# Electrons and the Periodic Table

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

https://dx.doi.org/10.1103/PhysRevLett.110.213001

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

But it is easier to use a simple diagram:

In the center (marked "H" for "Hydrogen") is the nucleus with 1 proton, and around that is our one electron.

Electrons are attracted to protons, and are in their lowest energy state:

• each wider "shell" (any dashed circle shown below) has a higher energy state
• within each shell there are also different energy states called orbitals (we give special colors in the animation)
• each orbital has at most two electrons together (with opposite spin)

Play with it below ... discover patterns for yourself ... then learn more later.

images/periodic.js

The electrons don't always go where you expect. It is like packing things into a box, the items find their lowest energy locations, sometimes with subtle shifting going on.

## It Causes Chemistry!

But the really cool thing is that the outermost electrons affect how each element bonds or reacts chemically with others.

Examples:

• Hydrogen (H) is happy to bond (with itself and others) as it has an empty electron spot
• Helium (He) does not like to bond, as its two electrons fill its first shell
• Lithium (Li), similar to Hydrogen, is happy to bond as it has one empty electron spot in its outer shell.
• etc

And so it makes sense to group all elements that behave in a similar way into columns (called groups).

And the rows (called periods) show us how many shells.

There are also 4 main blocks that correspond roughly with the type of oribital (shown as the color of the block).

Have another play with the table and more for yourself.

(Note: the block starting at 57 etc belongs in the table next to number 56. And the block starting at 89 etc belongs in the table next to number 88.)

## Conclusion:

• electrons naturally seek their lowest energy level around the nucleus
• the resulting outermost electrons ... cause chemistry!