SI units

Using Magnitude Prefixes and Standard Form

Chemists work with incredibly large numbers of atoms and extremely small distances. A Magnitude Prefix is placed before a unit to indicate a multiple or fraction based on powers of ten. You must know these prefixes:

• Tera ($T$): $10^{12}$
• Giga ($G$): $10^{9}$
• Mega ($M$): $10^{6}$
• Kilo ($k$): $10^{3}$
• Deci ($d$): $10^{-1}$
• Centi ($c$): $10^{-2}$
• Milli ($m$): $10^{-3}$
• Micro ($\mu$): $10^{-6}$
• Nano ($n$): $10^{-9}$

When processing data, you will frequently write these values using Standard Form ($A \times 10^n$, where $1 \le A < 10$). For example, a nanoparticle measuring $75\text{ nm}$ is $75 \times 10^{-9}\text{ m}$, which simplifies to $7.5 \times 10^{-8}\text{ m}$ in standard form.

Converting Volumes and Masses

Converting units is a frequent requirement in chemistry calculations, particularly for titration and reacting mass problems.

Volume Conversions The most crucial rule is remembering to cube the conversion factor: $1\text{ dm} = 10\text{ cm}$, so $1\text{ dm}^3 = 10^3\text{ cm}^3 = 1000\text{ cm}^3$.

Convert $150\text{ cm}^3$ into $dm^3$ and express the answer in standard form.

1. Identify the conversion factor: $1000\text{ cm}^3 = 1\text{ dm}^3$, so divide by $1000$.
2. Calculate decimal value: $150 \div 1000 = 0.15\text{ dm}^3$.
3. Standard form: $1.5 \times 10^{-1}\text{ dm}^3$.

Mass Conversions Mass is often provided in milligrams ($mg$) in medical or analytical contexts and must be converted to grams ($g$) for molar calculations.

Convert $500\text{ mg}$ into $g$ and express the answer in standard form.

1. Identify the conversion factor: $1\text{ g} = 1000\text{ mg}$, so multiply by $10^{-3}$ (or divide by $1000$).
2. Calculate decimal value: $500 \times 10^{-3} = 0.5\text{ g}$.
3. Standard form: $5.0 \times 10^{-1}\text{ g}$.

Naming Inorganic Compounds

Just as measurement units must be standardised, chemical names follow IUPAC Nomenclature rules to ensure clear global communication.

• Binary Ionic Compounds: The metal (cation) comes first, followed by the non-metal (anion) with its ending changed to -ide (e.g., sodium chloride).
• Polyatomic Anions: If the anion contains oxygen, the suffix changes to -ate (e.g., sulfate $SO_4^{2-}$, nitrate $NO_3^-$).
• Transition Metals: Because transition metals do not form ions with a single fixed charge, their systematic names must include their Oxidation State in Roman numerals. For example, $FeCl_2$ is Iron(II) chloride, while $FeCl_3$ is Iron(III) chloride.
• Covalent Molecules: Greek prefixes show the number of atoms: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), and hexa- (6).

Naming Organic Compounds

Organic compounds belong to a Homologous Series and their names are built systematically. The prefix (stem) tells you the length of the longest continuous carbon chain: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), and hex- (6).

The suffix identifies the compound's Functional Group:

• -ane: Alkane (only C-C single bonds)
• -ene: Alkene (contains a C=C double bond)
• -ol: Alcohol (contains an -OH group)
• -oic acid: Carboxylic acid (contains a -COOH group)
• -yl -anoate: Ester (contains a -COO- group, e.g., ethyl ethanoate)

When branches (like methyl ($-CH_3$) or ethyl ($-CH_2CH_3$) groups) or halogens are attached, use a Locant number to show their precise position on the carbon chain. Always number the chain starting from the end that gives the functional group or substituent the lowest possible number. Commas separate numbers, and hyphens separate numbers from letters (e.g., 2,2-dimethylbutane).