What is the isotopic distribution of gallium metal?
This site reports two common gallium isotopes,
Now for gallium,
The problem is that there are multiple isotopes of Gallium. What this means is that there are Gallium atoms with different amounts of neutrons.
You have to first figure out which isotope of Gallium you want to analyze before trying to calculate the amount of neutrons.
If we decide to pick the most abundant isotope, it is
The reason you might see a single number for an atomic mass of an element in a periodic table for example, is because those have averaged out the atomic masses of all the isotopes depending on how abundant they are. If the mass contains decimals, this is a big clue that it is not an atomic mass of a single isotope.