Question

A neutral atom of an element has 2K, 8L, and 5M electrons. What is the total number of unpaired electrons?
(A) 2
(B) 3
(C) 4
(D) 5

Answer 1

When the charges of proton and electron on an atom are balanced it is said to be a neutral atom. Electrons have negative charge and protons have positive charge. One electron has the same charge as a proton but with opposite signs.
For example,
C has 6 protons. The neutral C atom has 6 electrons & protons as the atomic number is 6.

Complete step by step solution:
The given question states that the total number of electrons in K, L, M shells is 2, 8, and 5.
Therefore, the total number of electrons = $2 + 8 + 5 = 15$
It is concluded that the given atom is Phosphorous. The electronic configuration is as follows;
$1s_22s_22p_63s_23p_3$
Unpaired electrons = ${\mathbf{3}}\;\;({\mathbf{3px}}\;{\mathbf{3py}}\;{\mathbf{3pz}})\;$
The unpaired three electrons are in ${\mathbf{3px}},\;{\mathbf{3py}},\;{\mathbf{3pz}}$ orbitals respectively.
Hence the answer is; the total number of unpaired electrons is three.

Additional Information:
Phosphorus (P) has atomic weight $15$ , atomic mass number as $30.975$ , it has $15$ protons, $16$ neutrons, $15$ electrons. it has a melting point of $44.1^\circ C$ and $280.0^\circ C$ boiling points. It belongs to the family of nonmetals placed in period three of the periodic table.

Note:
An electron shell or an orbit followed by electrons around the nucleus. The 1st shell that is closest to the nucleus also called as the K shell, followed by the second or the L shell, then the third or the M shell and so on are far away from the nucleus