The five linked pages introduce to the concept of a chemical bond and why atoms bond together, types of chemical bonds and which electron arrangements are particularly stable leading to stable chemical bonds. Through the use of dot and cross electronic diagrams is described and there are detailed notes on ionic bonding i.e. the mutual attraction of oppositely charged ions to give ionic bonds and the properties of ionic compounds, covalent bonds and the formation of small simple molecules and their properties, macromolecules like polymers and giant covalent structures like diamond, graphite and silica. Finally metallic bonding is described to explain the structure and physical properties of metals

When different elements (different types of atom) react and combine to form a compound (new substance) chemical bonds must be formed to keep the atoms together. Once these atoms are joined together its usually difficult to separate them.

The atoms can join together by sharing electrons in what is known as a covalent bond.

Or, they can transfer or accept electrons to form positive and negative ions and form an ionic bond.

These concepts are explained in more detail below with links to even more detailed notes with lots of examples.

Introduction to some important definitions in Chemistry eg atom, molecule, formula, element, compound etc. are all explained with examples.

Also see notes on How to write word & symbol equations, work out formula and name compounds formed by ionic or covalent bonding

Some atoms are very reluctant to combine with other atoms and exist in the air around us as single atoms. These are the Noble Gases and have very stable electron arrangements e.g. 2, 2,8 and 2,8,8 because their outer shells are full. The first three are shown in the diagrams below and explains why Noble Gases are so reluctant to form compounds with other elements.

All other atoms therefore, bond together to become electronically more stable, that is to become like Noble Gases in electron arrangement. Bonding produces new substances and usually involves only the 'outer shell' or 'valency' electrons and atoms can bond in two ways.

The phrase CHEMICAL BOND refers to the strong electrical force of attraction between the atoms or ions in the structure. The combining power of an atom is sometimes referred to as its valency and its value is linked to the number of outer electrons of the original uncombined atom (see examples later).

(a) IONIC BONDING – an ionic bond is formed by one atom transferring electrons to another atom to form oppositely charged particles called ions which attract each other – the ionic bond.

• An ion is an atom or group of atoms carrying an overall positive or negative charge

  • e.g. Na⁺, Cl^–, [Cu(H₂O)]²⁺, SO₄^2– etc.

• If a particle, as in a neutral atom, has equal numbers of protons (+) and electrons (–) the particle charge is zero i.e. no overall electric charge.

• The proton/atomic number in an atom does not change BUT the number of associated electrons can!

• If negative electrons are lost the excess charge from the protons produces an overall positive ion.

• If negative electrons are gained there is an excess of negative charge, so a negative ion is formed.

• The charge on the ion is numerically related to the number of electrons transferred i.e. electrons lost or gained.

• For any atom or group of atoms, for every electron gained you get a one unit increase in negative charge on the ion, for every electron lost you get a one unit increase in the positive charge on the ion.

• The atom losing electrons forms a positive ion (cation) and is usually a metal.

• The atom gaining electrons forms a negative ion (anion) and is usually a non–metallic element.

• The ionic bond then consists of the attractive force between the positive and negative ions in the structure.

• The ionic bonding forces act in all directions around a particular ion, it is not directional, as in the case of covalent bonding.

•  The sodium atom transfers an electron to the chlorine atom in forming sodium chloride

• For more detailed notes on this example and other examples see Ionic Bonding – compounds and properties

• This kind of bond or electronic linkage does act in a particular direction i.e. along the 'line' between the two nuclei of the atoms bonded together, this is why molecules have a particular shape.

• Hydrogen and oxygen atoms share electrons to form covalent O–H bonds to form water

• which has a 'bent' shape

  • More detailed notes on this example and other examples see the

    • Revision notes  Covalent Bonding – small simple molecules and properties

  • Covalently bonded structures can lead to the formation of some very BIG molecules

    • See Covalent Bonding – macromolecules and giant covalent structures

  • Advanced Level Chemistry notes on the shapes of molecules

Between all particles, but with particular reference to covalently bonded molecules, there always exists some very weak electrical attractive forces known as intermolecular forces or intermolecular bonding.

These constantly acting attractive forces or intermolecular bonds are very much weaker than covalent or ionic chemical bonds (approximately ¹/₃₀ to ¹/₂₀th in comparative attractive force).

For example, although the oxygen and hydrogen atoms are very strongly bonded in water to make a VERY stable molecule, BUT this does NOT account for the existence of liquid water and ice!

It is the weak intermolecular forces that induces condensation below 100^oC and freezing–solidification to form ice crystals below 0^oC.

In the reverse process, when ice is warmed, the intermolecular forces are weakened and at 0^oC the intermolecular bonds are weakened enough to allow melting to take place.

Above 0^oC (evaporation), and particularly at 100^oC (boiling), the intermolecular forces are weak enough for 'intact water molecules' to escape from the surface of the liquid water.

It is VERY important to realise that the chemical hydrogen–oxygen covalent bonds (O–H) in water are NOT broken and the state changes ...

solid <== freezing/melting ==> liquid <== condensing/boiling ==> gas ...

are due to the weakening of the intermolecular forces/bonds with increase in temperature OR the strengthening of the intermolecular bonds/forces decrease in temperature.

For more details see Covalent Bonding – small simple molecules and properties

As explained at the start of Part 1, NOBLE GASES are very reluctant to share, gain or lose electrons to form a chemical bond ie they do NOT readily form a covalent or ionic bond with other atoms. They are already electronically very stable. For most other elements the types of bonding and the resulting properties of the elements or compounds are described in detail in Parts 2 to 5. In all the electronic diagrams ONLY the outer electrons are shown.

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