C3.9–10 Bonding in Metals and Giant Metallic Structures

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: metallic bonding, delocalised electrons, the properties of metals, and why alloys are harder than pure metals. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe metallic bonding as the attraction between positive ions and delocalised electrons
• Explain why metals have high melting and boiling points
• Explain why metals are good conductors of electricity and heat
• Explain why pure metals can be bent and shaped
• Explain why alloys are harder than pure metals, and calculate the percentage of a metal in an alloy

Key words

• Metallic bond: The strong attraction between positive metal ions and the shared delocalised electrons.
• Delocalised electrons: Outer-shell electrons that are not tied to one atom and can move freely through the metal.
• Malleable: Can be hammered or bent into shape without breaking.
• Ductile: Can be drawn out into wires.
• Alloy: A mixture of a metal with one or more other elements, usually other metals (or carbon in steel).

Metallic bonding

Metals are giant structures of atoms arranged in a regular pattern (layers). The outer-shell electrons of the metal atoms are delocalised: they leave their atoms and are free to move through the whole structure. This leaves positive metal ions in a “sea” of electrons.

Key fact: A metallic bond is the strong electrostatic attraction between the positive metal ions and the shared, delocalised electrons. It acts throughout the structure.

The number of delocalised electrons depends on the number of outer electrons. Sodium (Group 1) gives 1 electron per atom and forms Na⁺ ions; magnesium (Group 2) gives 2 and forms Mg²⁺ ions.

Because the atoms are packed in a regular pattern, metals are made of crystals. You can sometimes see them, for example the large zinc crystals on galvanised steel lamp posts.

Model: You can think of the delocalised electrons as a “glue” holding the ions together. This model is useful, but it has a big drawback: real glue stays put, while the electrons are free to move through the whole metal. That movement is what lets metals conduct and bend.

Properties of metals

• high melting and boiling points (most are solid at room temperature); Explanation the strong metallic bonds act throughout the giant structure; lots of energy is needed to break them
• good conductors of electricity; Explanation the delocalised electrons can move through the metal and carry electrical charge
• good conductors of heat (thermal energy); Explanation energy is transferred by the delocalised electrons
• malleable and ductile (pure metals); Explanation the atoms are in layers which can slide over each other while the bonding stays in place

Worked example: why magnesium melts at a higher temperature than sodium

Sodium gives 1 delocalised electron per atom and forms Na⁺ ions. Magnesium gives 2 and forms Mg²⁺ ions.
So in magnesium there are more delocalised electrons and the ions have a bigger charge.
The electrostatic attraction between the ions and the electrons is stronger, so more energy is needed to overcome it.

Answer: magnesium has stronger metallic bonding, so a higher melting point (650 °C compared with 98 °C)

Exam tip: For conduction, say what moves (delocalised electrons) and what they do (carry charge / transfer energy). “Free electrons” is fine; “free ions” is wrong.

Alloys

Most pure metals are too soft for everyday use. An alloy is a mixture of a metal with other elements. The atoms of the added elements are a different size. They distort the layers, so it is more difficult for the layers to slide over each other. This makes alloys harder than pure metals.

• steel; Main metal + added element iron + carbon (and other metals); Example use buildings, bridges, cars
• brass; Main metal + added element copper + zinc; Example use door handles, musical instruments
• bronze; Main metal + added element copper + tin; Example use statues, medals
• 18 carat gold; Main metal + added element gold (75%) + copper / silver; Example use jewellery

Calculating the composition of an alloy

Worked example: percentage of a metal in an alloy

A 400 g brass door handle contains 268 g of copper. The rest is zinc.
% copper = (mass of copper ÷ total mass) × 100 = (268 ÷ 400) × 100 = 67%
Mass of zinc = 400 − 268 = 132 g, so % zinc = 100 − 67 = 33%

Answer: 67% copper and 33% zinc

Gold purity is measured in carats. Pure gold is 24 carat. So % gold = (carats ÷ 24) × 100. For example, 18 carat gold is (18 ÷ 24) × 100 = 75% gold.

Common mistakes

• Saying metals conduct because of free ions or moving atoms. It is the delocalised electrons.
• Saying alloys are harder because the bonds are stronger. It is because the layers are distorted and cannot slide easily.
• Describing metallic bonding as atoms sharing a pair of electrons (that is covalent bonding).
• Saying an alloy is a compound. It is a mixture.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows a model of the bonding in a metal. What are the particles labelled A? (Choose from: positive metal ions, negative ions, delocalised electrons, protons.)

Answer: positive metal ions

Working: A is a positive metal ion. B shows a delocalised electron. Metallic bonding is the attraction between the positive ions and the delocalised electrons.

Question 2. The diagram shows the atoms in two metals, X and Y. Which statement is correct? (Choose from: Y is harder because the different-sized atoms distort the layers, X is harder because its layers are regular, Y is softer because it has more atoms, X and Y are equally hard.)

Answer: Y is harder because the different-sized atoms distort the layers

Working: Y is an alloy. The larger atoms distort the layers, so they cannot slide over each other as easily. X is a pure metal with regular layers that slide easily.

Question 3. True or false? Metals conduct electricity because their positive ions move through the metal.

Answer: False

Working: False. The delocalised electrons move through the metal and carry the charge; the ions stay in place.

Related lessons

• Previous lesson: C3.8 Fullerenes and Graphene
• Next lesson: C4.1 Relative Masses and Moles
• In this chapter: C3.1 States of Matter
• In this chapter: C3.2 Atoms Into Ions
• In this chapter: C3.3 Ionic Bonding

Continue in the full course

This lesson is section C3.9–10 (AGCSC2009) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.