Electrons within atoms have fixed amounts of energy called quanta.
The discrete lines observed in atomic spectra can be explained if electrons, like photons, also display the properties of both particles and waves.
Stationary (or standing) waves are waves that vibrate in time but do not move in space.
Electrons behave as standing (stationary) waves in an atom.
There are different sizes and shapes of standing wave possible around the nucleus, known as orbitals.
Orbitals can hold a maximum of two electrons.
The different shapes of orbitals are identified as $s$, $p$, $d$ and $f$.
→ The $s$ orbital has the following shape.
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→ The different $p$ orbitals have the following shapes, and are labelled $p_x$, $p_y$, and $p_z$.
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→ The different $d$ orbitals have the following shapes, with the labels as shown.
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The following shows how the different principle quantum shells are split up into sub-shells.
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Each principle quantum shell can hold a maximum number of electrons
In an isolated atom, the orbitals within each subshell are degenerate — meaning orbitals are equal in energy.
Electrons within atoms are filled according to three main principles.
→ The aufbau ( ‘building up’ ) principle, states that electrons fill orbitals in order of increasing energy. This order can be found using the following diagram — $1s$ is filled first, then $2s$, all the way to $4s$, then $3d$.
