Periodic trends explained: electronegativity and atomic radius
Short answer: going left to right across a period, electronegativity increases and atomic radius decreases. Going down a group, electronegativity decreases and atomic radius increases. So fluorine (top right) pulls electrons hardest, and cesium and francium (bottom left) are the largest, least electronegative atoms.
Why across a period
Moving right, each element has one more proton in the nucleus, but the new electron goes into the same outer shell. The stronger positive charge pulls the shell in, so the atom gets smaller and holds bonding electrons more tightly.
Why down a group
Moving down, each element adds a whole new electron shell. The outer electrons are farther from the nucleus and shielded by the inner shells, so the atom is bigger and pulls on bonding electrons less.
Examples
- Period 2: Li 0.98 → C 2.55 → O 3.44 → F 3.98 (Pauling electronegativity)
- Group 17: F 3.98 → Cl 3.16 → Br 2.96 → I 2.66
- Group 1 radius grows from Li to Cs, which is why the reactivity of alkali metals increases down the group.
Exceptions worth knowing
- Noble gases usually have no Pauling value because they rarely bond.
- Across the transition metals, radius changes only slightly.
- Different data sources give slightly different radius values depending on how radius is defined (covalent, metallic, van der Waals).
See it on a heat map
In Elemora, switch the table view from Categories to Electronegativity or Atomic radius. The whole table becomes a heat map, so the diagonal from bottom left to top right is easy to see. Elements without data are shown in gray.
FAQ
Which element has the highest electronegativity?
Fluorine, about 3.98 on the Pauling scale. Oxygen (3.44) and chlorine (3.16) follow.
Why are noble gases often left out of electronegativity tables?
Helium, neon and argon form almost no bonds, so Pauling values are usually not given for them.
Does atomic radius always decrease across a period?
For main-group elements the trend is clear. Across the transition metals the radius changes only a little, because the added electrons go into an inner shell.