Understanding how chlorine (Cl) becomes the chloride ion (Cl⁻) by gaining an electron is fundamental to Class 10 Chemical Bonding.
The Cl⁻ ion is isoelectronic with Ar (Argon) — both have 18 electrons. Similarly, Na⁺ is isoelectronic with Ne (both have 10 electrons).
Chlorine atom (Cl):
Chloride ion (Cl⁻):
The number of valence electrons in Cl⁻ ion = 8
Cl⁻ now has the same electron configuration as the noble gas Argon (Ar) — 2, 8, 8 — making it stable.
Cl has 7 valence electrons — it needs just 1 more to complete its outer shell (octet). It is energetically favourable for Cl to gain 1 electron (rather than lose 7). This is why Cl has high electron affinity and forms Cl⁻.
Cl⁻ has a charge of **-1** (negative one) because it has gained one extra electron compared to the neutral Cl atom.
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