonded atoms in adjacent molecules. Used for noble gases and other situations where atoms are in contact but not bonded.

The atomic radius finds important applications in materials science:

Interstitial compounds

Transition metal borides, nitrides, and carbides — the ratio of the small atom’s radius to that of the transition metal predicts stability of these interstitial compounds.

Ultra-high-temperature ceramics

Transition metal borides survive 1700°C — used on hypersonic vehicles and re-entering spacecraft. A hypersonic vehicle can travel four times the speed of sound and helps launch satellites at low cost.

Cutting-tool coatings

Transition metal carbides are hard coatings that protect cutting tools from wear and erosion, extending their service life dramatically.

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Trends Across the Table

Groupwise Analysis

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Trend Analysis

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Note: Covalent radius is used for nonmetals, metallic radius for metals, and Van der Waals radius for noble gases. Where primary data is unavailable, the next available radius type is substituted.

References:

Beatriz Cordero et al., Covalent radii revisited. Dalton Transactions, 2008, doi:10.1039/b801115j
William M. Haynes, CRC Handbook of Chemistry and Physics, 95th ed., CRC Press, 2014
Kaye & Laby tables of physical & chemical constants (2017)
Royal Society of Chemistry — interactive periodic table

Periodwise Analysis

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Trend Analysis

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Across a period, atomic radius typically decreases due to increasing effective nuclear charge — with notable exceptions for noble gases (using VdW radius).

References:

Cordero et al., 2008
CRC Handbook of Chemistry and Physics, 95th ed.

Transition Metals Analysis

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Transition metal radius is dominated by d-orbital shielding effects rather than simple periodic trends.

References:

CRC Handbook of Chemistry and Physics, 95th ed.

Lanthanide / Actinide Analysis

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Lanthanide contraction is the steady decrease in atomic radius across the 4f series — small but cumulatively significant.

References:

Cordero et al., 2008