contain two electrons with opposite spins (ms=±1/2). This is called the Aufbau
principle. An example may be used to illustrate this. The element lithium,
atomic number 3, has 3 electrons. In the unexcited or ground state, these must
occupy the lowest energy levels, which are the 1sand 2slevels. Two electrons
fill the 1slevel and one goes into the 2s.
Figure 3 shows the sets of atomic energy levels with n =2, 3, 4, 5, 6 and 7 and
l=0, 1, 2 and 3. The diagram also shows that the most favorable transitions
occur when lchanges by ±1. It is worth noting that the transition shown in bold
is used to measure lithium in atomic emission spectrometry (see Topic E4). In an
excited state, the electron population is altered. In transition elementsthere are
many low-lying energy levels and excited states with similar energies.Molecules also possess energy levels defined by quantum numbers. When
atoms combine into molecules, their orbitals are changed and combined into
molecular orbitals. As an example, the atomic orbitals of carbon, hydrogen and
oxygen combine in the molecule of propanone, C 3 H 6 O, so that the three carbons
are linked in a chain by single (s) bonds, the two outer carbons are each linked
by sbonds to three hydrogens, while the central carbon is linked by a double
bond to the oxygen, that is by both a sand a pbond. Additionally, the oxygen
still has unpaired or nonbonded nelectrons.Molecular
energy levels
192 Section E – Spectrometric techniques
WavelengthEnergyWavenumber
cm–1110 –210 –410 –1210 –1010 –810 –610 –410 –2110210201018101610141012101010810610 –1310 –1510 –1710 –1910 –2110 –2310 –2510 –27100001000100101mÅ
MeVVacuum
ultraviolet
Ultraviolet
visible
Near infrared
InfraredFar infraredMicrowavesRadio wavesNuclear transitionsTransitions of inner
shell electronsTransitions of valence
electrons and
bonding electronsMolecular vibrationsMolecular rotationsElectron spin
transitionsNuclear spin
transitionsJ HzEnergy frequencyFig. 2. The electromagnetic spectrum from g-ray to radiowave.