Arrangement of ElementsCambridge IGCSE Chemistry: Revision notes

Section 1

How Is the Periodic Table Arranged?

The Periodic Table organises all known elements in a systematic way.

Elements are arranged in order of increasing proton (atomic) number, from left to right and top to bottom
Vertical columns are called groups; elements in the same group have the same number of electrons in their outer shell
Horizontal rows are called periods; elements in the same period have the same number of occupied electron shells
Elements with similar chemical properties are placed in the same group, because the pattern repeats periodically as proton number increases

Key termsperiodic tablegroupperiod

Exam tip

Group number = number of outer shell electrons (for Groups I–VII). Period number = number of occupied electron shells.

Section 2

How Does Character Change Across a Period?

Moving from left to right across a period, elements show a clear trend in character:

Elements on the left of a period are metals
Elements on the right of a period are non-metals
There is a gradual change from metallic to non-metallic character as you move across a period, with some elements near the middle/right showing intermediate (metalloid-like) behaviour
This links to electron arrangement — metals tend to lose electrons easily, non-metals tend to gain electrons easily

Example

In Period 3, sodium and magnesium (left) are metals, while chlorine and argon (right) are non-metals, with silicon in the middle showing some intermediate properties.

Section 3

How Does Group Number Relate to Ion Charge?

The group an element is in tells you the charge of the ion it typically forms, because it relates directly to the number of outer shell electrons.

Elements in Group I form ions with a 1+ charge (lose 1 electron)
Elements in Group II form ions with a 2+ charge (lose 2 electrons)
Elements in Group VI form ions with a 2− charge (gain 2 electrons)
Elements in Group VII form ions with a 1− charge (gain 1 electron)

In general, metals in Groups I–III lose electrons to form positive ions, while non-metals in Groups V–VII gain electrons to form negative ions.

Common mistake

Don't confuse group number with the charge sign — Groups I–III form positive ions (losing electrons), while Groups V–VII form negative ions (gaining electrons).

Section 4

Why Do Elements in the Same Group Behave Similarly?

Elements in the same group share similar chemical properties because they have the same number of outer shell electrons.

Chemical reactions mainly involve the outer shell (valence) electrons
Since elements in a group all have the same number of outer electrons, they tend to react in similar ways and form ions/compounds with similar formulae
This also means you can use an element's position in the periodic table to predict its properties, based on the trends shown by other elements in the same group or period

Think of it like this

Elements in the same group are like siblings — they share a 'family trait' (the same number of outer electrons) that makes them behave in a similar way chemically.

Must Know

The periodic table arranges elements in order of increasing proton number, in periods (rows) and groups (columns)
Group number equals the number of outer shell electrons; period number equals the number of occupied electron shells
Character changes from metallic (left) to non-metallic (right) across a period
Group number relates to the charge of ions formed (e.g. Group I → 1+, Group VII → 1−)
Elements in the same group have similar chemical properties because they have the same number of outer shell electrons
An element's position in the periodic table can be used to predict its properties from known group/period trends

That's the notes covered.

Carry on to the next subtopic.