For students doing A Level Chemistry under the OCR A specification.
This module is for AS Level too.
The main themes in this summary guide include:
The periodic table and periodicity;
Group 2 and the halogens;
Qualitative analysis;
Enthalpy changes;
Reaction rates and equilibrium (qualitat...
Periodic Table
Groups are vertical column in the periodic table. Elements in
a group have similar chemical properties and their atoms have
the same number of outer shell electrons.
Periods a horizontal row of elements in the periodic table.
Elements show trend in properties across a period.
Periodicity a repeating trend in properties of the elements
across each period of the periodic table.
, Ionisation Energy
First (2nd, 3rd) ionisation energy is the energy needed
to remove one electron from each atom (ion) in 1 mole
of gaseous atoms of an element to form 1 mole of
gaseous +1 ions (+2, +3). X(g) → X+(g) + e-
Three factors affect the attraction between the nucleus and
the outer electron of the atom, therefore, the ionisation
energy.
Atomic radius – greater the distance the less the nuclear
attraction. This has a large effect due to force of attraction
falling off sharply with increasing distance.
Nuclear charge – the more protons there are in the nucleus
the greater the attraction.
Electron shielding – inner-shell electrons repel outer-shell
electrons. The shielding effect reduces the attraction.
The first electron lost is always from the highest energy level
(shell). So, d-block elements lose electrons from the 4s first -
not the 3d.
Successive ionisation energies – involves removing all the
electrons from an atom, one at a time. These provide
evidence for energy levels (shells).
Noble Atomic Number of
Trend
gas radius inner shells
• Atomic radius increases
He 0 • Shieling increases
• Nuclear attraction
Ne 1 decreases
• First ionisation energy
decreases
Ar 2
Trend in first ionisation energy down a group
, General trend in first ionisation energy across Period 2
Atomic
radius
Element Li Be B C N O F Ne
Protons 3p+ 4p+ 5p+ 6p+ 7p+ 8p+ 9p+ 10p+
• Nuclear charge increases
• Shieling is same/similar
Trend • Nuclear attraction increases
• Atomic radius decreases
• First ionisation energy increases
Patterns in first ionisation (provide evidence for sub-shells).
Clear periodicity is seen here…
Broadly the values rise across
each period due to the number
of protons rising in the nucleus
but the shielding staying the
same. There is then a sharp
drop as a new, more distant and
better shielded electron shell is
started by a group 1 element.
The peak of each period is a
noble gas. The lowest point
(trough) of each period is group
1. More subtle patterns, such as,
the drop from group 2 to group
3 (e.g. Be to B) due to change from s to p-subshell. The p-
subshell is higher in energy than the s-subshell (a contributing
factor is it protrudes slightly further from the nucleus). There
is another small drop between group 5 and group 6 (e.g. N
and O; P and S). Going from p3 to p4 requires electrons to
pair up in an orbital to pair up leading to an increase in
repulsion making it slightly easier to remove the first electron
from a group 6 atom than a group 5 atom.
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