Step-by-step explanation:
ex:
39, the top number, is the mass of the element
19 is the number of protons in the element. it is also the number of electrons.
to know the number of neutrons, you would take the mass number, 39, and subtract it from the proton number, 19, which would equal 20.
This works because neutrons and protons are equal to 1 amu. Electrons are significantly lower in mass and do not really matter when calculating mass
sorry let me add more:
the bohr rutherford diagram would have, in our example, 19 protons and electrons in the neucleus (in the middle).
For the electrons, the layer can hold two electrons (H, He), so we put two electrons
the second layer holds 8, because when counting along the periodic table, there are eight elements: Li, Be, B, C, N, O, F, Ne.
So now we have placed 10 electron but need to get to 19, so 9 more.
The third layer hold 8, because we count arcoss the periodic table. and then put one more electron on the fourth layer to get to 19 electrons and total