Key concepts in chemistry Study Guide

The way atoms are bonded together and the overall arrangement of these particles—their structure—fundamentally determines the physical properties of a substance. Ionic compounds, formed between metals and non-metals, consist of a giant lattice of charged ions held by strong electrostatic forces. This leads to high melting points and the ability to conduct electricity only when molten or in solution, as ions become mobile. Simple molecular substances, like water, are composed of discrete molecules with strong covalent bonds within the molecules but weak intermolecular forces between them. These weak forces result in low melting and boiling points, and they generally do not conduct electricity. Giant covalent structures, such as diamond or silicon dioxide, involve a vast network of covalently bonded atoms, giving them exceptionally high melting points and hardness, and generally making them non-conductors. Metals feature a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons, explaining their excellent electrical conductivity (both solid and molten) and malleability, with relatively high melting points due to strong metallic bonding.

Ionic compounds are formed when atoms transfer electrons, creating positively and negatively charged ions. These oppositely charged ions are held together by strong electrostatic forces of attraction, forming a giant ionic lattice structure. Because these electrostatic forces are very strong and act in all directions within the lattice, a significant amount of thermal energy is required to overcome them. This energy input is needed to separate the ions, leading to the characteristic high melting and boiling points of ionic compounds. In the solid state, the ions are fixed in their positions within the lattice and cannot move; therefore, solid ionic compounds do not conduct electricity. However, when ionic compounds are molten or dissolved in water, the ions become mobile and are free to move. This mobility of charged particles (ions) enables the substance to conduct electricity in these states.