Answer:
Option A. 9
Explanation:
Isotope results when there is a change in the Neutron number of an atom.
From the question,
Mass number of the sodium isotope = 20
But the atomic number of isotopes are constant. Therefore, the isotope has:
Proton = 11
Mass number = Neutron + Proton
Neutron = Mass number — Proton
Neutron = 20 — 11 = 9
