P6
The Periodic Table
The Periodic Table is an important tool that chemists use to summarise the physical
and chemical properties of elements. The strength of the Periodic Table lies in its
ability to predict the properties of elements that have not yet been discovered.
Historical
development of the
Periodic Table
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Periodic law
Properties of elements varied periodically with their atomic
weights
Atomic number
Henry Mosley (1914) determined the atomic number of the
elements
Modified Periodic
law
Properties of the elements vary periodically with their atomic
numbers
Trend in atomic
radius
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Decreases from left to right across any period of the table
Increases down each group of the table
Ionisation energies
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First ionisation energy M(g)
Second ionisation energy M+(g)
Third ionisation energy
M2+(g)
Trend in ionisation
energies

The first ionisation energy increases from left to right
across each period of the table
The first ionisation energy decreases down each group of
the table
The lower its ionisation energy the more readily an
element forms positive ions
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Trend in melting
points across a period
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Trend in melting
points down groups
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Dobereiner described the triads of elements (1829)
Newlands described the law of octaves (1864)
Mendeleev and Meyer described the periodic law (1869)
M+(g) + eM2+(g) + eM3+(g) + e-
Each period begins with a low melting point element
Melting points tend to increase until a maximum is
reached (group 4 elements in the second and third
periods)
After the maximum melting points in any period decrease
The lowest melting point for any element in a period is
the melting point for the group 8 element
Melting points decrease down groups 2 and 3
Melting points increase down groups 7 and 8
Trend in metallic
character
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Valency and the
periodic table
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Metallic character decreases across each period
Metallic character increases down each group
The most metallic elements are found in the bottom left
hand side of the periodic table
The most non-metallic elements are found in the top right
hand side of the periodic table
The most common valency of elements from groups 1 to
4 is equal to the group number
The most common valency of elements from groups 5 to
8 is 8 minus the group number
Electronegativity
A measure of the ability of an atom of that element to attract
bonding electrons towards itself when it forms compounds
Trend in
electronegativities
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Ionic compounds
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Covalent compounds
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Electronegativities increase from left to right across each
period
Electronegativities decrease down each group
Electronegativities range from 0.7 (francium) to 4.0
(fluorine)
When the difference between the electronegativities of
two elements is greater than 1.5 an ionic compound is
formed
An ionic compound is formed between the atoms of a
metal and the atoms of a non-metal
When the difference between the electronegativities of
two elements is less than 1.5 a covalent compound is
formed
Covalent compounds are formed between the atoms of
different non-metallic elements