Electrons in atoms are arranged in electronic energy levels, subshells and orbitals.
One of the areas in which chemistry and physics interact is in the structure of the atom. Chemists came to realise over the course of the twentieth century that, in order to understand the way atoms combine as compounds, it was necessary to understand the way electrons were arranged in atoms. Quantum mechanics is the area of physics which deals with tiny particles. Newtonian mechanics, based on the laws of motion developed by Isaac Newton, work well for large bodies like rocks or planets, but not with tiny particles.
The Heisenberg Uncertainty Principle
The atom is made of a nucleus of protons and neutrons and outer shells of electrons. Chemists found that it was impossible to determine the location motion of these electrons as they move round the nucleus. In 1927 Werner Heisenberg proposed his uncertainty principle which, simply put, states that by trying to observe a tiny particle like an electron, we affect its motion and so our view of it is blurred.
Energy Levels and Orbitals
The solution to this problem was that, instead of predicting a precise location for each orbiting electron, the electrons were assigned to energy levels and orbitals. Quantum mechanics shows that electrons in an atom can only possess specific quantities of energy, called energy levels. Each level has certain areas of space containing electrons named orbitals. Each orbital can contain two electrons which are spinning in the opposite direction to one another.
Shells and Subshells
Atoms of different elements contain different numbers of electrons. These electrons fill the electronic energy levels, beginning at the lowest. So the first energy level or shell is at level 1. This level only has one subshell containing one orbital, and s-orbital. So a hydrogen atom, with one electron has that electron, unpaired in the 1s-orbital. Helium, with two electrons fills the 1s-orbital, one electron spinning one way and one the other way.
Filling from the Bottom
The next element is lithium. This fills the 1s-orbital and the left over electron goes into the 2s-orbital. The level 2 shell has an s-orbital and three p-orbitals, which have slightly higher energy than the s-orbitals. Beryllium is next and fills the 2s-orbital. Then come boron, carbon and nitrogen. These each have one more electron, which go in separate p-orbitals, electrons prefer to be unpaired when they can get away with it. Oxygen, the next element, has one full 2p-orbital and two half full.
d and f Orbitals
This pattern continues with the larger elements. Level three has one s, three p and five d-orbitals. Level four adds seven f-orbitals. It is at this stage that the orbitals start to cross over in terms of energy levels. The 4s-orbital is lower in energy than the 3d-orbitals, so it is filled first. The filling-order of atomic orbitals determines the properties of different groups in the periodic table, and hence the periodicity. For example, the elements in group 1 of the periodic table all have one electron in the outer s-orbital and the transition elements all have the d-orbitals outermost.
Molecular bonding
Each type of orbital; s, p, d, f; has a different shape and orientation around the nucleus (see this orbital website). This will be important when we look at the orbitals involved in molecular bonding.
