Number of electrons in the outermost shell of chloride ion ($ \text{Cl}^- $) is: · Atomic Structure MDCAT Past Paper MCQ

Chemistry Atomic Structure MDCAT 2013

PMDC Verified Question 91 of 95

Number of electrons in the outermost shell of chloride ion ($ \text{Cl}^- $) is:

$ 17 $

$ 7 $

$ 8 $

$ 1 $

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Propolis Cognitive Error Autopsy

Official Correct Choice:

Option C: $ 8 $

Concept: Formation of an anion involves the addition of electrons to the valence shell of a neutral atom to achieve a stable noble gas configuration.

Formula: $$ \text{Electrons} = Z - \text{Charge} $$

Solution:

A neutral Chlorine (Cl) atom has atomic number $ Z = 17 $.
Its electronic configuration is $ 1s^2, 2s^2, 2p^6, 3s^2, 3p^5 $ (or 2, 8, 7).
The outermost shell ($ n=3 $) has $ 2 + 5 = 7 $ electrons.
When it gains 1 electron to form the chloride ion ($ \text{Cl}^- $), the configuration becomes $ 3s^2, 3p^6 $.
The outermost shell now contains $ 2 + 6 = 8 $ electrons.

Why other options are incorrect: 17 is the total number of protons. 7 is the valence electrons in a neutral Cl atom, not the ion.

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