Recall from O level the idea of electrons arranged in shells (energy levels) known as the Bohr model.
When examined in further detail, this idea of shells is actually known as principle quantum shells which are described by principle quantum numbers n, i.e. n = 1 refers to the first principle quantum shell.
There are some flaws in this model such as not being able to explain fine structure and hyperfine structure in spectral lines.
1) This model of the atom assumes that all the 8 electrons in the n=2 quantum
shell are equivalent and of the same energy level.
2) It also assumes that the electrons are orbiting in a circular orbit around the
Both of these assumptions are wrong which will be addressed shortly.
1) The n=2 quantum shell is further split into two subshells known as the s and p subshell which are different in energy where the p subshell is higher in energy.
Note: n= 3 quantum shell would thus be split into three subshells, n=4 quantum shell will be split into four subshells, so on and so forth.
2) Subshells can be further split into orbitals. For example, a 2p subshell consists of three 2p orbitals, the 2px, 2py and 2pz orbitals. The subscript denotes the axis on which the orbital lie, for example, the 2px orbital would be aligned to the x-axis.
Note: 2px, 2py and 2pz orbitals are commonly referred to as 2p orbitals.
Hence, electrons do not have a circular orbit. In fact, they do not even orbit, they simply have a probability to be found in the region of space as denoted by the shape of the orbital.
Next, we will examine how we can write the electronic configuration of any atom following a few simple rules.