Electronic structure describes the placement and configuration of electrons around the nucleus in an atom. Shells are split in sub-shells which contain a finite number of orbitals. An atomic orbital is a region in space where there is a high probability of finding up to 2 electrons with opposite spins. Orbitals are classified into s, p, d and f. A subshell holding s-orbitals could only hold 1 s-orbital, whereas a subshell holding p-orbitals could hold 3 p-orbitals, hence holding a maximum of 3 x 2 = 6 electrons.
Mass spectrometry is used to find the abundance and mass of each isotope and find the molecular mass of substances. There are 4 stages to this process: Ionisation, Acceleration, Flight Tube and Detection. Sample is first ionised either by electron impact or electrospray ionisation, then it is accelerated using an electric field to gain the same kinetic energy. Lighter ions travel faster and reach the negatively charged detector plate first than heavier ones.
There are different models of the atom that have been improved and changed over the years.