Question

Which statement best explains why most atomic masses on the Periodic Table are decimal numbers?
(1) Atomic masses are determined relative to an H–1 standard.
(2) Atomic masses are determined relative to an O–16 standard.
(3) Atomic masses are a weighted average of the naturally occurring isotopes.
(4) Atomic masses are an estimated average of the artificially produced isotopes.

Answer 1

(3) Atomic masses are a weighted average of the naturally occurring isotopes.

Here, we found this answer my looking at the key words 'naturally occurring isotopes'

An isotope is a different amount of neutrons, making the atomic mass change. 

Answer 2

♥(3) Atomic masses are a weighted average of the naturally occurring isotopes.
(3) Atomic masses are a weighted average of the naturally occurring isotopes. is the best statement for why most atomic masses on the Periodic Table are decimal numbers.