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The number of d-electrons in Fe2+ (Z = 26) is not equal to the number of electrons in which one of the following?
(a)- d-electrons in Fe (Z = 26)
(b)- p-electrons in Ne (Z = 10)
(c)- s-electrons in Mg (Z = 12)
(d)- p-electrons in Cl (Z = 17)

Answer
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Hint: First write the configuration of Fe in the ground state and then in Fe2+, there is a removal of two electrons. Now compare all the configuration of the elements Fe, Ne, Mg, and Cl.

Complete answer:
Iron is the element that belongs to d-block so its configuration in ground state is given below:
1s22s22p63s23p64s23d6
When there is an excitation of two electrons then the Fe converts into Fe2+. So from the above configuration, two electrons will be removed from the 4sorbital. So the configuration becomes:
1s22s22p63s23p63d6
Now, the electronic configuration of Fe having atomic number 26 is:
1s22s22p63s23p64s23d6
So, it has the same number of electrons in d-orbital as that of an electron in d-orbital of Fe2+, i.e., 6 electrons.
In Ne having atomic number 10, the electronic configuration is:
1s22s22p6
So, it has the same number of electrons in p-orbital as that of an electron in p-orbital of Fe2+, i.e., 6 electrons.
In Mg having atomic number 12, the electronic configuration is:
1s22s22p63s2
So, it has the same number of electrons in s-orbital as that of an electron in s-orbital of Fe2+, i.e., (2+2+2) 6 electrons.
In Cl having atomic number 17, the electronic configuration is:
1s22s22p63s23p5
So, the number of p-electrons in Clis: 6+5=11. But in Fe2+, the number of p-electrons is: 6+6=12. So they are not equal.

Therefore, the correct answer is an option (d)- p-electrons in Cl (Z = 17).

Note:
Magnesium is the element of s-block having atomic number 12 and its last electron comes in the s-orbital. Neon and chlorine are the elements of p-block having atomic numbers 10 and 17 respectively and their last electron comes in the p-orbital.
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