Electron Configurations And Orbital Diagrams

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Electron Configurations and Orbital Diagrams
According to the Quantum Mechanical model of the atom, every electron of an atom is described by four
quantum numbers. The quantum numbers describe the orbitals that the electrons are located in. Each
orbital has a unique size (n value), shape (l value), and spatial orientation (m
value). Each orbital can
l
hold at most two electrons, with a full orbital having two electrons with a different spin direction (m
s
value). The location of the electrons within the various orbitals is often expressed by orbital diagrams
and electron configuration symbols.
Electrons fill the orbitals of an atom starting with the lowest energy level. Once each orbital at the same
energy sublevel is filled, electrons begin filling the orbitals of the next energy sublevel. The ordering of
the energy of the various orbital
types is shown in the diagram at the
left. It might be surprising to observe
that the 4s orbitals of an atom have
slightly lower energy than the 3d
orbitals. While they are very close in
energy, the 4s orbital is slightly lower
in energy. The diagram at the right
represents a convenient way of
remembering the order in which the
orbitals fill. Simply follow the
arrows beginning with the lowest
one. A final rule for filling orbitals
with electrons is that each orbital at
the same energy sublevel must have
an electron before electrons begin
pairing up inside the same orbital;
this is known as Hund's rule.
The diagram below represents the so-called orbital diagram for chromium. The 24 electrons of a
chromium atom will fill each of the atomic orbitals in the manner shown.
An orbital diagram naturally leads to the writing of an electron configuration. The electron configuration
for chromium is:
2
2
6
2
6
2
4
1s
2s
2p
3s
3p
4s
3d
The orbital diagram above is formatted in such a manner as to place the various orbital types at different
energy levels. A similar format that is used in the textbook (and serves to save space) is the format below
in which the orbitals are listed in order of their energies but along the same line.

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