Chemical Formulae and Nomenclature
~9 min read · Tier toggle changes depth (T1 plain · T2 standard · T3 rigour)
You should be comfortable with basic algebra and the concept of ratios, as these form the mathematical backbone of stoichiometry. No prior knowledge of specific chemical reactions is required, as this lecture introduces the fundamental language used to describe them.
Why this matters
Chemical formulae are the concise shorthand that allows chemists to communicate complex structural and compositional information instantly.
Before we can calculate how much product forms from a given amount of reactant, we must be able to read and write the identities of the substances involved. This lecture establishes the rules for naming compounds and interpreting their formulae. It distinguishes between molecular compounds, which exist as discrete units, and ionic compounds, which form extended lattices. Understanding this distinction is crucial because it dictates how we interpret the numbers in a formula: as a count of atoms in a molecule, or as a ratio of ions in a crystal. Without this foundational clarity, subsequent calculations regarding molar mass and reaction yields become ambiguous and prone to error.
The Language of Composition
Imagine trying to describe a recipe without using names for the ingredients. You might say, "mix the white powder with the clear liquid," but this is useless if there are ten white powders and five clear liquids in the kitchen. Chemistry requires a precise vocabulary. The chemical formula is the name of the ingredient. However, unlike a person's name, a chemical formula also contains quantitative information. It tells us not just what the substance is, but what it is made of and in what proportions. There are two main families of substances we encounter in introductory chemistry: molecular compounds and ionic compounds. Molecular compounds are formed when non-metal atoms share electrons to form discrete molecules. Think of water, \(\mathrm{H_2O}\), or carbon dioxide, \(\mathrm{CO_2}\). In these cases, the formula represents a single, countable entity. One molecule of water contains exactly two hydrogen atoms and one oxygen atom. The ratio is fixed and absolute for that single unit. Ionic compounds are different. They are formed when metals transfer electrons to non-metals, creating positively charged ions (cations) and negatively charged ions (anions). These ions attract each other electrostatically to form a giant, repeating 3D lattice structure. There is no such thing as a single "molecule" of sodium chloride, \(\mathrm{NaCl}\). Instead, there is a crystal where every sodium ion is surrounded by chloride ions and vice versa. Therefore, the formula \(\mathrm{NaCl}\) does not represent a molecule; it represents the simplest whole-number ratio of ions in the lattice. It tells us that for every one sodium ion, there is one chloride ion. This distinction is vital. When we later calculate the mass of a "mole" of substance, we are counting formula units for ionic compounds, not molecules. Confusing these two concepts leads to errors in understanding what we are actually counting. The nomenclature rules reflect this difference: molecular compounds often use prefixes (mono-, di-, tri-) to indicate the number of atoms, while ionic compounds rely on the charges of the ions to determine the ratio, often requiring Roman numerals for metals with variable charges.
Naming Ionic Compounds
The naming of ionic compounds follows a strict logical sequence based on the charges of the constituent ions. The general rule is that the cation (positive ion) is named first, followed by the anion (negative ion). For main group metals, the name of the element is used directly. For example, \(\mathrm{Na^+}\) is the sodium ion, and \(\mathrm{Mg^{2+}}\) is the magnesium ion. The anion is always the name of the non-metal element with the suffix "-ide". Thus, \(\mathrm{Cl^-}\) is the chloride ion, and \(\mathrm{O^{2-}}\) is the oxide ion. When a metal can form ions with different charges, known as transition metals or post-transition metals, we must specify the charge using Roman numerals in parentheses. This is the Stock system. Iron can form \(\mathrm{Fe^{2+}}\) or \(\mathrm{Fe^{3+}}\). Therefore, \(\mathrm{FeCl_2}\) is named iron(II) chloride, while \(\mathrm{FeCl_3}\) is iron(III) chloride. The Roman numeral indicates the charge on the single metal ion, not the total charge of the compound. To write the formula from the name, we use the "criss-cross" method or, more rigorously, the principle of charge neutrality. The total positive charge must equal the total negative charge. Consider aluminium oxide. Aluminium is in Group 13, so it forms \(\mathrm{Al^{3+}}\). Oxygen is in Group 16, so it forms \(\mathrm{O^{2-}}\). To balance the charges, we need two aluminium ions (\(2 \times +3 = +6\)) and three oxide ions (\(3 \times -2 = -6\)). The formula is \(\mathrm{Al_2O_3}\). We do not write \(\mathrm{Al_6O_9}\), as formulae are always reduced to the lowest whole-number ratio. This reduction is a key feature of ionic formulae, distinguishing them from molecular formulae which may not be reducible (e.g., \(\mathrm{H_2O_2}\), hydrogen peroxide, is distinct from \(\mathrm{H_2O}\), water).
Naming Molecular Compounds
Molecular compounds consist of two or more non-metals. Since non-metals do not form ions in the same way, we cannot rely on charge balancing. Instead, we use Greek prefixes to indicate the number of atoms of each element present in the molecule. The prefixes are: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), and deca- (10). The element further to the left in the periodic table is named first. The second element is named with the suffix "-ide". The prefix "mono-" is typically omitted for the first element if there is only one atom, but it is included for the second element if there is only one atom. For example, \(\mathrm{CO_2}\) is carbon dioxide, not monocarbon dioxide. However, \(\mathrm{NO}\) is nitrogen monoxide, not nitrogen oxide, to distinguish it from \(\mathrm{NO_2}\) (nitrogen dioxide). Consider dinitrogen tetroxide, \(\mathrm{N_2O_4}\). The prefix "di-" indicates two nitrogen atoms, and "tetra-" indicates four oxygen atoms. The "a" in "tetra" is often dropped before "oxide" to make pronunciation easier, resulting in "tetroxide". This system is unambiguous. Unlike ionic compounds, where the ratio is determined by charge, the ratio in molecular compounds is determined by the specific bonding arrangement of the atoms. Two different compounds can have the same elements but different ratios, such as \(\mathrm{CO}\) (carbon monoxide) and \(\mathrm{CO_2}\) (carbon dioxide). These are distinct substances with different physical and chemical properties, and their names must reflect this difference precisely. The nomenclature system ensures that every unique molecular composition has a unique name.
Interpreting Formulae: Empirical vs Molecular
It is critical to distinguish between the empirical formula and the molecular formula. The empirical formula represents the simplest whole-number ratio of atoms in a compound. The molecular formula represents the actual number of atoms in a molecule. For many ionic compounds, the formula given is the empirical formula because they do not exist as discrete molecules. For example, the formula for sodium chloride is \(\mathrm{NaCl}\). This is the empirical formula, and it is also the formula unit. There is no larger "molecular" formula for \(\mathrm{NaCl}\) because the lattice extends infinitely. For molecular compounds, the empirical and molecular formulae may differ. Consider glucose, \(\mathrm{C_6H_{12}O_6}\). The ratio of C:H:O is 6:12:6, which simplifies to 1:2:1. Therefore, the empirical formula is \(\mathrm{CH_2O}\). The molecular formula is \(\mathrm{C_6H_{12}O_6}\). Both formulas describe the same substance, but they convey different levels of information. The empirical formula tells us the relative proportions, while the molecular formula tells us the absolute count in a single molecule. When performing stoichiometric calculations, we must be clear about which formula we are using. If we are calculating the molar mass of a substance, we use the molecular formula for molecular compounds and the formula unit mass for ionic compounds. The molar mass of glucose is calculated using \(\mathrm{C_6H_{12}O_6}\), not \(\mathrm{CH_2O}\). Using the empirical formula would give a molar mass that is one-sixth of the actual value, leading to significant errors in quantitative analysis. This distinction becomes particularly important in analytical chemistry, where the empirical formula is often determined experimentally from percentage composition data, and the molecular formula is then deduced using the molar mass determined by other means, such as mass spectrometry or colligative properties.
A common misconception is that the subscripts in a chemical formula always represent the number of atoms in a single molecule. This is true for molecular compounds like \(\mathrm{H_2O}\), but false for ionic compounds like \(\mathrm{NaCl}\). In \(\mathrm{NaCl}\), the formula represents the ratio of ions in the crystal lattice, not a discrete molecule. When we say "one mole of \(\mathrm{NaCl}\)", we mean \(6.022 \times 10^{23}\) formula units, not molecules. Confusing these terms can lead to conceptual errors in understanding the structure of solids and the basis of molar mass calculations. Always ask: is this a discrete molecule or an extended lattice? C
Worked example
Problem. Determine the correct chemical name and formula for the compound formed between iron and oxygen, given that the iron ion has a charge of \(+3\) and the oxide ion has a charge of \(-2\). Calculate the number of oxygen atoms present in \(2.50 \times 10^{22}\) formula units of this compound.
- Identify the ions: Iron(III) is \(\mathrm{Fe^{3+}}\) and oxide is \(\mathrm{O^{2-}}\).
- Determine the ratio for charge neutrality: The least common multiple of 3 and 2 is 6. We need two \(\mathrm{Fe^{3+}}\) ions (\(2 \times +3 = +6\)) and three \(\mathrm{O^{2-}}\) ions (\(3 \times -2 = -6\)).
- Write the formula: \(\mathrm{Fe_2O_3}\). The name is iron(III) oxide.
- Identify the number of oxygen atoms per formula unit: In one formula unit of \(\mathrm{Fe_2O_3}\), there are 3 oxygen atoms.
- Calculate the total number of oxygen atoms: Multiply the number of formula units by the number of oxygen atoms per unit. \(2.50 \times 10^{22} \times 3 = 7.50 \times 10^{22}\) oxygen atoms.
Units check. The input is a count of formula units (dimensionless), and the multiplier is a count of atoms per unit (dimensionless), so the result is a count of atoms (dimensionless), which is consistent.
Three quick tests
- What is the chemical formula for calcium nitrate?
Answer
\(\mathrm{Ca(NO_3)_2}\). Calcium is \(\mathrm{Ca^{2+}}\) and nitrate is \(\mathrm{NO_3^-}\). Two nitrate ions are needed to balance one calcium ion. - Why is \(\mathrm{CO_2}\) named carbon dioxide rather than carbon oxide?
Answer
To distinguish it from carbon monoxide (\(\mathrm{CO}\)). The prefix "di-" indicates two oxygen atoms, which is necessary for molecular compounds to specify the exact composition. - What is the empirical formula of \(\mathrm{C_6H_6}\)?
Answer
\(\mathrm{CH}\). The ratio of 6:6 simplifies to 1:1. The molecular formula is \(\mathrm{C_6H_6}\) (benzene), but the simplest whole-number ratio is 1:1.
Back to CU-103 →
Every lecture and every result in this unit, with prerequisites and threads.
Lecture notes drafted with a local model and checked line by line by the editor; every number in the worked example was recomputed.
Results cited: — · Threads: structure