Arranging the Elements
In the modern Periodic Table, elements are arranged in order of their atomic numbers.
Each horizontal row is called a period.
Elements odten show trends in properites across a period in the table:

these trends are repeated across each period
repeating pattern of trends is called periodicity

Each vertical column is called a group, and contains the elements with similar properties.
The Periodic Table is the chemist's way of ordering the elements to show patterns of chemical and physical properties.

Semi-metals or metalloids display properties display properties between those of a metal and a non-metal - boron, silicon, germanium, arsenic and tellurium.

Periodicity
Periodicity is the trend in properties that is repeated across each period. Using periodicity, predictions can be made about the likely properties of an element and its compounds.
Trends down a group may also affect the periodic trends:

across each period, elements change from metals to non-metals.
as you move down the Periodic Table, the changes take place further to the right
so, for example, at the top of Group 4, carbon is a non-metal, but at the bottom of Group 4 tin and lead are metals
so trends in properties can exist vertically down a group as well as horizontally across a period.

Variation in Electron Structure
Chemical reactions involve electrons in the outer shell. Any similarity in electron configuration will be reflected in the similarity of chemical reactions.
Similar elements are placed in vertical groups - this is a key principle of the Periodic Table.

elements within a group have atoms with the same number of electrons in their outer shells. This explains their similar chemical behaviour.
this repeating pattern of similarity is caused by the underlying repeating pattern of electron configuration

Each element within a vertical group has:

the same number of electrons in the outer shell
the same type of orbitals

So, elements in the same group react in a similar way because they have similar electron configurations.

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Undergraduate Chemistry - Part 1 (The Periodic Table) Notiz am The Modern Periodic Table, erstellt von siobhan.quirk am 20/05/2013.