The periodic table of elements will give you an atom's atomic number. That is the number of protons (and electrons for neutral atoms) in an atom. In order to figure out the number of protons in the atom, you need to know what the atom's mass number is. The mass number is the sum of the protons and neutrons within an atom. The simplest and most direct way of finding an atom's mass number is to round off the atomic mass listed in the periodic table. Take the rounded off atomic mass (which is the mass number) and subtract the atomic number from it. The difference is the number of neutrons.
Let's take carbon for example. It's atomic number is 6. That means it has 6 protons. It's atomic mass is listed as 12.011. That would round down to 12. 12 - 6 = 6. Carbon has six neutrons.
Now let's look at boron. It's atomic number is 5. It's atomic mass is 10.81. That rounds up to 11. 11 - 5 = 6. Boron has 6 neutrons.
To get the number of neutrons, you need to subtract the number of protons (atomic number) from the atomic mass.
Say for example you had C-12, and you wanted to find the number of protons.
1. Atomic number of Carbon is 6.
2. Subtract 6 from 12, so you have 12-6=6
3. Therefore, there are 6 neutrons in Carbon-12.