Structures and properties notes

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  • Created by: Aman
  • Created on: 21-02-11 14:58

Ionic compounds

·         Require lots of energy to break the lattice apart.

·         The ionic compounds have a very high melting and boiling point.

·         Once the lattice has been broken, the ionic solid becomes a liquid.

·         The ions are free to move in this liquid therefore the liquid is able to carry electrical charge (through the molten liquid).

·         The solid lattice cannot carry an electrical charge because the ions in the lattice are held together tightly with strong electrostatic forces in the ionic bonds and cannot move freely as a result. They can only vibrate in a fixed position.

·         Many ionic compounds will dissolve in water.

·         When an ionic compound is dissolved in water, the lattice is split up by water   molecules and the ions can move more freely. 

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Simple molecules

·         Substances that are made up of chemically bonded molecules tend to have low melting and boiling points.

·         As a result, substances with simple molecules are usually a gas or liquid at room temperature.

·         Covalent bonds are very strong. So the atoms in each molecule are held tightly together. However, each molecule tends to be quite separate from its neighbouring molecules.

·         Covalent compounds have a very small attraction between the individual molecules. We say they are weak intermolecular forces between molecules. Overcoming these forces does not take much energy.

·         Substances with simple molecules will not conduct electricity. This is because there is no overall charge in the substance and this makes it impossible for simple molecules to conduct electricity. 

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Giant covalent structures

·         Giant covalent substances such as diamond; are very hard, have a high melting and boiling point and they are chemically unreactive.

·         Carbon isn’t always diamond

·         For example, we can get graphite. Graphite is carbon atoms arranged in giant layers. There are also very weak intermolecular forces between the layers therefore they can slide over each other easily.

·         The carbon atoms in graphite’s giant layers are arranged in hexagons. This means that each carbon atom bonds to three others. This leaves one spare outer electron on each carbon atom. This is the electron that becomes delocalised along the layers of carbon atoms.

·         The free electrons in its structure allow it to conduct electricity which diamond cannot.

·         Giant covalent structures containing cage structures are called fullerenes. 

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Giant metallic structures

·         We can hammer and bend metals into shapes because the layers of atoms in a pure metal are able to slide easily over each other.

·         Metals are good conductors of electricity because they have a sea of delocalised electrons which can carry the charge around the whole lattice.

·         Because the delocalised electrons can move throughout the lattice, they hold the ions together and at the same time they enable the lattice to distort so that the metal atoms can move past one another.

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