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Higher Chemistry
Periodicity
NEW LEARNING
REVISION
Elements are the simplest
substances and consist of
one type of atom.
They are arranged in the
Periodic table in increasing
atomic number.
Grouping elements by bonding
and structure – focus on
Titanium
Grouping compounds by bonding
and structure.
Periodic trends
Starter Task
Lesson 3: Trends in Elements
Today we will learn to
Describe trends in different properties
of elements.
We will do this by
Defining and analysing covalent radius,
ionisation energy and electronegativity.
We will have succeeded if
We can show the pattern of increasing
trends on a periodic table.
Trends in Elements
To keep oxygen away from the titanium when it was extracted it was
important to convert the titanium oxide into another compound that did not
contain any oxygen which could then have pure titanium extracted from it. In
order to achieve this chemists needed to find another non metal that was
more reactive than oxygen and able to displace non-metals from compounds.
ACTIVITY 1.10 Halogen Displacement Reactions and the reactivity of the
alkali metals. Your teacher might demonstrate displacement reactions of
the halogens and/or the reactivity of the alkali metals.
DISCUSS
After discussion with your teacher and others can you describe the pattern
in reactivity for both the alkali metals and halogens as you go down the group:
NOTES
Briefly describe how the reactivity of alkali metals and then the halogens
changes as you go down the group.
Trends in Elements
Trends in Elements
Trends in Elements
Trends in Elements
Trend Going Across the Periodic Table
Name
Atomic Number
Atomic Radius
Lithium
3
Beryllium
4
134 129
Boron
5
Carbon
6
Nitrogen
7
Oxygen
8
Fluorine
9
90
77
75
73
71
1st I.E
526
905
807
1090
1410
1320 1690
Electronegativity
1.00
1.50
2.00
2.50
3.00
3.50 4.00
Trend Going Down the Periodic Table
Name
Atomic Number Atomic Radius
1st I.E
Electronegativity
1690 4.00
Fluorine
9
71
Chlorine
17
99
1260
3.00
Bromine
35
114
1150
2.80
Iodine
53
133
1020
2.60
Neon
10
2090
Trends in Elements
Atomic Size
• As we go across a period, the nuclear charge and
the number of outer electrons increase.
• As we go down a group, the number of electron
shells or energy levels increases but the number of
outer electrons stays the same.
• The covalent radius of an element is half the
distance between the centres (nuclei) of 2 bonded
atoms.
• The trend in atomic size (covalent radius) is:
– Across a period the atomic size decreases as the nuclear
charge increases and attracts the outer electrons closer to
the nucleus.
– Down a group the atomic size increases as an extra
electron shell is added.
Trends in Elements
Ionisation Energy (Enthalpy)
• The ionisation energy is the energy absorbed when
1 mole of electrons is removed from a mole of
atoms in the gaseous state.
• The 1st ionisation energy is the energy needed to
remove the first mole of electrons and the 2nd
ionisation energy is the energy needed to remove
the second mole of electrons, etc.
e.g. the ionisation energies for magnesium are:
1st
2nd
3rd
Mg(g)
Mg+(g)
Mg2+(g)
Mg+(g) + eMg2+(g) + eMg3+(g) + e-
∆H= 744 kJmol-1
∆H= 1460 kJmol-1
∆H= 7750 kJmol-1
• Why the big jump from 2nd to 3rd ionisation energy
for Mg?
NB Successive ionisation energies increase as the atom
becomes more positive.
There is a large jump in ionisation energy when the
electron to be removed comes from a new shell,
closer to the nucleus.
e.g. between the 2nd and 3rd ionisation energy for
magnesium.
Examples
Use your Data Book to calculate the energy
required for the following changes:
a) Ca(g)
b) Al(g)
•
Ca2+(g) +
Al3+(g) +
2e3e-
The total energy to remove more than 1 mole of
electrons is equal to the sum of each mole added
together (as above).
Trends in Ionisation energy
• Across a period ionisation energies increase.
 This is because the nuclear charge increases and holds the outer
electrons more strongly.
 Within each period the noble gas has the highest value for the 1st
ionisation energy explaining the stability of full electron shells.
• Down a group ionisation energies decrease.
 This is because the outer electrons are further away from the
nucleus. The screening effect of the inner electron shells reduces
the nuclear attraction for the outer electrons, despite the
increased (positive) nuclear charge.
Trends in Elements
Electronegativities
• Electronegativity is a measure of an atom’s attraction for electrons
in a bond.
e.g. A covalent bond in the element chlorine:
Both atoms have the same electronegativity so the electrons are
shared equally.
e.g. A covalent bond in the compound hydrogen chloride:
The bonded atoms have different electronegativities so the chlorine
atom has a bigger attraction for the shared electrons than the
hydrogen atom.
As the electrons are attracted closer to the chlorine it becomes
slightly negative (δ-) and the hydrogen atom becomes slightly
positive (δ+).
The trend in electronegativity is:
• Across a period, electronegativity increases.
– This is because the nuclear charge increases, attracting the
electrons more strongly.
• Down a group, electronegativity decreases.
– This is because the atoms are bigger, bonding electrons are
further away from the nucleus and the screening effect of the
inner electron shell decreases the nuclear attraction for
electrons.
Exit Task
Complete Quick Test 3 in your booklet
and we will review as a class. You have 5
minutes!
Answers
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Quick Test 3
1. B
2. A
3. B
4. D
5. C
6. D