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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
Contents
Topic 3 - Trends in the Periodic Table
1
Ionisation Energy
1
Electronegativity
3
Polar Molecules and Hydrogen Bonds
4
Trends in the Periodic Table Progress Checklist
6
Answer to SAQ
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
Topic 3 - Trends in the Periodic Table
By the end of this section you should be able to:

Describe ionisation energy trends in the periodic table

Describe electron attachment enthalpy trends in the periodic table

Describe electronegativity trends in the periodic table
In this section we are going to investigate trends in the periodic table. The Periodic
Table is the culmination of years of observation and investigation. As you know, it is
not a random list of elements. Their arrangement is defined by their atomic structure.
The main historical character associated with the modern periodic table is Dimitri
Mendelev. If you are interested you could start by visiting this website:
http://www.wou.edu/las/physci/ch412/perhist.htm
As the groupings within the periodic table are based on the atomic structure of atoms,
various trends are found within it.
We are going to look at some of these trends:



Ionisation Energy/ Enthalpy
Electron Attachment Enthalpy
Electronegativity
Ionisation Energy
The ionisation energy is a measure of the energy required to remove one mole of
electrons from an atom in the gaseous state. This results in the formation of a
positively charged ion.
E(g)
E+(g) + e-
Ionisation energy values can be found in your data booklet. The energy values are
measured in kJmol-1. It is useful to remember that within a period the noble gases will
always have the highest ionisation energy, this is to be expected as noble gases have
full outer electron shells and are very stable, removing an electron will result in the
atom becoming less stable as it will no longer have a full outer electron shell.
The first ionisation energy for sodium is shown
Na(g)
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Na+(g) + e- and has a value of 502kJmol-1
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
Further electrons can be removed and it is possible to measure the energy required to
remove further electrons. For example the energy required to remove a second mole
of electrons from sodium is called the second ionisation energy and for sodium would
be shown
Na+(g)
Na2+(g) + e- has a value of 4560kJmol-1
Why do you think the second ionisation energy for sodium is so much higher
than the first?
It is perhaps more difficult to get used to seeing the ionisation energy for elements that
we are more used to seeing form negative ions for example chlorine.
Cl+(g) + e- which has a value of 1260kJmol-1
Cl(g)
As general rule, elements in the lower left of the periodic table have low first ionisation
energies and those on the upper right have higher values.
Ionisation energy increases
Ionisation energy decreases
Ionisation energy values increase as more and more electrons are removed. This is
due to in relative increase in the positive pull of the protons in the nucleus. Electrons
furthest away from the nucleus are less ‘tightly’ held due to their comparative distance
from the nucleus and from the ‘shielding’ effect of electrons closer to the nucleus.
The noble gases have the highest first ionisation energies, and these values fall as
atomic mass increases.
He
2380
Ne
2090
Ar
1530
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Kr
1351
Xe
1170
2
Rn
1037
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
With the aid of your periodic table predict which of the following will have the lowest
first ionisation energy value.
1
Lithium or Fluorine
_________________
Phosphorus or Magnesium
_________________
Oxygen or Potassium
_________________
Check your answers with those given at the end of this booklet.
Electronegativity
Electronegativity is a measure of an element’s ability to attract electron pairs in a
covalent compound; the higher the value, the greater the power of attraction.
Atoms with high ionisation energy are unlikely to lose an electron to another atom and
tend to have high electronegativities. These elements can often remove electrons
from other elements. The values for electronegativity can be found in your data
booklet.
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
Polar Molecules and Hydrogen Bonds
The electronegativity of the elements in a compound can be used to predict the polarity
of the bonds that form.
Electrons will accumulate towards the atom with the higher electronegativity. This
gives them a slightly negative charge. If both atoms have similar electronegativities,
then there will be comparatively equal sharing of the electrons.
If there is a large difference between the electronegativities, such as between Na (0∙9)
and Cl (3∙00), the bonding will be ionic as two ions will be formed Na+ and Cl-.
The electronegativity of oxygen is 3∙5 and that of hydrogen is 2∙2, so in a O-H bond the
oxygen has a negative charge and the H a positive charge. This results in the
formation of Polar molecules and Hydrogen bonds.
O δ  Hδ 
The more electronegative Oxygen effectively retains a greater share of the shared
electrons, so giving it an overall slightly negative charge, leaving the Hydrogen atom
with a slightly positive charge.
This unequal sharing results in intermolecular bonds being formed; these are known as
hydrogen bonds. A single water molecule H2O will contain polar bonds. You should
remember this from earlier in this unit.
Polar Bonds in a Water Molecule
OδHδ+
Hδ+
When two or more water molecules come together, the negative charge on the
Oxygen and positive charge on the Hydrogen attract each other and form a bond
between the two molecules - as shown in the diagram below.
Polar Bonds in a Water Molecule
Hδ+
Hδ+
Hδ+
Oδ-
Hδ+
OδHδ+
Hydrogen Bond
Hδ+
Oδ-
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
A polar bond is one with a net dipole moment. That is, it has positive areas and
negative areas within the molecule. The hydrogen bonds that form between the water
molecules are weak bonds that are much more readily broken than the strong covalent
bonds that are found within the molecule.
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
Trends in the Periodic Table Progress Checklist
Tick the boxes only if you can:
Topic
Describe ionisation enthalpy trends in the periodic table
Describe electron attachment enthalpy trends in the periodic table
Describe electronegativity trends in the periodic table
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Understand(?)
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Fundamental Chemistry: Theory and Practice
Topic 4 – Trends in the Periodic Table
Carnegie College
Answer to SAQ
Answer to SAQ 1
Lithium
Magnesium
Potassium
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