Chemistry Unit 2

ATOMS, MOLECULES AND IONS

Chemical Bonding

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

                Copper Sulphate is made of Cu²⁺ and SO₄^2- 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).

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:

Particle

Mass

Charge

PROTON   

1

+1

NEUTRON            

1

0

ELECTRON

Very Small

-1

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).

·         Atomic Number = Number of Protons

·         Number of Protons = Number of Electrons

·         Mass of Atom = Protons + Neutrons

Electronic 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)

C2.1 STRUCTURE AND BONDING

Ionic and Covalent Bonding

Noble gases have 2 things in common:

·         They are very unreactive (stable)

·         They have complete outer shells of electrons.

Other elements have a tendency to change their electronic structures to become more like noble gases and therefore…