Chemistry

A covalent bond is formed between non metal atoms, which combine together by sharing electrons. Covalent compounds have no free electrons and no ions so they don't conduct electricity.

The Periodic Table is an arrangement of the elements in order of atomic number. Elements in the same vertical column are in the same group or family and have similar chemical properties.

Non metals combine together by sharing electrons. The shared pair of electrons holds the two atoms together. It's called a covalent bond. The group of atoms bonded together in this way is called a molecule.

The types and numbers of atoms in a molecule are shown in its formula.

NameStructureModel Hydrogen (H₂) Water (H₂O) Ammonia (NH₃) Methane (CH₄)

Metals form giant structures in which electrons in the outer shells of the metal atoms are free to move. The metallic bond is the force of attraction between these free electrons and metal ions. Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points.

Metals are good conductors of electricity and heat, because the free electrons carry a charge or heat energy through the metal. The free electrons allow metal atoms to slide over each other, so metals are malleable and ductile

Ionic bonding occurs between positive and negative ions, which attract each other and bind together to form ionic compounds. For example, sodium chloride consists of Na⁺ ions and Cl^- ions bound together.

Each ion is surrounded by oppositely charged ions held in place by electrostatic attraction and forming an ionic crystal lattice. The ions in a crystal lattice are very strongly bonded - a high temperature is required to melt the crystal.

There are several ways in which atoms chemically combine together to make compounds. One of these ways is called ionic bonding. Atoms turn into ions when they lose or gain electrons.

Metal ions

In some circumstances metal atoms may lose electrons. The atom then has more protons than electrons and so it will be positively charged, a positive ion.

Example: A magnesium atom may lose two electrons and become a Mg²⁺ ion.

Non-metal ions

Non-metal atoms may gain electrons and become negatively charged.

Example: An oxygen atom may gain two electrons and become an O^2- ion.

Simple molecular substances consist of molecules in which the atoms are joined by strong covalent bonds. However, the molecules are held together by weak forces so these substances have low melting and boiling points. They do not conduct electricity.

Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.

Properties of simple molecular substances

• Low melting and boiling points - This is because the weak intermolecular forces break down easily.
• Non-conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge.