We are given this element:
a
X
b
When we look at the periodic table and see an element, the number that we have at the top is the mass number and the one that we have at the bottom is the atomic number.
So: a = mass number (A) b = atomic number (Z)
Then by definition the mass number (A) is the number of protons plus neutrons in the nucleus. And the atomic number (Z) is the number of protons in the nucleus.
A = number of protons + number of neutrons
Z = number of protons
Answer: (a) a represents: b) correct and d) correct
(b) b represents: a) correct and c) correct
