PeriodicTable.world · Chemistry reference
Periodic trends reference
A printable guide to atomic radius, ionization energy and electronegativity, with the reasoning behind the patterns and key exceptions.
| Property | Across a period → | Down a group ↓ |
|---|---|---|
| Covalent radius | Usually decreases | Usually increases |
| First ionization energy | Generally increases | Generally decreases |
| Electronegativity | Generally increases | Generally decreases |
| Metallic character | Generally decreases | Generally increases |
How to use it
- Moving left to right across a main-group period, the nucleus generally pulls outer electrons more strongly. The extra electrons in the same shell do not fully screen the attraction from the added protons.
- Moving down a group, added electron shells generally place the outer electrons farther from the nucleus. Inner electrons also help screen the nuclear attraction.
- First ionization energy is the energy needed to remove one electron from an isolated gaseous atom in its ground state. Electronegativity describes attraction for shared electrons in a bond.
- Isoelectronic ions have the same number of electrons. Within such a series, the ion with more protons generally has the smaller radius.
Worked example
Largest to smallest: O²⁻ > F⁻ > Na⁺ > Mg²⁺. Each ion has 10 electrons; their proton counts increase from 8 to 9 to 11 to 12.
Remember the exceptions
- First ionization energy: Be > B and N > O. Subshell energies and electron pairing create exceptions to the simple across-period arrow.
- Compare the same radius definition. Do not mix covalent, metallic and van der Waals radii.
- Electron affinity has a less regular pattern and different sign conventions; it is not included in these arrows.