8.1The Periodic Table

Arrangement of elements — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

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📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision

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8.1 Arrangement of elements — Notes — IGCSE Chemistry notes: Describe the layout of the Periodic Table, relate position to electronic configuration and ions, and explain trends in metallic character.Order: elements are arranged in order of increasing atomic (proton) number, not r…

Proton number
Transition elements
Outer electrons
Periods: horizontal rows, numbered 1 to 7
Groups: vertical columns, I to VII and VIII (also called 0), in which elements have similar properties
Metals are on the left and in the centre, and non-metals are on the right

Part 1 of 6Layout of the Periodic Table

Order: elements are arranged in order of increasing atomic (proton) number, not relative atomic mass,

and each element has one more proton than the one before.

Periods: horizontal rows, numbered 1 to 7.

Groups: vertical columns, I to VII and VIII (also called 0), in which elements have similar properties.

Metals are on the left and in the centre, and non-metals are on the right.

Group 0 contains the noble gases, which are unreactive.

Hydrogen and helium are both in Period 1.

Hydrogen is placed on its own because it does not fit the properties of any single group.

Transition elements: the central block between Group II and Group III,

which are all metals with high melting points and densities, coloured compounds and catalytic activity.

Part 2 of 6Period and group numbers

Period number: the number of occupied electron shells, so Period 2 elements have two shells and Period 3 elements have three.

Group number: for Groups I to VII, the number of outer electrons, so Group IV atoms have 4 and Group VI atoms have 6.

Group VIII (0) has a full outer shell.

Reading the configuration: the number of numbers gives the period and the last number gives the group.

Chlorine, 2,8,7, has 3 shells and 7 outer electrons, so it is in Period 3 and Group VII.

Element	Proton number	Configuration	Period	Group
Lithium	3	2,1	2	I
Nitrogen	7	2,5	2	V
Magnesium	12	2,8,2	3	II
Chlorine	17	2,8,7	3	VII
Argon	18	2,8,8	3	VIII (0)
Potassium	19	2,8,8,1	4	I

Method
- Find the proton number of the element.
- Count the shells to give the period.
- Count the outer electrons to give the group.
- Decide metal (left) or non-metal (right).
- Predict ion charge and reactions from the group.

Part 3 of 6Group number, similar properties and ions

Chemical reactions involve the outermost electrons, so elements with the same number of outer electrons react in similar ways.

Lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) all react with Group VII elements to form ionic compounds.
All Group I metals react with water to give hydrogen and an alkaline hydroxide.

Down a group a full shell of electrons is added to each subsequent element.

Metals lose outer electrons to form positive ions, and for Groups I to III the charge equals the group number (Na⁺, Mg²⁺, Al³⁺).

Non-metals in Groups V to VII gain electrons to form negative ions of charge (8 − group number), such as O²⁻ and Cl⁻, or bond covalently by sharing.

Group VII gains 1 electron to form 1− ions and Group VI gains 2 electrons to form 2− ions.

Noble gases have full outer shells, are very unreactive and exist as single atoms (monatomic).

Part 4 of 6Metallic character across and down

Across a period metallic character decreases, with elements changing from metallic to non-metallic from left to right.

Down a group metallic character increases,

as in Group IV where carbon is a non-metal, silicon and germanium are metalloids,
and tin and lead are metals.

The staircase (zig-zag) line separates the metals on the left from the non-metals on the right, running from boron to astatine.

Elements next to the line are metalloids (semi-metals) and show some properties of both.

Property	Metal	Non-metal
Outer electrons	1–3 (more in lower periods)	4–7
Bonding	Metallic	Covalent
Electrical conductivity	Good conductors	Poor conductors
Oxide	Basic	Acidic
Reaction with acids	Many react	Do not react
Physical properties	Malleable, high melting and boiling points	Brittle, low melting and boiling points

Aluminium oxide and zinc oxide are amphoteric, as they react with both acids and alkalis.

Period 3	Na	Mg	Al	Si	P	S	Cl	Ar
Type	Metal	Metal	Metal	Metalloid	Non-metal	Non-metal	Non-metal	Non-metal
Outer electrons	1	2	3	4	5	6	7	8
Ion or bonding	Na⁺	Mg²⁺	Al³⁺	Covalent (giant)	P³⁻ or covalent	S²⁻ or covalent	Cl⁻	None
Oxide	Basic	Basic	Amphoteric	Acidic	Acidic	Acidic	Acidic	None

Part 5 of 6Predicting properties and identifying trends

The position of an element allows prediction of boiling point, melting point, density and reactivity.

Trends to know

Group I elements react quickly with water, and their melting points decrease down the group.
Noble gases are unreactive.
Transition elements are denser than Group I elements.
Reactivity decreases going down Group VII.

Identifying trends from data (Supplement)

Put the data in order of position in the group.
Describe how the property changes and look for general patterns.
Extend the pattern to an unlisted element.

Group I reactivity increases down the group,

so rubidium explodes with sparks, caesium explodes violently, and francium is too reactive and radioactive to predict.

Part 6 of 6More from the specification

Observation
- What is seen, heard, smelt or measured

Key terms

Term	Meaning
Group	a vertical column of the Periodic Table, whose elements have similar properties.
Period	a horizontal row of the Periodic Table.
Proton number	the number of protons, which sets the order of the elements.
Metalloid	an element with some properties of both metals and non-metals.
Transition elements	the central block of metals between Group II and Group III.
Outer electrons	the electrons in the outermost shell, which decide chemical behaviour.

Avoid these mistakes

Confusing groups and periodsgroups are columns and periods are rows.

Using the period number as the outer electron count: the group number gives the outer electrons.

Saying the table is in order of atomic mass: it is in order of atomic number.

Saying elements in a group have the same number of electrons: they have the same number of outer-shell electrons.

Saying elements get more metallic across a period: they change from metallic to non-metallic.

Last updated: 14 October 2026