Periodic trends
Three properties change in regular patterns across the table.
- Atomic radius is the size of an atom.
- Ionization energy is the energy needed to remove an electron.
- Electronegativity is how strongly an atom attracts electrons in a bond.
| Property | Across a period, left to right | Down a group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionization energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
Two ideas explain all three.
Across a period, protons are added but the electrons go into the same energy level. The stronger nuclear pull draws the electrons in. Atoms get smaller and hold electrons more tightly.
Down a group, each element has one more energy level. The outer electrons are farther from the nucleus and shielded by inner electrons. Atoms get larger and hold outer electrons more loosely.
Fluorine, at the upper right, is the most electronegative element.
Worked example: which is larger, Na or Cl?
Both are in period 3. Sodium is on the left and chlorine is on the right. Radius decreases across a period, so sodium is larger.
Common mistake
Assuming atoms get bigger across a period because they have more electrons. The added electrons are in the same level, and the added protons pull them closer.