Answer:
Atomic Number
Neutral atoms of an element contain an equal number of protons and electrons. The number of protons determines an element's atomic number (Z) and distinguishes one element from another.
Explanation:
H2(g) + Cl2(g) = 2 HCl(aq) (balanced equation)
1, 1, 2 (coefficients)
Becoming an ion involves gaining or losing electrons
Electrons have a relative mass of 0
So adding or removing them doesn't change the mass of the atom
So the answer is (3) remains the same
Amino acids is the building blocks