Orbitals and electrons in boxes
Electrons in boxes
- We can draw each orbital as a box and each electron as an arrow.
- This shows spin and how orbitals fill.
- It also reveals free radicals — species with unpaired electrons.
Spin and filling rules
- Two electrons in the same orbital must point opposite ways (opposite spin).
- Within a sub-shell, electrons fill empty orbitals singly first (parallel spins) before pairing up.
- Spreading out keeps the negative electrons apart, lowering repulsion.
For nitrogen ($1\text{s}^2\,2\text{s}^2\,2\text{p}^3$): the three 2p orbitals each take one electron first.
Practice
Two electrons sharing one orbital must:
Electrons in the same orbital have opposite spins; this is why a box holds at most two arrows pointing opposite ways.
Practice
Within a sub-shell, electrons:
Filling singly first keeps the electrons apart, reducing repulsion (e.g. nitrogen's three 2p orbitals each get one).
Orbital shapes
- an s orbital is a sphere centred on the nucleus.
- a p orbital is a dumbbell (two lobes); the three p orbitals point along the $x$, $y$ and $z$ axes.
Practice
The shape of a p orbital is:
An s orbital is a sphere; each p orbital is a dumbbell, and the three point along x, y and z.
Free radicals
- A free radical is a species with one or more unpaired electrons.
- Free radicals are very reactive.
Practice
A free radical is a species that:
Free radicals carry unpaired electrons, which makes them highly reactive.
You've got it
Key idea
- paired electrons in one orbital have opposite spins; sub-shells fill singly first (parallel) then pair
- s orbital = sphere; p orbital = dumbbell along $x$, $y$, $z$
- a free radical has unpaired electrons and is very reactive