– The vaporised sample is passed through an electron beam, from an electron gun, forming cations.
– The cations are attracted to negatively charged plates, passing through a small hole to focus the beam.
– The magnetic field of an electromagnet deflects the beam, so lighter ions are deflected more. The ions passing out of the electromagnet will be of the same mass.
– The ions reaching the detector are counted, to give the relative abundance of each isotope
Relative Atomic Mass
Relative atomic mass (Ar ) = the average mass of an atom of the element 1/12 of the mass of a carbon-12 atom
E.g. naturally occurring chlorine consists of atoms of relative isotopic masses 35 (75%) and 37 (25%). Its relative atomic mass is 35.5.
Ar = (75/100 x 35) + (25/100 x 37) = 35.5
The average mass can be determined from a mass spectrum – the peaks show the relative abundances of each isotope, which can be multiplied by the mass of each to give the average mass.
Electron shells are arranged into sub-orbitals (s, p, d and f), each of which can contain two electrons spinning in opposite directions due to the Pauli exclusion principle. When filling orbitals the lowest energy orbitals will be filled first (e.g. 1s before 2s).You cannot go to the next orbital unless the previous orbital is half or completely filled.
Orbital How many orbitals does it hold Max Number of Electrons
S 1 2 P 3 6 D 5 10 F 7 14
E.g. electron configuration of Carbon (6 electrons): 1s2 2s2 2p2 Exceptions: Chromium (24 electrons) 1s2,2s2,2p6,3s2,3p6,3d5,4s1 Copper (29 electrons) 1s2 2s2 2p6 3s2 3p6 4s1 3d10