Chemistry revision notes. Atomic Structure and Bonding, Electrolysis, Acids and Alkalis.

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Atomic Structure and Bonding (F)

Atoms, Molecules and Ions.

AN ATOM is the smallest particle of an element. They cannot be split into smaller particles in chemical reactions. Iron is made of iron atoms (Fe). Sulphur is made of sulphur atoms (S)

A MOLECULE is a small group of atoms joined together.

 The atoms may be the same (e.g. O2) or different (e.g. H2O). The chemical formula shows the number and type of atoms present. Non-metal compounds are made of molecules:

        Carbon dioxide contains CO2 molecules

        Methane (natural gas) contains CH4 molecules

AN ION is an atom or group of atoms with an electrical charge (+ or -). Metal compounds such as sodium chloride or copper sulphate contain ions.

        Sodium chloride is made of Na+ and Cl- ions

        Magnesium Oxide is made of Mg2+ and O2- ions

        Note that metals form positive ions while non-metals form negative ions.

        A solid is represented by (s). e.g. H2O(s) is ice.

        A liquid is represented by (l) e.g. Fe(l) is molten iron.

        A gas is represented by (g) e.g. H2O(g) is steam.

        A solution in water is represented by (aq). Salt dissolved in water is NaCl(aq).

You should remember that the common gases are diatomic (have 2 atoms in each molecule). These are Oxygen O2; Hydrogen H2; Nitrogen N2; and Chlorine Cl2.

Elementary Particles

Atoms are made up of smaller particles called protons, neutrons and electrons.

The protons and neutrons cluster together in a small nucleus at the centre of the atom while the electrons orbit the nucleus.

The main properties of the particles are:


        Every element has an atomic number, which is the number of protons in the nucleus.

        Atoms are neutral, so that the number of electrons is the same as the number of protons.

The mass of an atom is almost entirely made up of protons and neutrons, which have the same mass as each other (the mass of electrons can be ignored).

        These relations can be summarised as:





Electron Structure

The electrons orbit the nucleus in 'shells'. These can hold the following numbers of electrons:

        The innermost shell can contain up to 2 electrons

        The next shell can contain up to 8 electrons

        The next shell can contain up to 8 electrons (although this can be expanded up to 18)

Fluorine has atomic number 9. Its electronic structure is 2.7

Sodium has atomic number 11. Its electronic structure is 2.8.1

Calcium has atomic number 20. Its electronic structure is

Ionic and Covalent Bonding

Elements have a tendency to change their electronic structures to become more like noble gases and therefore more stable. They can do this by losing, gaining or sharing electrons.

Ionic Bonding

1. Ionic bonding takes place between a metal and a non-metal.

2. The metal atoms lose their outer shell electrons to form positive ions.

3. The non-metal atoms gain these electrons to fill their outer shells and form negative ions

4. The positive and negative ions attract each other in a lattice

Covalent Bonding

1. Covalent bonds are formed between 2 non-metallic elements.

2. The atoms share electrons in order to complete their outer shells.

3. The atoms all attain noble gas structure (complete outer shells).

4. The new particles formed are neutral molecules.

Structures of Substances

        There are 4 main structures which substances can have. These are known as:


Simple Molecular.

        Simple molecular substances have small molecules, such as H2O or CO2 . The atoms in these molecules are held together by strong forces called covalent bonds, but there are only very weak forces between the molecules. This means:

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                1) They have low melting and boiling points (many are liquids or gases).

                2) They tend to be soft and/or have little strength.

                3) They do not conduct electricity

        Simple molecular substances are nearly always non-metallic elements or their compounds.

        e.g. Water (H2O), Iodine (I2), Carbon dioxide (CO2), Sugar (C12H22O11).

Giant Covalent Substances

        In these substances, strong covalent bonds join atoms together in large numbers to make giant structures. Sand (silicon dioxide), diamond and graphite (both forms of carbon) are examples.

        Diamond has a 3 dimensional structure where each carbon is bonded to 4 others. Diamond is extremely ...

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