Atomic and ionic radius
Atomic and ionic radius
- The atomic radius is the size of an atom.
- It changes in a clear pattern across the Periodic Table.
- Ions have their own sizes too.
Across a period and down a group
- Across a period (left → right), the radius gets smaller. The nuclear charge rises but shielding stays about the same, so the outer shell is pulled in.
- Down a group (top → bottom), the radius gets larger. Each step adds a new shell, with more shielding.
Practice
Across a period, the atomic radius decreases because:
Electrons go into the same outer shell, so rising nuclear charge with similar shielding pulls them closer.
Practice
Down a group, the atomic radius increases because:
Adding shells increases distance and shielding, so the radius grows down a group.
Ion sizes
- a cation (positive ion) is smaller than its atom — it has lost its outer shell.
- an anion (negative ion) is larger than its atom — extra electrons repel each other.
- among ions with the same number of electrons, the one with more protons is smaller.
Practice
Compared with its atom, a cation (positive ion) is:
Losing the outer shell (and electrons feeling a stronger pull each) makes a cation smaller than its atom.
Practice
Among ions with the same number of electrons, the smallest is the one with:
More protons pull the same number of electrons in more tightly, giving a smaller ion.
You've got it
Key idea
- across a period: radius decreases (more nuclear charge, similar shielding)
- down a group: radius increases (extra shells, more shielding)
- cation < atom; anion > atom
- among isoelectronic ions, more protons → smaller