What is the valency of an element with the electronic configuration 2, 8, 7?

Analyze the question You're asked for the valency of an element whose electrons are arranged as 2, 8, 7. This is chlorine (atomic number 17).

Explain the concept Valency is the number of electrons an atom gains, loses, or shares to achieve a stable outer shell (the octet, 8 electrons).

• Atoms with 1 to 3 valence electrons usually lose them, so valency = number of valence electrons.
• Atoms with 5 to 7 valence electrons usually gain electrons, so valency = 8 − number of valence electrons.

Solution

1. Configuration 2, 8, 7 means the outer shell has 7 electrons.
2. It needs 1 more electron to complete the octet (8).
3. Valency = 8 − 7 = 1.

Chlorine gains one electron to form Cl⁻, which has the configuration 2, 8, 8. This is why it forms compounds like NaCl and HCl.

Answer: B (1)

Option A (7) is the number of valence electrons, not the valency. Students often pick it by mistake.

Exam tip For elements in Groups 5, 6, and 7, use "8 minus the valence electrons." For Groups 1, 2, and 3, valency equals the group number. Halogens (F, Cl, Br, I) always have valency 1.