Electron Configurations


8.2 Electron Configurations
• Building-up (aufbau) principle – as new
electrons are added to the atom, they are
placed in the lowest energy available orbital
• Closed shell configuration represents a
(minimization of the total energy of the atom)
completely filled principal shell (He → 1s
– Electron configuration – a list of the occupied
subshells and the number of electrons on them
– Orbital diagrams – each orbital is represented
by a box; the electrons are shown as up or down
arrows depending on the spin quantum number
(+1/2 or -1/2)
• Degenerate orbitals – orbitals with equal energies
– All orbitals in a subshell are degenerate (same n
and l) → the three 2p-orbitals are degenerate
• Hund’s rule – in filling degenerate orbitals,
electrons enter the empty orbitals having identical
spins before pairing in one of them (minimization of
the repulsion between the electrons)
• Outer electrons – electrons in the outermost
occupied principal shell
• Inner (core) electrons – inner shells
• Condensed e
configurations – inner shells
(or part of them) can be abbreviated with the
symbol of the previous noble gas in brackets
→ abbreviated as [He]


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