Elements, compounds and mixtures Study Guide

The way a material behaves – its melting point, electrical conductivity, strength, and more – is directly linked to the type of chemical bonds present and how those bonds are arranged. Ionic compounds, like table salt (sodium chloride), are formed between metals and non-metals through the transfer of electrons, creating strong electrostatic forces between positive and negative ions arranged in a giant lattice. This strong bonding means they have high melting and boiling points and are typically hard. They only conduct electricity when molten or dissolved because their ions are then free to move. Simple molecular substances, such as water or oxygen, are made of discrete molecules with strong covalent bonds within each molecule, but weak intermolecular forces between molecules. These weak forces require little energy to overcome, resulting in low melting and boiling points, and they do not conduct electricity. Giant covalent structures, like diamond or silicon dioxide, consist of vast networks of atoms held together by strong covalent bonds throughout the entire structure. This continuous strong bonding leads to very high melting points, extreme hardness, and lack of electrical conductivity (except for graphite, which has delocalised electrons). Polymers are large molecules made of repeating units (monomers) joined by strong covalent bonds. The bulk properties depend on the strength of intermolecular forces between polymer chains. Thermosoftening polymers have weak intermolecular forces, making them melt and soften when heated. Thermosetting polymers have strong covalent cross-links between chains, making them rigid and preventing them from melting. Finally, metals are composed of positive metal ions arranged in a regular lattice surrounded by a 'sea' of delocalised electrons. This metallic bonding is strong, giving metals high melting points and making them good conductors of heat and electricity because the delocalised electrons can move freely throughout the structure.

Carbon is a unique element due to its ability to form four covalent bonds. This characteristic is critical in allowing carbon to form a vast array of compounds, including the complex molecules found in living organisms. A covalent bond forms when two atoms share a pair of electrons, typically when both atoms need to achieve a stable electron configuration, like a full outer shell. Carbon's atomic structure includes four electrons in its outermost shell. To achieve a stable electron arrangement (a noble gas configuration, such as that of neon), carbon atoms need to gain four more electrons. They achieve this by sharing their four outer electrons with electrons from four other atoms. This sharing results in four strong covalent bonds. These bonds can be formed with other carbon atoms, creating long chains, branched structures, and rings, or with atoms of other elements like hydrogen, oxygen, nitrogen, and sulfur. The ability to form four stable covalent bonds means carbon atoms can link together in many different ways, leading to the diversity and stability of organic compounds.