General Periodicity

General Periodicity

Periodicity

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What is periodicity?

Periodicity is the study of the trends in chemical and physical

properties of the elements and their compounds relative to their

positions in the periodic table.

The periodic table has been developed over the last 150 years. The

basic structure of the table is very closely related to our current

understanding of the electronic structure of the elements. However,

when the periodic table was first constructed (by Mendeleev, in the

1800's ), the electronic structure of the elements had not been

devised.

Instead, early forms of the periodic table relied on using the

physical properties of the elements known at that time. It must have

been a great revelation to the chemists from the early part of this

century to realise that there really was such a strong correlation

between chemical properties of the elements and their physical

structure.

The periodic table.

An example of a periodic table is given below :

[IMAGE]

The elements in the columns down the periodic table form groups. The

major groups are numbered but often the numbers are given as Roman

numerals (the first group, the blue column in the above diagram, is

referred to as group I etc).

Elements within the same group similar chemical properties but often

show a systematic variation in extent or speed of reaction as the

group is descended.

For example all of the group one elements (the alkali metals) react

vigorously with water to form water soluble alkalis with the

subsequent production of hydrogen gas.

However, the speed of reaction increases down the group (lithium only

reacts relatively slowly with water but francium and rubidium are

explosive in the presence of water)

The rows across the periodic table are known as periods. The chemistry

of the elements within each period will tend to change quite

dramatically but we can still determine trends in some physical

properties and the nature of some common reactions.