Question 9
Argon has atomic number 18 and relative atomic mass 40; potassium has atomic number 19 and mass 39. What does this show?
The right answer is the one marked above.
That mass does not always rise with atomic number, because neutrons count towards it too. The atomic number gives only the number of protons.
This is a real fact, not an error. Being a noble gas or a metal has no direct bearing on the mass.