Structure, bonding and the properties of matter | Year 9 Chemistry (Cambridge) PDF Download

Chemical Bonds

Chemical bonds are interactions that hold atoms together in molecules or compounds.
There are three main types of chemical bonds:

• Ionic Bonds: Formed between ions of opposite charges. For example, sodium chloride (NaCl) is held together by ionic bonds where sodium (Na⁺) and chloride (Cl^-) ions attract each other.
• Covalent Bonds: Formed by sharing electrons between atoms. For instance, in water (H₂O), each hydrogen atom shares electrons with the oxygen atom to form covalent bonds.
• Metallic Bonds: Found in metals where electrons are shared among a lattice of metal atoms, giving metals their unique properties like conductivity and malleability.

Simple and Giant Structures

Materials can have simple or giant structures depending on their atomic arrangement:

• Simple Molecular Structures: Consist of small molecules held together by weak intermolecular forces. Examples include water (H₂O) and oxygen (O₂), which have low melting and boiling points due to weak intermolecular forces.
• Giant Covalent Structures: Made of large networks of atoms bonded by strong covalent bonds throughout the structure. Examples include diamond (carbon atoms in a tetrahedral lattice) and graphite (layers of carbon atoms in hexagonal rings), which have high melting points due to strong covalent bonds.
• Metallic Structures: Feature a lattice of metal cations surrounded by a 'sea' of delocalized electrons. This structure accounts for properties such as conductivity and ductility in metals.

Density

Density is a measure of how tightly packed the particles of a substance are:

• Definition: Density (ρ) is calculated as mass per unit volume
• Effect of Structure: Materials with compact atomic arrangements (e.g., metals) typically have higher densities compared to materials with more open structures (e.g., gases).
• Applications: Density is crucial in various applications such as determining the purity of substances, understanding buoyancy (e.g., why objects float or sink in fluids), and engineering materials with specific properties.

Examples

• Ionic Bond Example: Sodium chloride (NaCl) forms when sodium (Na⁺) ions and chloride (Cl^-) ions attract each other due to their opposite charges.
• Covalent Bond Example: Water (H₂O) molecules form when each hydrogen atom shares electrons with the oxygen atom to complete its outer electron shell.
• Giant Covalent Structure Example: Diamond consists of carbon atoms bonded in a tetrahedral lattice structure through strong covalent bonds, resulting in its hardness and high melting point.
• Density Example: Gold has a high density (19.3 g/cm³) due to its tightly packed atomic structure, making it useful in jewelry and electronics.