Question

The atomic number of magnesium is 12. How many protons are there in the nucleus of the magnesium atom? How many electrons and protons does an ion of magnesium which has a charge of +2 contain?
(A) 10, 10
(B) 10, 13
(C) 11, 12
(D) 10, 12

Answer 1

Hint: Atomic number gives the number of protons in the nucleus of an atom. To maintain the electrical neutrality of an atom, the number of protons is equal to the number of electrons in an atom.

Complete step by step solution:
The atomic number is usually represented by Z.
The given atomic number of magnesium, Z = 12.
As we know that atomic number is equal to the number of protons in the nucleus of an atom. Therefore, we can write the number of protons in the nucleus of the Mg atom is equal to
The number of protons in Mg = atomic number (Z) of Mg = 12.
We further know that an atom is electrically neutral and thus contains an equal number of protons and electrons. Mg atom has 12 protons then the number of electrons in the Mg atom will be:
Number of protons in Mg atom = number of electrons in Mg atom = 12
Now, when the Mg atom loses two electrons, the number of electrons will decrease by 2. Since the number of protons remains unchanged, there will be 2 extra protons in the nucleus. Thus, Mg atom converts to ion having 2 units of positive charge, i.e. $M{g}^{2+}$.
Therefore, in Mg ion having +2 charge,
Number of electrons = 12– 2 = 10.
The number of protons = 12.

Hence, the correct option is (D).

Note: Note that decrease in the number of electrons from a neutral atom leads to an exact increase in the number of protons (and hence, the positive charge) in the nucleus. Loss of electrons by an element leads to an increase in its oxidation number. Magnesium loses two electrons and its oxidation number increases from 0 to +2.