Advanced Higher Chemistry
Unit 1
Writing electronic structures
Electronic Configuration
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The electronic configuration for
hydrogen is 1s1, for helium 1s2 and for
lithium 1s2 2s1
This means that for hydrogen there is
one electron in the 1s orbital, for helium
2 electrons in the 1s orbital and for
lithium, 2 electrons in the 1s orbital and
1 in the 2s orbital.
Orbital box notation
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Hydrogen has one electron
(represented by an arrow).
This is found in the 1s
subshell.
Helium has two electrons.
These are also found in the
1s subshell. Due to Hund’s
rule these electrons have
opposing spins (represented
by the arrows pointing in
opposite directions).
1s
1s
Orbital box notation
The following diagrams represent the orbital
box notation for lithium and carbon
Orbital Box Notation
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The orbital box notation for oxygen is
below.
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Electrons fill the 4s orbital before the 3d
orbital because the 4s orbital has a lower
energy.
There are two exceptions to this rule
- chromium which has one electron in its 4s
orbital and five in its 3d orbitals.
- copper which has one electron in its 4s
orbital and ten in its 3d orbitals.
(See later slides on ionisation energies for an
explanation)
Periodic Table
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Groups 1 and 2 of the Periodic Table
are known as the s-block as the
outermost electrons can be found in s
orbitals.
Groups 3 to 0 are known as the p-block
elements as the outermost electrons
can be found in p orbitals.
The transition metals are d-block
elements.
f-block elements are elements 57 to 71
and 89 to 103.
Ionisation Energy
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First ionisation energy = energy
required to remove one mole of
electrons from one mole of gaseous
atoms.
In GENERAL, ionisation increases across
a period due to the increasing positive
charge of the nucleus and it decreases
down a group as the outer electrons are
shielded from the nucleus by the inner
electrons.
Ionisation Energy
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Be has a 1st I.E. of 905 kJ mol-1
B has a 1st I.E. of 807 kJ mol-1
This is because of the electronic
configuration:
Be 1s2 2s2
B 1s2 2s2 2p1
The first ionisation energy of Boron is lower
as the electron is removed from an
incomplete orbital which is less stable than if
it were complete.
Complete orbitals are more stable than
incomplete orbitals
Ionisation Energy
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Nitrogen also has a higher first
ionisation energy than oxygen as the
electron is removed from a half full
subshell.
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Half full subshells are more stable.