How Electrons Are Arranged
Electrons do not simply orbit the nucleus randomly. They occupy shells (also called energy levels), each of which can hold a limited number of electrons:
| Shell | Maximum electrons |
|---|---|
| 1st (closest to nucleus) | 2 |
| 2nd | 8 |
| 3rd | 8 (for the first 20 elements) |
Electrons always fill the lowest available shell first.
Writing Electronic Configurations
The electronic configuration shows how many electrons are in each shell, written with commas:
| Element | Atomic number | Configuration |
|---|---|---|
| Hydrogen | 1 | 1 |
| Oxygen | 8 | 2, 6 |
| Sodium | 11 | 2, 8, 1 |
| Calcium | 20 | 2, 8, 8, 2 |
The number of electrons in the outer shell (valence electrons) determines how an element reacts. Elements in the same group of the periodic table have the same number of outer electrons — that's why they behave similarly.
Full Outer Shells and Stability
Atoms with a full outer shell are very stable and unreactive. These are the noble gases (helium, neon, argon, etc.). Most other atoms react in order to achieve a full outer shell, either by:
- Sharing electrons (covalent bonds)
- Gaining or losing electrons (ionic bonds)
Linking Shells to the Periodic Table
The periodic table is structured around electron shells:
- Period 1 (H, He): electrons only in the 1st shell
- Period 2 (Li → Ne): electrons filling the 2nd shell
- Period 3 (Na → Ar): electrons filling the 3rd shell
- Groups 1–7: show how many electrons are in the outer shell
- Group 0: full outer shell, noble gases
Real world: Noble gases in lighting
Neon (2,8) and argon (2,8,8) both have completely full outer electron shells. This makes them almost totally unreactive — they do not bond with other atoms. This is why neon is used in neon signs (it glows without reacting with anything) and argon is used in light bulbs (to prevent the filament oxidising).
Real world: Sodium reacting with water
Sodium (2,8,1) has just one electron in its outer shell. Having nearly-empty outer shell makes it extremely reactive — it desperately "wants" to give away that one electron to achieve the stable 2,8 configuration of neon. This is why sodium fizzes violently when placed in water, releasing hydrogen gas.
Real world: Chlorine in swimming pools
Chlorine atoms (2,8,7) have 7 electrons in their outer shell — one short of the stable 8. This makes chlorine very reactive and a strong oxidiser. Chlorine is added to swimming pools to kill bacteria (by oxidising their cell membranes). The chlorine's reactivity is entirely due to its nearly-full outer electron shell.
Where do you see this in real life? The reactivity of all chemicals in daily life — from the sodium in your table salt to the chlorine in disinfectants to the neon in illuminated signs — comes directly from how many electrons are in the outer shell of each atom.