Chemistry Notes on Covalent Bonding Small Molecules A covalent bond is a shared pair of electrons. Covalent bonding forms molecules. Substances with small molecules have low melting and boiling points, and do not conduct electricity. Covalent bonds Forming a covalent bond A covalent bond is formed when two atoms share a pair of electrons. Covalent bonding occurs in most nonmetal elements, and in compounds formed between non-metals. These shared electrons are found in the outer shells of the atoms. Usually each atom contributes one electron to the shared pair of electrons. Below is a covalent bond forms between a hydrogen atom and a chlorine atom, making hydrogen chloride. The hydrogen atom has bonded with the chlorine atom, meaning there is now a shared pair of electrons Molecules Most covalently bonded substances consist of small molecules. A molecule is a group of two or more atoms joined together by covalent bonds. Molecules of the same element or compound always contain the same number of atoms of each element. The atoms in a molecule are always joined together by a covalent bond. Substances that are made up of ions do not form molecules. Sizes of atoms and simple molecules A small molecule contains only a few atoms, so atoms and small molecules have a similar range of sizes. They are very small, typically around 0.1 nm or 1 × 10-10 m across. Individual atoms and molecules are too small to see even with the most powerful light microscope. Some electron microscopes can produce images of atoms and simple molecules. A water molecule, H2O, is about 0.3 nm (3 × 10-10 m) across: The chemical formula of a substance with small molecules shows the number of atoms of each element in one molecule of the substance. Substance Formula Number of atoms Hydrogen H2 Two hydrogen atoms Oxygen O2 Two oxygen atoms Nitrogen N2 Two nitrogen atoms Hydrogen chloride HCl One hydrogen atom and one chlorine atom Water H2O Two hydrogen atoms and one oxygen atom Ammonia NH3 One nitrogen atom and three hydrogen atoms Modelling molecules Dot and cross diagrams A dot and cross diagram can show the bonding in a small molecule: ● the outer shell of each atom is drawn as a circle ● circles overlap where there is a covalent bond ● electrons from one atom are drawn as dots, and electrons from another atom as crosses For example, a hydrogen molecule, H2, forms when two hydrogen atoms each share their outer electron. A dot and cross diagram to show the bonding in hydrogen An ammonia molecule, NH3, forms when one nitrogen atom shares its outer electrons with three hydrogen atoms. There are two types of dot and cross diagram - one without circles, and one with. These dot and cross diagrams show the bonding in ammonia Structural formulae A structural formula shows the bonds between the atoms in a molecule. In these formulae: ● each atom is shown by its chemical symbol ● each covalent bond is shown as a straight line For example, an ammonia molecule has three covalent bonds. The diagram shows its structure. Three-dimensional structures The structure of a small molecule can also be shown as a three-dimensional ball and stick model. These models show how the atoms and bonds are arranged in space. The diagram shows a ball and stick model of ammonia. Drawing dot and cross diagrams How many bonds? Atoms form covalent bonds by sharing electrons to get a full outer shell. This means that the number of covalent bonds an atom can form is the same as the number of electrons needed to get a full outer shell. For most elements a full outer shell is eight electrons. The table below shows the number of bonds formed by elements in groups 4 to 7. Group Example Electrons Covalent bonds 4 Carbon, C 4 4 5 Nitrogen, N 5 3 6 Oxygen, O 6 2 7 Chlorine, Cl 7 1 Hydrogen atoms only have one electron and form one covalent bond as they only need one more for a full outer shell. How many circles? To work out how many circles to draw for a simple molecular substance and how to label them, look at the formula. For example, the formula for ammonia is NH3. For this, draw four circles, one labelled N and three labelled H. Each of the three H circles overlaps the N circle. It is often easiest to draw circles at 90° or 180° to each other This incomplete dot and cross diagram shows only the bonding pairs of electrons Nitrogen is in group 5 so it forms three covalent bonds. There are three shared spaces between the circles, so add a dot and cross to each one. Finally, add in the non-bonding outer electrons. Nitrogen atoms have five outer electrons. Three of these are shared, which leaves two electrons that do not take part in bonding. Interpreting dot and cross diagrams Dot and cross diagrams show how electrons are shared in covalent bonds. The table shows dot and cross diagrams for an element, chlorine, and two compounds. Double and triple bonds Some molecules contain a double bond, which consists of two shared pairs of electrons. For example, oxygen molecules consist of two oxygen atoms joined together. Oxygen atoms can form two covalent bonds, so to link the two oxygen atoms together, a double bond forms between them. A nitrogen molecule is made up of two nitrogen atoms joined together. Nitrogen atoms can form three covalent bonds, so a triple bond forms between them. The structure of nitrogen is N≡N, showing that it has three shared pairs of electrons. Properties of substances with small molecules A substance with small molecules has strong covalent bonds that hold the atoms together in its molecules. There are weak forces between molecules and their neighbours. The structure and bonding of substances with small molecules explains their properties. Low melting points and boiling points Most substances with simple molecules have low melting points and boiling points. Many are in the liquid or gas state at room temperature. Melting and boiling Melting and boiling are changes of state. Energy must be transferred to a substance to make it melt or boil. This energy overcomes the attractive forces between the particles in the substance: ● some forces of attraction are overcome during melting, allowing molecules to move over each other ● more of the forces of attraction are overcome during boiling, allowing the molecules to move freely away from each other The more energy that is needed, the higher the melting point or boiling point. Explanation There are intermolecular forces between small molecules. Intermolecular forces are much weaker than the strong covalent bonds in molecules. When small molecular substances melt or boil, it is these weak intermolecular forces that are overcome. The covalent bonds are not broken. Relatively little energy is needed to overcome the intermolecular forces, so small molecular substances have low melting and boiling points. Substance Melting point Boiling point State at 20°C O2 -218°C -183°C Gas H2O 0°C 100°C Liquid The intermolecular forces between water molecules are stronger than those between oxygen molecules. In general, the bigger the molecule, the stronger the intermolecular forces, so the higher the melting and boiling points. Conduction of electricity A substance can conduct electricity if: ● it contains charged particles, and ● these particles are free to move from place to place Small molecules have no overall electric charge, so they cannot conduct electricity, even when liquid or dissolved in water. When simple molecular substances melt or boil, their weak intermolecular forces are overcome, not their strong covalent bonds.
