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The Periodic Table » Metals and Non-metals

What you'll learn this session

Study time: 30 minutes

AQA spec: 4.1.2.3

What makes an element a metal or a non-metal
Where metals and non-metals sit in the periodic table
How outer electrons decide whether an element forms positive ions
The main differences in physical and chemical properties

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Metals and non-metals: the chemist's definition

You probably already think of metals as hard, shiny things like spoons and coins. That is a good start, but chemists use a sharper test. It is all about the ions an element makes when it reacts.

Remember, an ion is an atom that has lost or gained electrons, so it has a charge. Losing electrons gives a positive ion. Gaining electrons gives a negative ion.

Key terms:

Metal: an element that reacts to form positive ions.
Non-metal: an element that does not form positive ions.
Positive ion: a particle with more protons than electrons, made when an atom loses one or more electrons.
Negative ion: a particle with more electrons than protons, made when an atom gains one or more electrons.

So calcium is a metal because it reacts to form Ca²⁺ ions. Sulfur is a non-metal because it never forms positive ions. When it reacts with a metal it forms S^2- ions instead.

Where they are in the periodic table

Most elements are metals, like the copper, iron and aluminium here - they sit on the left and bottom of the periodic table

The majority of elements are metals. If you shade every metal on a periodic table, most of the table is covered.

↙ Metals

Found to the left and towards the bottom of the periodic table. This includes Groups 1 and 2, the transition metals in the middle block, and heavy elements like lead at the bottom.

↗ Non-metals

Found towards the right and top of the periodic table. This includes elements like carbon, nitrogen, oxygen, the Group 7 elements and the Group 0 elements.

Many printed periodic tables show a zig-zag "staircase" line running from boron down towards the bottom right. Metals are to the left of the line and non-metals are to the right.

Why position links to atomic structure

Remember, the group number tells you the number of outer electrons, and atoms react to get a full outer shell. Now put those two ideas together.

➖ Metals: few outer electrons

Most metals have 1, 2 or 3 outer electrons. It takes much less energy to lose a few electrons than to gain five or more. So these atoms lose their outer electrons and become positive ions. The shell underneath is already full.

➕ Non-metals: nearly full outer shells

Most non-metals have 4 to 7 outer electrons. Losing all of them would be far too hard. Instead they gain electrons from metals to form negative ions, or share electrons with other non-metals. Group 0 already has full shells, so its elements hardly react at all.

Here are two examples:

Calcium (2,8,8,2) loses 2 electrons and becomes Ca²⁺ with the structure 2,8,8. Metal.
Sulfur (2,8,6) gains 2 electrons and becomes S^2- with the structure 2,8,8. Non-metal.

A few heavy metals near the bottom, such as lead, have 4 outer electrons but still lose electrons to form positive ions. This is why the metal area reaches further right lower down the table.

Both calcium and sulfur ions end up with a full outer shell. The difference is the direction the electrons move. You will see how metal and non-metal ions join together in Ionic Bonding.

From atomic number to reactions

The spec wants you to explain how the reactions of an element depend on its electrons, and so on its atomic number. The chain of reasoning is always the same:

Atomic number gives the number of protons, and so the number of electrons in the atom.
The electrons fill the shells, giving the electronic structure.
The last number is the number of outer electrons.
Few outer electrons: the atom loses them and forms positive ions, so it is a metal. Many outer electrons: the atom gains or shares electrons, so it is a non-metal.

Worked example

Element with atomic number 3. It has 3 electrons, so the structure is 2,1. One outer electron, so it is in Group 1 on the far left. It loses that electron to form a 1+ ion. It is a metal (lithium).

Element with atomic number 7. It has 7 electrons, so the structure is 2,5. Five outer electrons, so it is in Group 5 on the right. It needs 3 more for a full shell, so with a metal it gains 3 electrons to form a 3- ion. It is a non-metal (nitrogen).

Element with atomic number 17. Structure 2,8,7. Seven outer electrons, so it gains 1 electron to form a 1- ion. It is a non-metal (chlorine).

Differences in physical properties

Shiny and great at conducting electricity - that's why copper is the metal inside nearly every wire, while non-metals like sulfur are dull and brittle

Physical properties are things you can observe or measure without changing the substance. Typical differences are:

⚙ Typical metals

Shiny when freshly cut or polished
High melting and boiling points, so most are solids at room temperature
Good conductors of heat and electricity
Malleable (can be hammered into shape) and ductile (can be drawn into wires)
Usually high density
Sonorous (ring when struck)

☁ Typical non-metals

Dull when solid
Low melting and boiling points, so many are gases at room temperature
Poor conductors of heat and electricity (insulators)
Brittle when solid, so they shatter rather than bend
Usually low density
Not sonorous

Everyday examples: copper is used for electrical wires because it conducts well and can be drawn into wires. Sulfur is a yellow, brittle solid that does not conduct.

These are typical properties and there are exceptions. Mercury is a metal but a liquid at room temperature. Bromine is a non-metal that is also a liquid. Carbon as graphite conducts electricity, and carbon as diamond has a very high melting point. Why metals have their properties is explained in Metallic Bonding and Properties of Metals and Alloys.

Differences in chemical properties

Chemical properties describe how an element reacts. This is where the definition really shows.

⚡ Ions

Metals form positive ions. Non-metals form negative ions when they react with metals, or share electrons when they react with other non-metals.

🔥 Oxides

Metal oxides are bases: they react with acids, and those that dissolve in water make alkaline solutions. Non-metal oxides, like sulfur dioxide, dissolve in water to make acidic solutions.

🔗 Compounds

A metal reacting with a non-metal makes an ionic compound. Two non-metals reacting make a compound of molecules held by shared electrons.

Physical properties have exceptions, so the chemical test is more reliable. Graphite conducts, but carbon never forms positive ions, so it is still a non-metal.

Common mistakes

"Metals gain electrons to become positive." No. Electrons are negative, so losing them leaves a positive ion.

"It conducts electricity, so it must be a metal." Not always. Graphite conducts and is a non-metal. Use the ion test.

Mixing up left and right. Metals are on the left and bottom. Non-metals are on the right and top.

Exam-style question

Element A has the electronic structure 2,8,3. Element B has the electronic structure 2,8,7.

(a) Which element is a metal? Explain your answer in terms of electrons. [3 marks]

(b) Give two physical properties you would expect element B to have. [2 marks]

Model answer

(a) Element A is the metal (1). It has 3 outer electrons, which it loses when it reacts (1), so it forms positive ions, 3+ ions (1).

(b) Any two from: low melting and boiling point (or a gas at room temperature); poor conductor of electricity; poor conductor of heat; dull; brittle if solid (1 mark each).

Exam tip

When asked to explain metal or non-metal "in terms of electrons", always say three things: how many outer electrons, whether they are lost or gained, and the charge of the ion formed.

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