The "electronic structure" of atoms and molecules is extremely important in chemistry as it determines the nature of the bonding between atoms, and the physical and chemical properties of the atom or molecule.
There are two important postulates to remember when dealing with electronic structure and electron density:
- The exact position and momentum of an electron can not be known precisely, this is the Heisenberg uncertainty principle which states that the uncertainty in one variable increases as the certainty in the other increases. So, you can only know either the exact position or the exact momentum of quantum particles such as electrons, not both. Thus it is more accurate to deal with electrons in terms of probabilities.
- Quantum Mechanics provides a means of calculating the probability of finding an electron at a specific location in space, and this is called the probability or electron density. In terms of the electron's charge (-1), since we can not know the position of the electron, it is better to think of the charge being spread out non-uniformly over a region of space. Thus a map of the total electron density provides a model of the atom's or molecule's electron distribution and hence the electronic structure.
Molecular Orbital and Valence Bond theories, both make use of atomic (AO) or molecular (MO) orbitals to describe the location of electrons in atoms or molecules. These AOs or MOs have no real existence as such. You can not measure an orbital. They arise out of the mathematical description of Quantum Mechanics. The electron density on the other hand is a measurable phenomena, i.e. it is something we can actually measure and study directly.