Answer is: boron (¹¹B).
1) Carbon (¹²C) is element with atomic number 6 and mass number 12. Atomic number (Z) is the number of protons, which is characteristic of a chemical element.
Z(C) = p⁺ = 6.
A = p⁺ + n°; mass number is the total number of protons and neutrons in a nucleus.
A(C) = 12.
n°(C) = 12 - 6.
n°(C) = 6; number of neutrons.
2) Boron (¹¹B) is element with atomic number 5 and mass number 11.
Z(B) = p⁺ = 5.
A(B) = 11.
n°(B) = 11 - 5.
n°(B) = 6; number of neutrons in boron.
