Y11 Chemistry - Metallic Bonding

Metallic bond

The non-directional force of attraction between metal ions and a sea of delocalised electrons.

How do metallic bonds form?

The metal atoms release their valence electrons in order to reach noble gas configuration, forming positive ions in a sea of negative delocalised electrons.

Delocalised electrons

Electrons that are free top move randomly throughout the metallic lattice.

What is the structure of metallic bonds?

A closely packed 3D lattice/network of cations, surrounded by a sea of delocalised valence electrons.

Why are metals good at conducting electricity?

The delocalised electrons are free charged particles that make metals conductive. (They move towards the positive terminal of a power source.)

Why are metals good heat conductors?

The delocalised electrons can carry kinetic energy throughout the entire lattice, the metal ions also vibrate.

Why are metals malleable and ductile?

Metallic bonds are non-directional, meaning that they stay bonded even if the ions move around each other.

Why are metals (excl. Hg) solid at room temp.?

The bonds between the metal ions and the sea of delocalised electrons is quite strong, so it takes moderate-high temperatures to melt them and disturb the lattice (network). Same applies to boiling point.

Why are transition metals harder, denser, and harder to melt than metals in groups 1 and 2?

They have greater core charge, so hold their electrons closer (smaller atomic radius) so more atoms can be packed into the same area.

Metal reactivity generally increases…

Left and down on the periodic table. This is due to their lower ionisation energy and larger atomic radius.

What is an alloy?

A solidified mixture of a metal with another metal or carbon

How do the properties of alloys generally differ from that of pure metals.

Due to the disruption of the metallic lattice, the alloy is harder, as the ions can not move around in the lattice as a pure metal can. The melting point is also lower, since the disturbed lattice is easier to break.

How does steel work?

A small amount of smaller carbon atoms are distributed into the small gaps between Iron cations in the lattice. This is called an interstitial alloy.

How does stainless steel work?

Nickel and Chromium cations replace some of the iron cations in the lattice. This is called a substitutional alloy and allows the steel to resist corrosion, and makes it stronger and harder.

What are some examples of alloys?

Brass - Copper + Zinc

Bronze - Copper + Tin

Electrum - Gold + Silver

Pewter - Tin + Lead

Steel - Iron + Carbon

What are metal crystals?

A region of solid metal where ions are arranged in a regular way. Crystals are arranged randomly with respect to one another, so there are gaps in the lattice between crystals.

How do crystals impact a metals properties?

Smaller crystals generally lead to a harder and more brittle metal as cations are less able to move around each other, and have most areas of disruption between them.

What is work hardening?

By hammering or otherwise working a cold metal, you can harden it by flattening and pushing the crystals together. This can change the properties of the metal e.g. Bending a paperclip too much will make it become malleable and snap.

What happens when a metal is heated and then cooled?

The crystals merge when the metal is heated, and reform when it is cooled, the speed of cooling determines the size of the new crystals, the longer it takes for the metal to cool, the larger the resultant crystals will be.

What are the three types of heat treatment?

Annealing, Quenching, and Tempering.

What is annealing?

A metal is heated and allowed to cool slowly, leading to larger crystals and increased ductility and softness

What is quenching?

A metal is heated and cooled rapidly, leading to smaller crystals forming and an increase in hardness and brittleness

What is tempering?

A quenched metal is heated to a lower temperature (than for quenching) and allowed to cool, allowing medium sized crystals to form, and reducing its brittleness.