Atom Structure: Electron Shells
Learn why electron shells are responsible for the properties of elements, and whether they are stable or have violent reactions.
- An element's reactivity is determined by the number of electrons in the outer shell of its atom.
- To be perfectly stable, an atom requires a full outer shell of electrons.
- The inner shell of every atom can hold only two electrons - as the shells move further away from the nucleus, they can accommodate more electrons.
- Potassium, with a single outer shell electron, will react with other elements to leave it with a full outer shell.
What makes the element potassium violently reactive?
While the element gold can remain unchanged for thousands of years...
The key to an element's reactivity lies with the number and arrangement of its electrons.
In all atoms, negative electrons move in a relatively huge space around the positive nucleus.
But crucially, the electrons organise into a set of discrete regions, called shells.
If this football were the nucleus of the atom, even the closest electrons would be far outside the stadium.
But scaled down, the electron shells are shown here as rings.
Each shell can only accommodate a set number of electrons.
So inside the first shell of every atom – whether potassium or gold, there can only be a maximum of 2 electrons.
The second shell can hold up to 8 electrons.
And atoms with more electrons can then fill the third shell with up to 8 electrons.
As the shells become further away from the nucleus they can accommodate a growing number of electrons.
1st shell = 2 electrons. 2nd shell = 8 electrons. 3rd shell = 8 electrons. 4th shell = 18 electrons
The innermost shell is always filled first. So a sodium atom, which always has 11 electrons, has 2 in the first shell, 8 in the second, and a single outer electron.
A fluorine atom has 9 electrons. So it has 7 in its outer shell.
It's the number of electrons in the outer shell that determines how reactive the element is.
Because to be as stable as possible, an atom requires a full outer shell of electrons and it will react with other atoms to achieve this.
So, given the chance, potassium, with its single outer shell electron, will react with other elements to lose this electron, leaving it with a full outer shell.
By contrast the noble gases, like neon and argon, already have a complete outer shell, explaining why they are so unreactive.
So the number of electrons in the outer shell explains why some elements don't react together... while other combinations can be explosive.