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Question

Which of the following does not have eight valence electrons?

A
Ca+
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B
Rb+
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C
Xe
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D
Br
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Solution

The correct option is A Ca+
Finding the number of valence electrons in:
(a) Ca+
Calcium has an atomic number of 20.
The electronic configuration of Ca is [He]2s2 2p6 3s2 3p6 4s2

In the case of Ca+ (with 19 electrons):
The electronic configuration will be: [He]2s2 2p6 3s2 3p6 4s1
Configuration of the valence shell: 4s1
It has only one electron in its valence shell. Hence, Ca+ does not have 8 electrons in its valence shell.

To attain 8 electrons in its valence shell, it should lose two electrons from its valence shell and attain the configuration [He] 2s2 2p6 3s2 3p6
Configuration of the valence shell: 3s2 3p6 Total number of valence electrons = 8

(b) Rb+
Rubidium has an atomic number of 37
The electronic configuration of Rb+ will be: [Ar]3d10 4s2 4p6
Since Rb+ has 8 electrons in its valence shell and resembles the configuration of a noble gas, Krypton, it has a complete octet.

(c) Xe
The atomic number of Xenon is 54
The electronic configuration of Xe is [Kr] 4d10 5s2 5p6
It has 8 electrons in its valence shell, and thus is a noble gas.

(d) Br
The atomic number of Bromine is 35
The electronic configuration of Br is [Ar]3d10 4s2 4p6
It has 8 electrons in its valence shell.

Hence, Ca+ does not have eight electrons in its valence shell.

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