Key concepts in chemistry Revision Notes

Covalent Bonding Basics

Covalent bonds form between non-metal atoms when they share pairs of electrons. This sharing allows each atom to achieve a stable outer electron shell, similar to that of noble gases. The shared electrons are attracted to the nuclei of both atoms, holding the molecule together. These bonds are strong within the molecule.

Representing Covalent Bonds with Dot & Cross Diagrams

Dot and cross diagrams illustrate the outer shell electrons of atoms in a molecule. Dots represent electrons from one atom, and crosses represent electrons from another. Shared electron pairs are placed in the overlapping regions of the atomic shells. Unshared (lone) pairs of electrons are also shown in the outer shells of the atoms. Understanding these diagrams helps visualise the electron arrangements in molecules like H₂, HCl, H₂O, CH₄, O₂, and CO₂.

Examples of Simple Molecular Structures

Hydrogen (H₂): Each H atom shares 1 electron, forming 1 single bond. Hydrogen Chloride (HCl): H shares 1 with Cl, Cl shares 1 with H, forming 1 single bond. Water (H₂O): O shares 2 electrons (one with each H), and each H shares 1 with O, forming 2 single bonds. Methane (CH₄): C shares 4 electrons (one with each H), and each H shares 1 with C, forming 4 single bonds. Oxygen (O₂): Each O atom shares 2 electrons, forming 1 double bond. Carbon Dioxide (CO₂): C shares 4 electrons (2 with each O), and each O shares 2 with C, forming 2 double bonds.

Ionic Structures and Properties

Ionic compounds are made of giant ionic lattices, where positive and negative ions are held together by strong electrostatic forces. This strong attraction necessitates a large amount of energy to overcome, resulting in high melting and boiling points. They are typically soluble in water. In solid form, the ions are fixed in position, so they cannot conduct electricity. However, when molten or dissolved in water, the ions become mobile and can carry charge, allowing electrical conductivity.

Simple Molecular Structures and Properties

Simple molecular substances consist of discrete molecules with strong covalent bonds *within* each molecule but weak intermolecular forces *between* molecules. Only a small amount of energy is needed to overcome these weak intermolecular forces, leading to low melting and boiling points. Most are insoluble or sparingly soluble in water and do not conduct electricity because they have no free charged particles.

Giant Covalent Structures and Properties

Giant covalent structures, such as diamond or silicon dioxide, feature a continuous network of atoms held together by strong covalent bonds. These strong bonds throughout the entire structure require immense energy to break, leading to very high melting and boiling points. They are generally insoluble in water and, with the exception of graphite, do not conduct electricity as they lack delocalised electrons or mobile ions.

Metallic Structures and Properties

Metals have a giant structure of positive metal ions arranged in a lattice, surrounded by a 'sea' of delocalised electrons. This metallic bonding is strong, giving metals high melting and boiling points. The delocalised electrons are free to move throughout the structure, making metals excellent conductors of electricity and heat in both solid and molten states. Metals are generally insoluble in water unless they react chemically, such as alkali metals.

High Melting and Boiling Points

Ionic compounds have high melting and boiling points due to the strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is needed to overcome these forces and break down the giant ionic lattice structure.

Electrical Conductivity

Ionic compounds do not conduct electricity when solid because their ions are fixed in the lattice and cannot move. However, they do conduct electricity when molten or in aqueous solution. In these states, the ions are free to move and carry charge, allowing for electrical conduction.

Structure of Ionic Compounds

Ionic compounds form a crystal lattice structure where positive and negative ions are arranged alternately. This arrangement maximises attractive forces and minimises repulsive forces between ions.

Low Melting and Boiling Points

Simple molecular compounds have strong covalent bonds *within* their molecules, but weak intermolecular forces *between* molecules. Little energy is needed to overcome these weak intermolecular forces, resulting in low melting and boiling points.

Poor Electrical Conductivity

Simple molecular compounds do not conduct electricity because they lack free-moving charged particles. All electrons are involved in covalent bonding, and there are no delocalised electrons or mobile ions available to carry an electrical charge.