Periodic table trends
Atoms get smaller across a period and bigger down a group.
The common mistake
Thinking more protons means a bigger atom across a period.
Instead: Across a period, more protons pull the same shells in tighter (smaller). Down a group, each new shell makes atoms bigger.
Worked example with Guide me
Which is bigger: sodium or chlorine?
- Sugar · Are they in the same period?Student · Yes, period 3.
- Sugar · Which is further left?Student · Sodium.
Answer: Sodium — fewer protons pulling the same shells.
Try 3 practice questions
1. Bigger atom: lithium or potassium?
Show answer
Potassium
2. More reactive: sodium or potassium?
Show answer
Potassium
3. More reactive: fluorine or iodine?
Show answer
Fluorine
Quick answers
- What is the most common mistake with periodic table trends?
- Thinking more protons means a bigger atom across a period.
- How do I get periodic table trends right?
- Across a period, more protons pull the same shells in tighter (smaller). Down a group, each new shell makes atoms bigger.
- Can you show a worked example?
- Which is bigger: sodium or chlorine? Sodium — fewer protons pulling the same shells.
Related concepts
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