Question

The species Ar, K+ and Ca2+ contain the same
number of electrons. In which order do their radii
increase ?
(1) Ca2+ < Ar < K+ (2) Ca2+ < K+ < Ar
(3) K+ < Ar < Ca2+ (4) Ar < K+ < Ca2+
Please explain the answer thoroughly by solving every option.

Answer 1

Their radii increase in the order (2) Ca2+ < K+ < Ar

1. We compare the proton/electron ratios

2. Ca2+ = 20/18 , K+= 19/18 , Ar = 18/18

3. The species having higher p/e ratio will have lesser size as the electron cloud will become smaller due to stronger attraction by +ve charge in the nucleus.

Answer 2

The correct order of increase in their radii is Ca2+ < K+ < Ar.

Option (B) is correct.

Explanation:

• Size of isoelectronic species -can be calculated as:
• Smaller the value of  z/e, larger the size of that species.
• Smaller z means effective nuclear charge is small hence size is large.
• In case of isoelectronic species, radius decreases with increase in nuclear charge.

Thus the correct order of increase in their radii is Ca2+ < K+ < Ar.

Also learn more

What are ISOELECTRONIC SPECIES and give examples.  ?

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