The electron shells, levels

The electrons of an atom, each with a negative charge, need a well organized space around the nucleus. They all are negative, so they will not stick toghether. On the contrary. Their behaviour and their position must be carefully arranged.
The positive nucleus attrackt them, keeps the electons toghether in the atom. But there is not only attraction, there is also repulsion between the electrons themselves. You could say: they all have to compete with each other and fight for a place. Here it is also very important to know that every electron possesses more or less amounts of energy.
Consider electrons as charges in movement, and in fact, that is the definition of electrical current. Moving charges cause an electromagnetic field and that field, in its turn, will influence the position of all electrons present in the atom.
All this contributes to the complicated arrangement of electrons in an atom, for example, the division of the electrns over the different levels. Electron levels.

We give now some simplified rules for the electron division inside the atom, in accordance to the atom theory.

Electrons have energy, and the amount of energy depends on various factors, like the distance of an electron to the nucleus.

The further from the nucleus, the more energy has an electron.

The own energy of an electron
that suffers the influence of the forces of an atom nucleus,
determines in what energy level that electron may stay.

Better to speak about energy level, main level and sublevel of an electron, than about electron shells.

Below three basic rules for the main levels: (note: do not automatically fit all atoms!!)
  1. The nucleus attracts electrons; dependent on their eneregy and on the available space, the electrons stay on a certain distance from the nucleus. The inner shell, main level, most close tot the nucleus, is called number 1; the next is number 2, etcetera.
    We use for that number the letter n = main quantum number.
  2. The maximum number of electrons in level n equals 2n2;
    This number has another impact; this number is also the preference number of level n
  3. The most outside main level has a maximum of 8 electrons.

That last rule knows a couple of exeptions in elements with a very low atom number.

An atom of Sodium has a total of 11 electrons to divide. In level 1 can be put 2 electrons, level 2 is full with 8 electrons. Remains one electron for lever 3. There is nothing more to divide.
That third level is then the outer shell of the Sodium atom and has only one electron. This electron gets the name of "valency electron" and is very very important for the properties of Sodium. (more about this later)

Now you come to look at it a bit more precisely. Now you get some rules that cover all atoms, without exeptions:
The main levels are subdivised in sublevels.
Main level 1 has only one sublevel, of the type s with a maximum electron number of: 2
Main level 2 has 2 sublevels of the type s and p with a maximum electron number of: 2 and 6
Hoofschil 3 has 3 sublevels of the type s, p and d with a maximum electron number of: 2, 6 and 10
Main level 4 has 4 sublevels of the type s, p, d and f with a maximum electron number of: 2, 6 , 10 and 14
The main levels 5 to 7 - theoretically - could be divised in 5, 6 and 7 sublevels.
But no such big atoms exist; nature did not realise them. so:
Main level 5 divides itself in 4 sublevels of the type s, p, d and f with a maximum electron number of: 2, 6, 10 and 14
Main level 6 divides itself in 3 sublevels of the type s, p and d with a maximum electron number of: 2, 6 and 10
Main level 7 divides itself in two sublevels ofthe type s and p with a maximum electron number of: 2 and 6

When creating a sublevel, you must imagine that the filling of it always starts as close as possible to the nucleus (+ and - do attract each other). The closer to the nucleus, the less energy the electron posssesses.
The complication starts from sublevel 3d/4s on, where the sublevels start to overlap.
And always: after filling one of the sublevels with electrons, according to the rules, a small rest remains (exept at noble gas atoms). That rest of electrons takes position in the outer sublevel. That outer sublevel is mostly important for the properties of the atom of the substance. This sublevel determines also the position of the element in the periodic table. (see next module)

An example:
The Sodium atom has the following electronic division in sublevels:
1s2 2s2 2p6 3s1.

So, the atom has 3 main levels and in the outer main level 1 (valency) electron.
In this case we come to the same result with the simple 2n2 -rule.

Every main level has at least one sublevel.

NB: Remember those data: number of valency electrons and number of main levels; they characterize the atom. So they are important.