Structure and bonding


Chemical bonding

Elements react to form compounds by gaining or losing electrons.

Atoms of metals in Group 1 combine with non-metals in Group 7 by transferring electrons to form ions that have electronic structures of noble gases.

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Ionic bonding

Ionic compounds are held together by strong forces between the oppositely charged ions. This is called ionic bonding. 

The ions form a giant structure or lattice. The strong electrostatic forces of attraction act throughout the lattice. 

Each ion in the giant structure or lattice is surrounded by ions with the opposite charge and so is held firmly in place.

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Formulae of ionic compounds

The charges of ions in an ionic compound always cancel each other out. Ionic compounds are neutral.

The formula of an ionic compound shows the ratio of ions to present in the compound.

Sometimes we need brackets to show the ratio of ions in a compound eg. magnesium hydroxide, (Mg(OH)2.

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Covalent bonding

A covalent bond is formed when two atoms share a pair of electrons. Substances that have atoms held together by covalent bonding are called molecules.

The number of covalent bonds an atom forms depends on the number of electrons it needs to achieve a stable electronic structure.

Many substances containing covalent bonds consist of simple molecules, but some have giant covalent structures.

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The atoms in a metal are closely packed together and arranged in regular layers. 

When metal atoms pack together the electrons in the highest energy level (outer shell) are delocalised. This produces a lattice of positive ions in a 'sea' of moving electrons.

The strong electrostatic forces between these electrons and the positively charged metal ions hold the metal together.

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Giant ionic structures

Ionic compounds have high melting points and they are all solids at room temperature.

Ionic solids have high melting points because a lot of energy is needed to overcome the ionic bonds. 

When an ionic compound is melted, the ions are free to move. This allows them to carry electrical charge, so the liquids conduct electricity. 

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Simple molecules

Substances made up of simple molecules have low melting points and boiling points.

Simple molecules have no overall charge, so they cannoot carry electrical charge and do not conduct electricity.

The weak intermolecular forces between simple molecules are why substances made of simple molecules have low melting points and boiling points.

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Giant covalent structures

Covalently bonded substances with giant structures have very high melting points.

Diamond is a form of carbon whose atoms each form four covalent bonds.
Graphite is another form of carbon where the carbon atoms form layers have no bonds so they can slide over each other. Graphite can conduct electricity because of the delocalised electrons in its structure.

Carbon also exists as fullerenes

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Giant metallic structures

When we bend and shape metals the layers of atoms in the giant metallic structure slide over each other.

Alloys are mixtures of metals and are harder than pure metals because the layers in the structure are disorted.

Alloys are more useful than pure metals as they are harder and they can be made to have special properties, such as the shape memory alloys.

Delocalised electrons in metalds enable metals to conduct heat and electricity well.

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The properties of polymers

The properties of polymers depend on the monomers used to make them.

Changing reaction conditions can also change the properties of the polymer that is produced

Thermosoftening polymers soften/melt easily when heated.

Thermosetting plastics do not soften or melt when heated because the strong covalent bonds form cross-links between their polymer chain.

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Nanoscience is the study of small particles that are between 1 and 100 nanometers in size.

A nanoparticle is a very small particle that is a few nanometers in size and made of a few hundred atoms. 

Nanoparticles behave differently than the bulk materials they are made from. 

Their very small sizes give them large surface areas and new properties that can make them very useful materials.

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