Electron Density Definition:

The electron density is the probability or more specifically the probability density of finding an electron in a particular region of space.

Electron Density Explained:

From Quantum mechanics, we know that the square of the wave function (Ψ) at a particular point indicates the probability of finding an electron at that point. However, since the electron exhibits wave-particle duality, we must imagine the electrons as being spread over a region in space; just like a water wave is spread over the surface of the water. This wave of water is our electron density. Moving on with this analogy, imagine a water wave in a large rectangular pool, what do you see? A wave of water in a particular region on the surface of the water. Similarly, if the pool represents the space around the atom, then the region occupied by the wave is the space where we are most likely to find an electron. This region occupied by the electron density – the water wave – is also called the atomic orbital.

Other aspects to note are –

• The region where the wave exists may be of different arbitrary shapes and hence the different shapes of atomic orbitals. The atomic orbital does not have defined edges and represents a region of space that contains 90–95% of the electron density. Beyond this region, the remaining 5–10% of the electron density tapers off but never ends. Imagine a p orbital which tapers since the probability of finding the electron in the tapering region is very low.
• Although the waterway comprises billions of water particles, the atomic orbital comprises one electron only (the particle nature of the electron)

*In layman terms, we know that density is mass per unit volume. The electron density can thence be imagined as the electron per unit region. And the region where this value is the highest is the atomic orbital.