GCSE Chemistry – Chemical Bonding

Ionic Bond

  • Bonding between metals and non-metals
  • Involves losing and gaining of electrons in order to get full outer shells like noble gases

E.g. Sodium chloride. Sodium 2,8,1 loses one electron (easier to lose one electron than to gain seven), chlorine gains one electron to get a full outer shell. Chlorine 2,8,7 gains one electron from the sodium to get a full outer shell.

  • When an atom loses an electron they are left a positive charge (Na+).
  • When an atom gains an electron they are left with a negative charge (Cl-).
  • During chemical bonding ions are formed.
  • An ionic bond is an electrostatic force of attraction between oppositely charged ions.
  • There is a quick way to work out what the charge on an ion should be:

the number of charges on an ion formed by a metal is equal to the group number of the metal

the number of charges on an ion formed by a non-metal is equal to the group number minus eight

E.g. Magnesium forms Mg2+ ions, Oxygen forms O2- ions

Ionic bonding can be represented by dot and cross diagrams

 

Covalent Bond

  • Bonding between non-metals and non-metals
  • The atoms share electrons in order to get full outer shells.
  • The new particles formed are neutral molecules.

1) Simple Molecular Structures

Small molecules containing few atoms (ee..gg.. H 2 O,, CO 2 )

Strong covalent bonds between atoms

Forces between molecules are fairly weak (weak intermolecular forces))

Typical Properties:

Low melting point and boiling point because forces between molecules are weak

Do not conduct electricity

2) Giant Ionic Structures

Ions held together by strong ionic bonds so have high melting points

Ions are not free to move in solid so do not conduct electricity

Ionic compounds conduct electricity when heated to become molten or dissolved in water because ions are free to move

3) Giant Covalent Structures

Large network of bonds – giant covalent Substances such as: diamond, graphite and silicon dioxide

Held together by very strong covalent bonds

High melting and boiling points because they have strong covalent bonds.

Unreactive chemically

Graphite

has free delocalised electrons so conducts electricity.

Fullerenes: carbon’s ability to make large cage like structures. Important in nanoscience and industry

Diamond

Carbon based

Each C joins to 4 others VERY hard No electrical conductivity because no free mobile electrons

Graphite Carbon based Each C joins to 3 others 1 free electron per carbon Soft and slippery because layers can slide over each other Good electrical conductor because there are free delocalised mobile electrons Silicon dioxide Each silicon joins to 4oxygens Each oxygen joined to 2 silicon

4) Giant Metallic Structures

The atoms in metals are in layers which can slide over each other, this makes it possible to bend them or beat them into shape.

The atoms in metals share their outer electrons with all the other metal atoms, so that a metal consists of positive ions surrounded by a sea of delocalised electrons. The outer shell electrons are free to move throughout the structure.

The forces of attraction between the positive ions and outer shell electrons(called metallic bonds)are very strong.

The main properties of metals are:

  • Metals are strong because of strong metallic bonding
  • Most metals have high melting points because of strong metallic bonding
  • Metals are malleable (they can be bent of beaten into different shapes) because metal atoms can slide over each other easily
  • Metals are good conductors of electricity because the delocalised electrons are free to move 5) Metals are lustrous (shiny)

Alloys

An alloy is a mixture of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties which are usually different from those of its components.

Pure metals have atoms arranged in a regular pattern and the layers of atoms can easily slide over one another when a force is applied making them too soft for use in construction.

Alloys are harder and stronger than pure metals Alloys are mixtures of metals. They are stronger than pure metals.

SMART ALLOYS

Some alloys have a special property.. If we bend them into a different shape or heat them,, they return to their original shape by themselves – these are called smart or shape memory alloys.

Smart alloys can change shape because of the way the atoms re-arrange into different forms at different temperatures

e.g. Thermostat in kettles: (when it starts to boil steam makes the alloy change shape and switch off the current)