It is generally observed that atomic mass is the double of atomic number So why does potassium have - Science -

Question

It is generally observed that atomic mass is the double of atomic
number So why does potassium have atomic mass 39 while its atomic
number is 19?

Geetha · Accepted Answer

Dear Joshi
Its not applicable to all cases Generally the mass number or atomic
weight depends on the number of proton + number of neutron
Here in potassium, the proton number is 19 and neutron number is
20
So its mass number or atomic weight = 19 + 20 = 39

It is generally observed that atomic mass is the double of atomic number . So why does potassium have atomic mass 39 while its atomic number is 19?