Question

An element A has an atomic number 6 and mass number 12, while element B has an electronic configuration of 2, 8, 2. Which of the following options is incorrect?

• A.
  Atomic number of ‘B’ is 12.
  
• B. ‘B’ is a metal.
• C.
  Valency of ‘A’ is 4.
  
• D. ‘A’ and ‘B’ are isotopes.

Answer 1

The correct option is D ‘A’ and ‘B’ are isotopes.

Atomic number of A = 6
Electronic configuration of A = 2, 4
So, A can share its 4 valence electrons.
⇒ Valency of A = 4 (number of electrons in the outermost/valence shell)

Electronic configuration of B = 2, 8, 2
Atomic number = 2 + 8 + 2 = 12 (Total number of electrons/protons in a neutral atom)

The atomic number of B resembles the valence shell electronic configuration of group 2 elements. It can lose 2 electrons to obtain the stable electronic configuration of 2, 8. Metals have the tendency to lose valence electron(s) and thus, B is a metal.

Now,
Atoms of an element having the same atomic number but different mass numbers (due to the differences in the number of neutrons) are known as isotopes.
Elements A and B have an atomic number of 6 and 12 respectively.
Thus, A and B are not isotopes.