1.5 - Compounds, Molecules & Mixtures
Definition of molecules
A molecule forms when two or more atoms join together through chemical bonds. These bonds hold the atoms in a stable arrangement, creating a single unit that behaves as one particle. Molecules can consist of atoms from the same element or different elements, and they represent the smallest unit of many substances that can exist independently.
Key examples of molecules:
- Diatomic elements - These are molecules made of two atoms of the same element, such as O2 (oxygen gas), where two oxygen atoms bond together
- Simple molecules - Examples include H2O (water), which has two hydrogen atoms bonded to one oxygen atom
- Larger molecules - These might involve more atoms, like CO2 (carbon dioxide), with one carbon atom bonded to two oxygen atoms
This bonding distinguishes molecules from loose collections of atoms, as the atoms in a molecule share electrons or form other types of chemical connections.
Definition of compounds
A compound is a substance made up of two or more different elements chemically combined in fixed proportions. This means the elements in a compound always join in the same ratio, giving the compound consistent properties that differ from those of its individual elements. Compounds have specific chemical formulae that show this fixed composition.
Characteristics of compounds:
- Fixed proportions - For example, water (H2O) always has two hydrogen atoms for every one oxygen atom
- Consistent formulae - Each compound has a unique formula that represents its composition, such as NaCl for sodium chloride (table salt)
- Chemical bonding - The elements are held together by chemical bonds, making compounds pure substances that cannot be separated by physical means alone
Compounds form through chemical reactions, and their properties, like melting point or reactivity, are different from the elements they contain.
Writing chemical formulae using element symbols and subscripts
Chemical formulae provide a shorthand way to represent the composition of molecules and compounds. They use element symbols from the periodic table combined with subscript numbers to show how many atoms of each element are present. A subscript is a small number written below and to the right of an element symbol, indicating the count of that atom in the formula.
Rules for writing basic chemical formulae:
- Element symbols - Use the standard one- or two-letter symbols, such as H for hydrogen or Na for sodium
- Subscript numbers - These show the number of atoms; for example, H2 means two hydrogen atoms. If no subscript appears, it implies one atom (like the O in H2O)
- Order of elements - Typically, the less electronegative element comes first, followed by the more electronegative one, but the key is accuracy in atom counts
For instance, the formula for carbon dioxide is CO2, showing one carbon atom and two oxygen atoms bonded together.
Interpreting brackets in formulae for polyatomic groups
In more complex formulae, brackets group together polyatomic ions or groups, which are clusters of atoms that act as a single unit. These brackets help show how many of these groups are present in the compound, especially when combined with subscript numbers outside the brackets.
How to interpret brackets:
- Polyatomic groups - These are sets of atoms bonded together that carry a charge or function as a unit, such as the hydroxide group (OH)
- Brackets with subscripts - For example, in Al(OH)3, the brackets around OH indicate the hydroxide group, and the subscript 3 means there are three of these groups attached to one aluminum atom
- Overall formula meaning - This results in a total of one Al, three O, and three H atoms, maintaining the compound's fixed proportions
Brackets ensure clarity when a compound contains repeating polyatomic units, preventing confusion in counting atoms.
Structure and formulae of ionic compounds
Ionic compounds form through the attraction between positively and negatively charged ions, rather than sharing electrons like in molecular compounds. They exist as large lattice structures, which are three-dimensional arrangements of ions held together by strong electrostatic forces. The formulae for ionic compounds represent the simplest ratio of ions needed to achieve electrical neutrality, not individual molecules.
Key features of ionic compounds:
- Lattice structures - Ions arrange in a repeating pattern, creating a crystal-like solid; for example, in NaCl, sodium ions (Na+) and chloride ions (Cl-) alternate in a vast network
- Formula ratios - The formula shows the ion ratio, such as MgCl2, indicating one magnesium ion (Mg2+) for every two chloride ions to balance charges
- No discrete molecules - Unlike molecular compounds, ionic compounds do not have separate molecules; the formula unit is just a representation of the ratio
This structure gives ionic compounds high melting points and the ability to conduct electricity when dissolved or melted.
Definition of mixtures
A mixture is a combination of two or more substances that are physically combined, without any chemical bonding between them. The components retain their individual properties and can be present in varying proportions, unlike the fixed ratios in compounds.
Types of mixtures:
- Homogeneous mixtures - Components are evenly distributed, like salt dissolved in water
- Heterogeneous mixtures - Components are not evenly mixed, such as sand and water
Mixtures can include elements, compounds, or both, and they form through physical processes like stirring or shaking.
Distinguishing mixtures from compounds and molecules
Mixtures differ fundamentally from compounds and molecules because they lack chemical bonds between components. While compounds have fixed compositions and new properties from chemical reactions, mixtures keep the properties of their original substances and can vary in composition.
Key distinctions:
- Bonding - Mixtures have no chemical bonds; compounds and molecules do
- Separation - Mixtures separate physically; compounds require chemical reactions to break apart
- Proportions - Mixtures can have variable ratios; compounds have fixed ones
- Properties - Mixture properties are a blend of components; compound properties are unique
For example, air is a mixture of gases like N2 and O2, easily separable, while water (H2O) is a compound that cannot be separated physically.
Physical methods for separating mixtures
Mixtures can be separated using physical methods that exploit differences in the physical properties of the components, such as size, solubility, or boiling point. These methods do not involve chemical changes and allow recovery of the original substances.
Common physical separation methods:
- Filtration - Separates solids from liquids by passing the mixture through a filter that traps insoluble particles; for example, filtering sand from water
- Crystallization - Separates soluble solids from solutions by evaporating the solvent, leaving behind crystals; for instance, obtaining salt crystals from saltwater by heating to remove water
These techniques rely on the fact that no chemical bonds need to be broken, making separation straightforward based on physical characteristics.