Question

Why is it that elements have fractional atomic mass?

Answer 1

• Many elements have fractional atomic mass because many elements have isotopes.
• Isotopes are those elements that have the same atomic mass but different atomic mass.
• For example, chlorine has two isotopes with atomic masses 35 and 37 with a natural abundance ratio of 3:1.
• $\frac{35\times 3+37\times 1}{4}=35.5$
• The weighted average atomic mass is 35.5.