Atomic and ionic radius
Introduced| English | Meaning |
|---|---|
| atomic radius/əˈtɒmɪk ˈreɪdɪəs/ | A measure of an atom's size based on a stated convention, such as half the distance between bonded identical atoms. |
| nuclear charge/ˈnjuːklɪə tʃɑːdʒ/ | The total positive charge of an atomic nucleus, determined by its number of protons. |
| shielding/ˈʃiːldɪŋ/ | The reduction in nuclear attraction experienced by an electron because of repulsion from other electrons. |
| cation/ˈkætaɪən/ | An ion with a positive electric charge. |
| anion/ˈænaɪən/ | An ion with a negative electric charge. |
| isoelectronic/ˌaɪsəʊlɪkˈtrɒnɪk/ | isoelectronic |
How big is an atom?
- The atomic radius 原子半径 is the size of an atom.
- It changes in a clear pattern across the Periodic Table.
- Ions have their own sizes too.
A positive ion (cation) is smaller than its parent atom.
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Losing the outer-shell electrons reduces shielding and shrinks the ion.
Across a period and down a group
- Across a period (left → right), the radius gets smaller. The nuclear charge 核电荷 rises but shielding 屏蔽 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.
Atomic radius shrinks across a period (stronger pull on the same shell) and grows down a group (new shells)
Atomic and ionic radius trends
Atomic radius falls across a period (rising nuclear charge pulls the same shell in) and rises down a group (an extra shell each time). Step across Period 3 to see it.
Across a period, the atomic radius decreases because:
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Electrons go into the same outer shell, so rising nuclear charge with similar shielding pulls them closer.
Down a group, the atomic radius increases because:
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Adding shells increases distance and shielding, so the radius grows down a group.
Match each change to its effect on radius.
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Pulling power vs number of shells sets the size; ions differ from their atoms by the electrons gained or lost.
Ion sizes
- a cation 阳离子 (positive ion) is smaller than its atom — it has lost its outer shell.
- an anion 阴离子 (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.
A scanning tunnelling microscope can image individual atoms
Compared with its atom, a cation (positive ion) is:
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Losing the outer shell (and electrons feeling a stronger pull each) makes a cation smaller than its atom.
Among ions with the same number of electrons, the smallest is the one with the most ______.
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More protons pull the same number of electrons in more tightly, giving a smaller ion.
Watch the trend direction
- Atomic radius decreases across a period (more protons pull the same shell in) but increases down a group (extra shells).
- A cation is smaller than its atom; an anion is larger.
A common trap: radius decreases across a period even though electrons are being added — because the growing nuclear charge pulls the same shell inwards. Only adding a new shell (down a group) makes atoms bigger.
You've got it
- 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 等电子 ions, more protons → smaller