Example 1 · level 1
What is the valency of phosphorus (P, atomic number 15)?
Show solution
- P: 2, 8, 5; valence electrons = 5.
- Valency = 8 − 5 = 3.
Answer: 3
CLASS 10 SCIENCE · PERIODIC CLASSIFICATION OF ELEMENTS
Periodic table: configuration, valency and trends
Shells fill 2, 8, 8 for the first 20 elements. Valency = valence electrons (if ≤ 4) or 8 − valence electrons. Across a period (left → right) atomic size and metallic character decrease; down a group both increase.
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Worked examples
Example 2 · level 2
An element has electronic configuration 2, 8. Where is it in the modern periodic table?
- Period 3, group 18
- Period 2, group 8
- Period 2, group 18
- Period 8, group 2
Show solution
- Period = 2.
- Group = 18.
- It is neon.
Answer: Period 2, group 18
Example 3 · level 3
These elements are all in period 3. Which has the largest atomic radius?
- chlorine (Cl)
- phosphorus (P)
- sodium (Na)
- magnesium (Mg)
Show solution
- Order left → right: Na, Mg, P, Cl.
- The largest atomic radius: sodium (Na).
Answer: sodium (Na)