Why does sodium have a larger atom than magnesium even though the two elements sit beside each other? Atomic radius follows a clear periodic pattern once you track two competing ideas: how many occupied shells an atom has, and how strongly the nucleus pulls on the outer electrons.
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Lithium and fluorine are in the same period (Period 2). Lithium has a larger atomic radius than fluorine. Sodium and chlorine are in the same period (Period 3), and sodium also has a larger atomic radius than chlorine. Yet sodium is larger than lithium, and chlorine is larger than fluorine. What is causing the atomic radius to decrease across a period, and what is causing it to increase down a group?
Before reading on, write your best answer. Think about the number of electron shells and the strength of attraction from the nucleus.
Core Content
Wrong: Atomic radius increases across a period because more electrons mean a larger atom.
Right: Atomic radius decreases across a period because the increasing nuclear charge pulls electrons closer to the nucleus. The added electrons enter the same shell and do not shield each other effectively, so the effective nuclear charge increases.
| Direction | Trend | Main reason |
|---|---|---|
| Across a period (left to right) | Atomic radius decreases | More protons are added, but electrons go into the same main shell. Shielding changes only slightly, so the stronger nuclear attraction pulls the outer electrons closer. |
| Down a group | Atomic radius increases | A new electron shell is added each step down the group. Outer electrons are further from the nucleus and more shielded, so the atom is larger. |
For full-mark responses, avoid saying only “it gets bigger” or “it gets smaller.” State what happens to shells, shielding, and nuclear attraction.
This trend sets up the next lesson. Smaller atoms generally hold outer electrons more tightly, which helps explain why ionisation energy often increases across a period.
Atomic radius questions often connect to ions. When an atom loses electrons to form a cation, it becomes smaller. When an atom gains electrons to form an anion, it becomes larger.
| Species change | Size effect | Reason |
|---|---|---|
| Atom → cation | Smaller | There are fewer electrons and often one fewer occupied shell. Electron-electron repulsion drops, so the remaining electrons are pulled in more strongly. |
| Atom → anion | Larger | Extra electrons increase repulsion in the outer shell, so the electron cloud expands. |
Worked Example
Step 1: All three elements are in Period 3, so their outer electrons are in the same main shell.
Step 2: Across a period, proton number increases from Na to Mg to Al, so effective nuclear charge increases.
Step 3: Stronger attraction pulls the outer electrons closer, so radius decreases left to right.
Look back at what you wrote in the Think First section. What has changed? What did you get right? What surprised you?
Practice
1. Which atom is larger: Li or K? Explain using shell number and shielding.
2. Which atom is smaller: P or Cl? Explain using effective nuclear charge.
3. Which is larger: Na or Na+? Explain why.
K is larger. It is lower in Group 1, so it has more occupied shells. The outer electron is further from the nucleus and more shielded by inner electrons.
Cl is smaller. P and Cl are in the same period, so the outer electrons are in the same main shell. Cl has more protons, so the effective nuclear charge is greater and the outer electrons are pulled closer.
Na is larger than Na+. Forming Na+ removes the 3s valence electron, leaving one fewer occupied shell and reducing electron-electron repulsion.
Return to your Think First response. You should now be able to explain both trends clearly:
Put your knowledge of atomic radius, ionisation energy and electronegativity trends to the test. Answer correctly to deal damage — get it wrong and the boss hits back. Pool: lessons 1–17.
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