3.
9 Electron Configurations
The electron configuration describes how the electrons are
distributed in the various atomic orbitals.
In a ground state hydrogen atom, the electron is found in the 1s
orbital.
Ground state electron
configuration of hydrogen
principal (n = 1) number of electrons in
1s 1 the orbital or subshell
Energy
2s 2p 2p 2p
angular momentum (l = 0)
The use of an up arrow indicates an electron
with ms = +
1s
Electron Configurations
If hydrogens electron is found in a higher energy orbital, the atom
is in an excited state.
A possible excited state electron
configuration of hydrogen
2s 1
Energy
2s 2p 2p 2p
1s
Electron Configurations
In a multi-electron atoms, the energies of the atomic orbitals are split.
Splitting of energy levels refers to
the splitting of a shell (n=3) into
subshells of different energies
(3s, 3p, 3d)
Electron Configurations
According to the Pauli exclusion principle, no two electrons in an
atom can have the same four quantum numbers.
The ground state electron
configuration of helium
2p 2p 2p
1s 2
Energy
2s
Quantum number
Principal (n) 1 1
1s describes the 1s orbital Angular moment (l) 0 0
Magnetic (ml) 0 0
describes the electrons in the 1s orbital Electron spin (ms) +
Electron Configurations
The Aufbau principle states that electrons are added to the lowest
energy orbitals first before moving to higher energy orbitals.
Li has a total of 3 electrons
The ground state electron
configuration of Li
2p 2p 2p 1s22s1
Energy
2s The third electron must go in the
next available orbital with the
1s lowest possible energy.
The 1s orbital can only accommodate 2
electrons (Pauli exclusion principle)
Electron Configurations
The Aufbau principle states that electrons are added to the lowest
energy orbitals first before moving to higher energy orbitals.
Be has a total of 4 electrons
The ground state electron
configuration of Be
2p 2p 2p 1s22s2
Energy
2s
1s
Electron Configurations
The Aufbau principle states that electrons are added to the lowest
energy orbitals first before moving to higher energy orbitals.
B has a total of 5 electrons
The ground state electron
configuration of B
2p 2p 2p
1s 2s 2p
2 2 1
Energy
2s
1s
Electron Configurations
According to Hunds rule, the most stable arrangement of electrons
is the one in which the number of electrons with the same spin is
maximized.
C has a total of 6 electrons The ground state electron
configuration of C
1s22s22p2
2p 2p 2p
Energy
2s The 2p orbitals are of equal energy, or degenerate.
1s Put 1 electron in each before pairing (Hunds rule).
Electron Configurations
According to Hunds rule, the most stable arrangement of electrons
is the one in which the number of electrons with the same spin is
maximized.
N has a total of 7 electrons The ground state electron
configuration of N
1s22s22p3
2p 2p 2p
Energy
2s The 2p orbitals are of equal energy, or degenerate.
1s Put 1 electron in each before pairing (Hunds rule).
Electron Configurations
According to Hunds rule, the most stable arrangement of electrons
is the one in which the number of electrons with the same spin is
maximized.
O has a total of 8 electrons The ground state electron
configuration of O
1s22s22p4
2p 2p 2p
Energy
2s
Once all the 2p orbitals are singly occupied, additional
electrons will have to pair with those already in the
1s orbitals.
Electron Configurations
According to Hunds rule, the most stable arrangement of electrons
is the one in which the number of electrons with the same spin is
maximized.
F has a total of 9 electrons The ground state electron
configuration of F
1s22s22p5
2p 2p 2p
Energy
2s
When there are one or more unpaired electrons, as
1s in the case of oxygen and fluorine, the atom is
called paramagnetic.
Electron Configurations
According to Hunds rule, the most stable arrangement of electrons
is the one in which the number of electrons with the same spin is
maximized.
Ne has a total of 10 electrons The ground state electron
configuration of Ne
1s22s22p6
2p 2p 2p
Energy
2s
When all of the electrons in an atom are paired, as
1s in neon, it is called diamagnetic.
Electron Configurations
General rules for writing electron
configurations:
1) Electrons will reside in the available
orbitals of the lowest possible energy.
2) Each orbital can accommodate a
maximum of two electrons.
3) Electrons will not pair in degenerate
orbitals if an empty orbital is available.
4) Orbitals will fill in the order indicated
in the figure.
Worked Example 3.10
Write the electron configuration and give the orbital diagram of a calcium (Ca)
atom (Z = 20).
Setup Because Z = 20, Ca has 20 electrons. They will
fill in according to the diagram at right. Each s subshell
can contain a maximum of two electrons, whereas each p
subshell can contain a maximum of six electrons.
Solution
Ca 1s22s22p63s23p64s2
1s2 2s2 2p6 3s2 3p6 4s2
Think About It Look at the figure again to make sure you have filled the
orbitals in the right order and that the sum of electrons is 20. Remember
that the 4s orbital fills before the 3d orbitals.
3.10 Electron Configurations and the Periodic Table
The electron configurations of all elements except hydrogen and
helium can be represented using a noble gas core.
The electron configuration of potassium (Z = 19) is
1s22s22p63s23p64s1.
Because 1s22s22p63s23p6 is the electron configuration of argon, we
can simplify potassiums to [Ar]4s1.
The ground state electron configuration of K:
1s22s22p63s23p64s1
[Ar] [Ar]4s1
Electron Configurations and the Periodic Table
Elements in Group 3B through Group 1B are the transition metals.
Electron Configurations and the Periodic Table
Electron Configurations and the Periodic Table
There are several notable exceptions to the order of electron filling
for some of the transition metals.
Chromium (Z = 24) is [Ar]4s13d5 and not [Ar]4s23d4 as
expected.
Copper (Z = 29) is [Ar]4s13d10 and not [Ar]4s23d9 as expected.
The reason for these anomalies is the slightly greater stability of d
subshells that are either half-filled (d5) or completely filled (d10).
Cr [Ar]
4s 3d 3d 3d 3d 3d
Greater stability with half-filled
3d subshell
Electron Configurations and the Periodic Table
There are several notable exceptions to the order of electron filling
for some of the transition metals.
Chromium (Z = 24) is [Ar]4s13d5 and not [Ar]4s23d4 as
expected.
Copper (Z = 29) is [Ar]4s13d10 and not [Ar]4s23d9 as expected.
The reason for these anomalies is the slightly greater stability of d
subshells that are either half-filled (d5) or completely filled (d10).
Cu [Ar]
4s 3d 3d 3d 3d 3d
Greater stability with filled 3d
subshell
Worked Example 3.11
Write the electron configuration for an arsenic atom (Z = 33) in the ground state.
Setup The noble gas core for As is [Ar], where Z = 18 2
for Ar.
2 6
The order of filling beyond the noble gas core is 4s, 3d,
and 4p. Fifteen electrons go into these subshells because 2 6 10
there are 33 18 = 15 electrons in As beyond its noble gas 2 3
core.
Solution
As [Ar]4s23d104p3
Think About It Arsenic is a p-block element; therefore, we should
expect its outermost electrons to reside in a p subshell.
