Chemistry

4.2 - The Modern Periodic Table

8 min

Periods & groups

Groups 1, 7 & 0

Metals & table layouts

note 1 of 3

Ordered by increasing atomic number

The elements in the periodic table are placed in order of increasing atomic number (the number of protons in an atom).
It's read left to right along each row, then down to the start of the next row (hydrogen is 1, helium is 2, lithium is 3, and so on).
Atomic number is also called proton number.

A periodic table with columns labelled G1 to G7 and G0 and rows labelled P2 to P7, under a left-to-right arrow labelled increasing atomic number; each cell gives a symbol and its atomic number, and the Group 0 column is tinted and labelled elements with similar properties.

Why the order matters

Arranging the elements by atomic number causes similar chemical properties to recur at regular intervals.
Properties that repeat regularly like this are described as periodic.
The table is laid out so that elements with similar properties line up beneath one another in columns.

note 2 of 3

Groups, periods and the named groups

Groups - The vertical columns

The vertical columns are called groups.
Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.
Groups are numbered from Group 1 to Group 7, and the far-right column is Group 0.

Periods - The horizontal rows

The horizontal rows are called periods, numbered from period 1 at the top downwards.
A new row begins each time the pattern of properties repeats.
Period 1 contains only two elements: hydrogen and helium.

Named groups and blocks

Group 1 - The alkali metals (reactive metals)
Group 2 - The alkaline earth metals (reactive metals)
Group 7 - The halogens (reactive non-metals)
Group 0 - The noble gases (unreactive non-metals)
Transition metals - The large block of metals located between Group 2 and Group 3

A periodic table with columns labelled Group 1 (alkali metals), Group 2 (alkaline earth metals), Group 7 (halogens), and Group 0 (noble gases), a block labelled transition metals, and rows labelled P1 to P7.

note 3 of 3

Predicting properties from position

Metals vs non-metals - Metals lie on the left and centre; non-metals lie towards the right, along with hydrogen.
Group number - Elements in the same column have the same number of outer-shell electrons and react in similar ways.
The properties and compound formulae of an element resemble those of the elements above and below it in its group.

A periodic table shaded orange on the left and centre, labelled metals, and blue on the right, labelled non-metals, with hydrogen also blue and a staircase boundary running from boron down to astatine.

Worked example - Element with atomic number 20

What can be predicted about the element with atomic number 20?

Step 1 - Find the element in the periodic table

Element 20 is calcium, which sits in Group 2, below beryllium and magnesium.

Step 2 - Deduce its properties from its position

It's on the left of the table, so it's a metal.
Being in Group 2, it has two outer-shell electrons and reacts similarly to magnesium.
Because magnesium forms MgO and MgCl₂, calcium forms CaO and CaCl₂.

key terms

TermDefinition

Atomic numberThe number of protons in the nucleus of an atom.

GroupA vertical column in the periodic table containing elements with the same number of outer-shell electrons.

PeriodA horizontal row in the periodic table.

Alkali metalsThe metals in Group 1 of the periodic table, which each have one electron in their outer shell.

Alkaline earth metalsThe reactive metals in Group 2 of the periodic table.

HalogensThe non-metal elements in Group 7 of the periodic table, which each have seven electrons in their outer shell.

Noble gasesThe unreactive non-metal elements in Group 0 (also called Group 8 or VIII) of the periodic table, which have a full outer shell of electrons.

Transition metalsThe block of metals in the centre of the periodic table, between Group 2 and Group 3.

end of notes

Key points to remember

Groups, periods and the named groups

Groups - The vertical columns

The vertical columns are called groups.

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

Groups are numbered from Group 1 to Group 7, and the far-right column is Group 0.

Periods - The horizontal rows

The horizontal rows are called periods, numbered from period 1 at the top downwards.

A new row begins each time the pattern of properties repeats.

Period 1 contains only two elements: hydrogen and helium.

Named groups and blocks

Group 1 - The alkali metals (reactive metals)

Group 2 - The alkaline earth metals (reactive metals)

Group 7 - The halogens (reactive non-metals)

Group 0 - The noble gases (unreactive non-metals)

Transition metals - The large block of metals located between Group 2 and Group 3

Predicting properties from position

Metals vs non-metals - Metals lie on the left and centre; non-metals lie towards the right, along with hydrogen.

Group number - Elements in the same column have the same number of outer-shell electrons and react in similar ways.

The properties and compound formulae of an element resemble those of the elements above and below it in its group.