How do you find the number of neutrons for atomic structure?

I thought you just did the top number - the bottom number in the periodic table?

For the structures, you add together the relative atomic masses of all the atoms and then subtract the total number of protons present in the structure.

For H-2 given above, you need to be told the atomic mass first as it is an isotope.

your protons as you know never change, so you look at the "2" in H, which means that you now have a mass number of 2. H has 1 proton, 1 neutron, and 1 electron, and it now says you have 2, so that means you have 2 neutrons. just find the atomic number of an atom (AKA its number of protons) and than find the difference between that atomic number and the isotopic number, which will give you your new amount of neutrons

the atomic number is the amount of neutrons

The number of protons = atomic number
The number of neutrons = mass number - atomic number
The number of electrons = atomic number - charge (watch the sign)

²H has one proton and one neutron (and one electron if uncharged).

The atomic number, Z defines how many protons are in an element.

So for Carbon, which is the 6th element, you know that it has 6 protons, and thus 6 electrons, to balance the positive Columbic charge, making it neutral.

Atoms of the same element have the same number of protons, yet may have different masses.

The atomic mass number is given by A.

A = Z + N

Now remember, elements are characterized by Z, the number of protons, but they may have varying neutrons leading to the differences in mass. Subsets of elements with the same Z but differing A (atomic mass) and n (neutrons) values are called isotopes.

ANZ

I'll do an example taken from the Materials Science course from Edx by MIT.

Chlorine has two isotopes, 35Cl and 37Cl. For the anion Cl−, of the isotope 37Cl, specify the following:

number of protons, np = Cl's atomic number, Z, is 17, on the periodic table, thus there are 17 protons.

number of electrons, ne = To balance the charge, there must be an equal number of electrons. However, there is a twist in this problem. 37Cl has a negative charge, as it gained an electron, so instead of the expected 17, it is 18.

number of neutrons, nn= Since we know the formula A = Z + N, whereas A = 37 because of the isotope 37Cl, the placement of the 37 is the atomic mass number, A. Z, the number of protons is 17. 37 = 18 + N

The number of neutrons, n = 17

While all the answers above have merit, you want to quantitatively base your answers so you know exactly where you derive your answers, and can quickly and easily check them.

Now try and do the problem with deuterium.