What does atomic radius mean?An atom has no hard outer wall. Covalent radius is an evidence-based way to compare the size of its electron cloud.

Na · 166 pmElectron cloud

Drag to rotate · click a comparison atom to focus · scroll to zoom

e−

Advanced Atomic Trends Lab

Clouds, orbitals, ions and periodic evidence

Complete narrated simulation lesson

1. What does atomic radius mean?

An atom has no hard edge. We compare size using a measured boundary of its electron cloud.

Narration ready

Spoken narration Advance automatically

Key takeaway

Atomic radius describes the extent of an electron cloud using a chosen measurement definition.

The pattern to remember

Across → radius decreases

Down ↓ radius increases

The cinematic explains why. Use the Model and Analyze tabs afterward to test the pattern yourself.

1. Choose a 3D view

2. Select an element

Element166 pm

Live atom data

Electron configuration1s² 2s² 2p⁶ 3s¹Occupied shells3Covalent radius166 pmElectronegativity0.93First ionization energy496 kJ/mol

Camera & playback

Zoom1.0×

Simulation speed1×

Model nuclear pull100%

Model shielding100%

Custom comparison tray (up to 5)

Property plotter

Elements to plot8 points

Atomic radius Electronegativity Ionization energy Electron affinity

radiuselectronegativityionizationelectron affinity

Values are normalized within the displayed set so different units can share one graph. Hover or tap a point for actual values; click to select that atom.

10-electron isoelectronic series

All species have 10 electrons. More protons pull the same electron cloud inward, so size decreases from O²− to Mg²+.

Export findings

Rank ’Em: increasing atomic radius

Select the atoms from smallest to largest before revealing the data.

Mystery element

Loading clues…

Guided comparison

Begin with shell count. Extra occupied shells usually have the largest effect.

Learning objectives

Explain radius trends using shells, shielding and effective nuclear charge.
Relate atomic size to electronegativity and ionization energy.
Predict how cations, anions and isoelectronic species compare.
Distinguish orbital probability shapes from electron paths.

Across a period

Electrons enter the same main energy level while proton number rises. Effective nuclear charge usually increases, pulling the cloud inward.

Down a group

New occupied shells increase distance and shielding. These effects outweigh the larger nucleus, so radius usually increases.

Cations and anions

Losing electrons reduces electron-electron repulsion and may remove an outer shell, so cations are smaller. Gaining electrons increases repulsion, so anions are larger.

Isoelectronic species

When electron count is fixed, proton number becomes the main difference. More protons pull the same number of electrons closer.

Properties that correlate with size

Smaller atoms often attract bonding electrons more strongly and require more energy to remove an electron. Exceptions occur because subshell structure and electron pairing also matter.

What an orbital shows

An orbital is a mathematical probability distribution. The colored lobes show an isosurface containing a chosen fraction of electron probability, not a solid object or orbit.

Data and model note

Covalent radii follow Cordero and co-workers (2008). Ionization energies, Pauling electronegativities and electron affinities are embedded educational reference values. Effective charge uses a simplified Slater-rule estimate. Ion sizes and orbital surfaces are conceptual models and are labeled accordingly.