why is the relative atomic mass of argon higher than that of potassium?

Duche

2 months ago

This is a classic chemistry question.
The key reason:
Relative atomic mass is based on the average mass of all naturally occurring isotopes of an element, not its atomic number.
Explanation
Argon (Ar) has atomic number 18
Potassium (K) has atomic number 19
So you might expect potassium to be heavier — but:
Isotopic Composition
Argon exists mainly as Argon-40 (Ar-40) → mass ≈ 40
Potassium exists mainly as:
K-39 (most abundant)
Small amounts of K-40 and K-41
→ average ≈ 39.1
Conclusion
Even though potassium has a higher atomic number, argon’s most abundant isotope (Ar-40) is heavier than potassium’s average atomic mass.
Short Exam Answer
The relative atomic mass of argon is higher than that of potassium because argon has a heavier predominant isotope (Ar-40), while potassium’s average mass is lowered by its isotopic composition.