Electron Configurations

Electron Configurations
Atoms & Ions
Men & their Rules
• Aufbau Principle: Start at the beginning
(Electrons enter lowest energy level 1st)
• Pauli Exclusion Principle: Only 2 allowed
(Only 2 electrons allowed in any orbital)
• Hund’s Rule: Stay solo as long as possible
(For orbitals of equal energy, 1 electron
enters each until all orbitals contain one
before any pairs up.)
Consequences of Aufbau
• Electrons enter orbitals in order of increasing
energy levels.
• This order is shown in text on page 111.
• 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p
7s 5f 6d 7p
But there are two exceptions, namely Cu & Cr,
who subtract 1 from 4s and add it to 3d.
Consequences of Pauli exclusion
• The s orbital is filled with 2 electrons.
• The 3 different orientations of the p orbital,
px, py, & pz, each contain a max of 2 electrons
• The 5 different orientations of d orbitals, dxy
dxz, dyz, dx2-y2, & dz2, can have max of 2 each.
• The 7 different orientations of f orbitals also
only have 2 electrons for each suborbital.
Consequences of Hund’s Rule
• One electron is added to px, then 1 to py, then
1 to pz, before the second one is added to px.
The 5th electron is then added to py, & 6th to pz.
• The same pattern is observed for the d & f
• NOTE: The single electrons all have parallel
spins. When the 2nd electron is added to each
suborbital, it spins in the opposite direction.
Relating this to the periodic table:
• The “s block” = Groups IA & IIA
• The “p block” = Groups IIIB – VIII (13-18)
• The “d block” = Transition elements
(Groups IIIA-IIB; Groups 3-12)
• The “f block” = Rare earth metals
What this means…
• You merely need to follow the periodic
table to figure out electron configurations.
• The block (s, p, d, & f) indicates where the
last electrons go.
• Follow the period numbers, 1-7, because
these are your energy levels & then
remember suborbitals penetrate differently.
– d block starts at 3d; f block starts at 4f
How can we apply this?
• Follow the periodic table numerically
– 1s2 (takes you to He) 2s2 (to Be) 2p6 (to Ne) 3s2
(to Mg) 3p6 (to Ar) 4s2(to Ca) 3d1-3 (to V) ***
– Cr (1st exception): 1s22s22p63s23p64s13d5
– Cu (2nd exception): 1s22s22p63s23p64s13d10
• Mn: 1s22s22p63s23p64s23d5
• Zn: 1s22s22p63s23p64s23d10
– Ga: 1s22s22p63s23p64s23d104px1
Applications, continued
• La is beginning of the lanthanide series
– 1s22s22p63s23p64s23d104p65s24d105p66s25d1
• The pattern slightly changes with Ce
– 1s22s22p63s23p64s23d104p65s24d105p66s25d14f 1
• Pr puts it back on track
– 1s22s22p63s23p64s23d104p65s24d105p66s24f 3
– This is because 4f and 5d are close in energies
Noble Gas Configurations
• Instead of writing entire core electron
configuration of the noble gas, one can
abbreviate it [NG] & write only valence e• Al: 1s22s22p63s23px1 or [Ne]3s23px1
• Ag:1s22s22p63s23p64s23d104p65s24d9 or [Kr]5s24d9
• Am:1s22s22p63s23p64s23d104p65s24d105p66s24f 14
5d106p67s25f 7 or [Rn] 7s25f 7
• Sg: [Rn]7s25f 146d4 86+2+14+4 =106= Sb atomic#
Orbital Notations
This shows all 4 quantum numbers
n (principle quantum #) = energy lvl 1,2,3...
l (angular momentum Q.N.) = shape s,p,d,f
m (magnetic Q.N.) = x, y, z for p, etc…
s (spin Q.N.) = +1/2 (cw) or -1/2 (ccw)
shown as upward or downward arrows
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