The Chemical bond
Electron shells
The atom consists of electrons moving around a nucleus. The movement of the electrons is complex, but we can use a simple model to talk about it, and to study it, namely the shell model.
The animation of the Kr atom, which has 36 electrons, is shown below:
(a different, and a little bit more realistic representation of the atom, is shown here, with elliptical orbits. The representation of electrons orbits as being circular-shown in this page- is useful to understand the shells and energy levels)
The animation also shows that electrons closer to the nucleus move at higher speeds.
Each shell has a limited capacity, which is given by th formula 2n2, where n is the number of the shell (actual values are shown on the left hand side of the page). In an outer shell, however, the limit is 8 electrons, whatever the shell (the exception are the atoms that have around 2 electrons; in this case their outer shell will fill up with only 2 electrons).
The electrons in shells higher up have higher energies than the ones closer to the nucleus.
It is interesting to observe that electrons are not allowed in spaces in between shells.They must be in a shell. This fact looks very unusual but it is explained by the laws of quantum mechanics.
Because electrons are forced to occupy shells further away from the nucleus, as the closest shells get filled up, the atoms with larger numbers of electrons are larger in size.
© Ricardo Esplugas. All images in this site can be bought in an enlarged version. Please contact me on ricardochemistry@gmail.com