The electrons which surround the nucleus move in specific orbits and these orbits form a series of electronic shells. One can envisage a structure similar to an onion in which the skins correspond to the electronic shells.

The maximum capacity of the first shell is two electrons. Click here to see
Therefore Helium which has two electrons in its "shell" and is then full where as the third element Lithium has three electrons so two of these are in the first shell and the extra on occupies the next shell.

The maximum capacity of the second shell is eight electrons.Click here to see this

The maximum capacity of the third shell is eighteen Click here to see this.
but when there are eight electrons in it , the next electron in the element potassium enters the fourth shell . This is because the 2,8,8,1 arrangement is more stable than a 2,8,9 arrangement. The reason for this will now be discussed.

Energy levels and Sub levels

When we study the shells or energy levels that we have just discussed we discover that each of these principal shells can be broken into sub levels.

The number of sub levels is equal to the number of that particular shell therefore the first shell has 1 sub level and the second shell has two sub levels and so on. Each of these sub levels have been assigned a letter these are s, p, d, f and the electron content of each of these sub levels is fixed regardless of the number of the shell, these are as follows

Sub shell Electron capacity.

S 2

P 6

d 10

f 18

The energy levels are numbered that of the lowest energy being denoted by n=1 and that of the next greatest being n=2 and so on. Therefore when n=1 the electron capacity is=2 and when n=2 the electron capacity is 8 and when n=3 the capacity is 18 i.e. 2(nxn) . The main energy level number (the shell number) is known as the principal quantum number.

In any given level the energy of the electron in the sub level increases in the order s < p < d < f but in some cases the "d" sub level of a lower shell overlaps the "s" sub level this is due to the Aufbau princibal which states electrons occupy the lowest available energy level. This means that as atoms are being built up the, 1s sub level is the first to be filled and then the 2s then the 2p and then the 3s and so on. Therefore as electrons only enter higher energy levels after lower energy levels have been filled, this means as the 3d level is of higher energy than the 4s the electrons will enter into the 4s sub level i.e. the fourth shell before the 3rd shell is filled to its maximum capacity because electrons occupy the lowest available energy level first.

A very important aspect of this is the transition elements which have an incomplete d sub level these include metals like Nickel (28) Copper (29) and Zinc (30)

Atomic Orbitals

In order to fully conclude this area of electrons and their configuration we must look at how electrons move around the nucleus. The arrangement of electrons is governed by Pauili's exclusion principal which states that no two electrons in an neutral atom may have the same set of four quantum numbers.

Electrons are confined to regions called orbitals. There are 2 electrons in each orbital each with opposite spin.

There are 4 types of orbitals s , p ,d , f one characteristic of each sub level.

The orbitals are all different shapes and we will now look at these ..........remember each orbital holds two electron.

The "s" orbital. s orbital

The "p" orbital is dumb bellied shaped and each P sub level is made of three "p" orbitals (because the P sub level can hold 6 electrons and every orbital holds 2 electrons)

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Each d sub level consists of 5 orbitals, here we see two of the five orbitals

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The f sub level consists of 7 "f" orbitals.