ELECTRONS IN ATOMS
Quantum theory
The quantum theory is the basis for the electronic structure of atoms. It is a theory
about the behaviour of the electrons in atoms.
The theory:
 States that electrons, like light have properties of both waves and particles.
 Involves a mathematical treatment in which equations describing waves are
modified to take account of the mass of the electron. Solutions to these equations
are complex and require the use of computers and approximations for all atoms
except hydrogen. The solutions to these equations give information on the
potential energy of the electrons arising from their movement, attraction to
nucleus & (in bigger atoms) interaction with other electrons.
Energy and position of electrons
 The energy that the electrons can have is quantised i.e. it can only have certain
values
 The position of the electrons around the nucleus depends mainly on the energy
they have.
 The electrons are located in orbitals. These are regions in space around the
nucleus where electrons have a specific energy.
 Electrons with the most energy are, on average, further away from the nucleus,
than those with lesser energy.
 Outer electrons, which can interact with other atoms, determine the reactivity of
the atom. They are called valence electrons.
 Periodic properties can be explained by the periodic variation in the number of
valence electrons.
Arrangement of electrons in atoms
 The existence of orbitals was suggested by solutions of the equations of the
Quantum theory.
 Each orbital can hold two electrons.
 Each orbital is described by a number (principal energy level) and a letter
describing the sub-level. (All electrons in a level do not have the same energy)
Principle
Energy level
2p
sub-level
 the lowest energy sub-level in each energy level is called ‘s’.
 the next lowest energy sub-level in each energy level is called ‘p’.
 then there is ‘d’ and ‘f’.
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the first energy level contains 1 sub-level (n = 1)
the second energy level contains 2 sub-level (n = 2)
the third energy level contains 3 sub-level (n = 3)
the fourth energy level contains 4 sub-level (n = 4)
the ‘s’ sub-level contains 1 orbital.
the ‘p’ sub-level contains 3 orbitals.
the ‘d’ sub-level contains 5 orbitals.
the ‘f’ sub-level contains 7 orbitals.
The total number of orbitals in any energy level = n2.
 Thus in the first energy level (n = 1) there is 1 orbital – 1s
 In the second energy level (n = 2) there are 4 orbitals. The first is
called 2s and then there are three ‘p’ orbitals called 2p.
 Different orbitals have different shapes.
 To fully describe an electron in an atom there are a series of 4 numbers called
quantum numbers, and a number of simple rules.
 Pauli Exclusion Principle – no two electrons have the same set of four
quantum numbers.
 Heisenberg Uncertainty Principle – it is impossible to know both the
position and the angular momentum of an electron simultaneously.
 Aufbau Principle – this is used to assign the electrons to the orbitals.
Ground state configuration
 An orbital can contain 0, 1 or 2 electrons only.
 Electrons occupy the lowest energy orbitals available to them. They go to higher
energy orbitals only when the lowest energy orbitals are filled.
 For orbitals of the same energy, each orbital is occupied by a single electron of
the same spin, before any pairing of electrons in the orbitals occurs (Hund’s
Rule).
Thus if there are three 2p electrons they are arranged:
Note: in an orbital electrons can spin in two directions – clockwise and anticlockwise.
This is shown as:
If two electrons are in the same orbital they must have opposite spins (Pauli
exclusion principle):
Electron configuration
The arrangement of electrons within an atom or ion is known as the electron
configuration. An ‘electron in boxes’ diagram is often used to show the available
orbitals and the way they fill. Complete the diagrams below and on the next page for
neon and phosphorus.
n=5
5s
4p
3d
n=4
4s
3p
n=3
3s
n=2
n=1
2s
1s
2p
s
p
d
sublevels
d
sublevels
1. Neon – 10 electrons 1s2 2s2 2p6
n=5
5s
4p
3d
n=4
4s
n=3
3s
n=2
2s
n=1
1s
3p
2p
s
p
2. Phosphorus – 15 electrons 1s2 2s2 2p6 3s2 3p3
Although the 4s sub-level is further away from the nucleus than the 3d sub-level, it is
at a lower energy level because of the shape of the orbitals. The 4s sub-level is
therefore filled before the 3d sublevel.
Order of filling
5th shell
5s2
5p6
5d10
5f14
4th shell
4s2
4p6
4d10
4f14
3rd shell
3s2
3p6
3d10
2nd shell
2s2
2p6
1st shell
1s2
Fill first