When scientists write down a description of an element, they try to show all the information needed to draw the diagrams, and there is a method for doing this. Let's look at the standard way of writing Hydrogen, Oxygen, and Fluorine :

The letter is the CHEMICAL SYMBOL for the element.
The top number is the MASS NUMBER: the sum of the protons and neutrons.
The bottom number is the ATOMIC NUMBER: the number of protons or the number of electrons (as there are the same number of protons and electrons in an atom).

Using these descriptions we can work everything out for the third example, fluorine, even though we have not talked about flourine yet.

Flourine has the chemical symbol F. The ATOMIC number tells us that It has 9 protons and 9 electrons. The MASS number, representing protons and neutrons, is 19. We already know that there are 9 protons, so the other 10 are neutrons. Try to work out the same answers for Hydrogen (H) and Oxygen (O).



Stability.
Every atom wants to be stable, and stability for an atom is achieved by having the right number of electrons in its outermost shell. As we have seen, each shell has a maximum number of electrons that it can hold, but shells also have an ideal number of electrons. If the outermost shell of an atom contains this ideal number, it will be a stable atom.

If the outermost shell does not contain this ideal number, the atom will strive to achieve stability by gaining or losing electrons in their outer shell. If this cannot be achieved by shuffling their own electrons around, atoms interact or react with other atoms to take or share electrons to achieve their goal.

Atoms that already have the ideal number of electrons in their outer shell have no desire for more electrons, and don't want to lose any either. These atoms tend to be unreactive or inert. Atoms that that do not have the ideal number tend to be reactive, striving to get their hands on more, or get rid of some electrons.

The table below lists shells one to five, and shows the maximum number of electrons each shell can hold, but also the ideal number fot the atom to be stable if that shell is the outermost shell..

Shell
Can hold
is stable with an outer shell holding
1
2
2
2
8
8
3
18
8
4
32
8
5
50
8

Lets think about two atoms that we are familiar with: Helium and Oxygen.

Helium has 2 electrons in its outer shell.
This is the ideal number for the first shell, so as we would expect, Helium is unreactive and inert.

Oxygen has 2 electrons in the first shell, but only 6 in its second outer shell.
This is not the ideal of 8 electrons in the second shell. We would expect oxygen to do something about this, and try to get 2 more electrons from somewhere by reacting with another atom. This is indeed the case, and oxygen gets involved in many reactions with other atoms.