Atom Orbitals

An atom is a system of one or more electrons bound to a nucleus. Here is a quantum microscope image of a hydrogen atom:

atom hydrogen quantum

Hydrogen has just one electron, which exists as a fuzzy probability cloud.

That probability cloud is called an orbital (not orbit):

Orbital: a region where we expect an electron to be 90% of the time.

The simplest orbital is the "s" orbital (such as the single electron in Hydrogen):

s orbital
s orbital

But we can have more than one orbital, each with a different energy state.

First, play with the different vibration modes we get on a simple string:


Now play with the interesting vibration modes on a 2D surface like a drum:


(Note for the curious: the above animation uses the "Bessel Function" to calculate the different ways a surface can vibrate when it is fixed at its edges.)

Now imagine the vibration modes around a nucleus!

Well, here they are, with labels:

s orbital
orbitals s, p, d and f

  • An orbital can have up to two electrons (each with opposite "spin")
  • Electrons prefer to be at the lowest energy state possible.
  • The s orbital has the lowest energy state then p, d and f

But there is one more twist! An atom hosts electrons in shells:

carbon shells
Carbon atom with two shells

So, with different orbitals available at different shells ...


... where does each electron go?

Well, it is like packing things into a box, the items find their lowest energy locations.

For each electron we have the energy needed for the shell plus the energy needed for the orbital.

And we get this (notice how energy levels overlap):

s orbital
energy of shell plus orbital

The order goes 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, etc. But there are exceptions! Those pesky electrons don't like to be told what to do except by nature.

ionized atom

But other things can happen to an atom.

For example radiation can knock an electron out of the atom, read about radiation to learn more.


The arrangement of electrons, particularly the outermost electrons, causes chemical behavior.

Now it is time to learn about the Periodic Table and Electrons to see more of the story!