Chemical Formulas
Chemical Formulas
A chemical formula is a shorthand notation that describes the composition of a substance — what elements are present and in what amounts or ratios. Different types of chemical formulas convey different levels of information about a molecule’s composition and structure. Chemists use several types depending on what they need to communicate.
Molecular Formulas
A molecular formula gives the actual number of each type of atom present in one molecule of a substance. Each element is represented by its chemical symbol, and a subscript number after the symbol tells you how many atoms of that element are in one molecule. If no subscript is written, exactly one atom of that element is present.
Water: H₂O — two hydrogen atoms and one oxygen atom per molecule.
Acetic acid (vinegar): C₂H₄O₂— two carbons, four hydrogens, and two oxygens per molecule.
Glucose: C₆H₁₂O₆ — six carbons, twelve hydrogens, and six oxygens per molecule.
Ammonia: NH₃ — one nitrogen and three hydrogens per molecule.
Molecular formulas tell you the composition of the molecule but nothing about how the atoms are connected to each other. Two different molecules can have exactly the same molecular formula — these are called isomers, and they are covered in their own lecture.
Structural Formulas
A structural formula shows not just which atoms are present but how they are connected — which atoms are bonded to which. Bonds are represented as lines between atom symbols. The structural formula of water (H–O–H) shows that both hydrogen atoms are attached to the central oxygen, and that the hydrogens are not bonded directly to each other. This is information the molecular formula H₂O alone does not convey.
Structural formulas become especially important for organic molecules, where the same molecular formula can describe very different structures with very different properties. For example, ethanol (C₂H₅OH, the alcohol in beverages) and dimethyl ether (CH₃OCH₃) both have the molecular formula C₂H₆O, but their structural formulas show entirely different connectivity — and their properties (boiling points, reactivity, biological effects) are completely different as a result.
Figure 1: Structural Formula of Water
Structural formula of water
Space-Filling Models
A space-filling model is athree-dimensional representation of a molecule that shows the relative sizes ofthe atoms and how they are arranged in space. Each atom is depicted as a sphere scaled to its van der Waals radius, and spheres overlap where bonds exist. Space-filling models give the most realistic sense of the molecule’s actual shape and the amount of space it occupies, which is important for understanding how molecules interact with one another (for example, how a drug fits into a biological receptor).
Figure 2: Space-Filling Model of Water
The space-filling model of water illustrates the molecular structure of H2O, highlighting the larger oxygen atom in red and the two smaller hydrogen atoms in white.
Empirical Formulas
An empirical formula gives the simplest whole-number ratio of atoms in a compound — it is the Molecular formula reduced to its lowest terms. To find the empirical formula from a molecular formula, divide all subscripts by their greatest common divisor.
Table 1. Molecular and empirical formulas for selected compounds. Note that benzene and glucose have the same empirical formula (CH₂O) even though they are very different molecules.
Compound | Molecular Formula | Empirical Formula | Reduction |
|---|---|---|---|
Diphosphorus pentoxide | P₄O₁₀ | P₂O₅ | Divide all by 2 |
Butene (hydrocarbon) | C₄H₈ | CH₂ | Divide all by 4 |
Water | H₂O | H₂O | Already in simplest ratio |
Benzene | C₆H₆ | CH | Divide all by 6 |
Glucose | C₆H₁₂O₆ | CH₂O | Divide all by 6 |
Acetic acid | C₂H₄O₂ | CH₂O | Divide all by 2 |
An important point from the table: benzene (C₆H₆) and glucose (C₆H₁₂O₆) reduce to CH and CH₂O respectively— completely different empirical formulas for very different molecules. But acetic acid and glucose both reduce to CH₂O, even though they are not the same compound at all. This shows that the empirical formula is less informative than the molecular formula: you cannot uniquely identify a compound from its empirical formula alone.
Figure 3: Empirical vs. Molecular Formula
Explore the differences between empirical and molecular formulas, using hydrogen peroxide as an example, to understand how compounds are represented by their simplest ratio versus the actual number of atoms.
Going from Empirical to Molecular Formula
If you know both the empirical formula and the molar mass of a compound, you can determine the molecular formula. The molecular formula is always a whole-number multiple of the empirical formula:
MolecularFormula = n × (Empirical Formula)
Example: A compound has the empirical formula CH₂ and a molar mass of 56 g/mol. What is its molecular formula?
Empirical formula mass of CH₂ =12 + 2(1) = 14 g/mol.
n = 56 / 14 = 4.
Molecular formula = 4 × CH₂ =C₄H₈ (butene). ✔
Summary: Types of Chemical Formulas
Table 2. Summary of the four main types of chemical formulas.
Formula Type | What It Shows | Example (for water) |
|---|---|---|
Molecular formula | Actual number of each atom per molecule | H₂O |
Structural formula | Which atoms are bonded to which | H–O–H (angle structure) |
Space-filling model | 3D shape and relative atomic sizes | (3D sphere model) |
Empirical formula | Simplest whole-number atom ratio | H₂O (same in this case) |
Summary
Chemical formulas are shorthand representations that communicate the composition and structure of substances. Molecular formulas provide the actual number of each type of atom present in a molecule, while structural formulas reveal how those atoms are connected. Space-filling models provide realistic three-dimensional representations of molecular shape and size, and empirical formulas describe the simplest whole-number ratio of atoms in a compound.
Because different formula types communicate different information, chemists use each for specific purposes. Understanding these representations provides a foundation for studying molecular structure, chemical bonding, and chemical reactivity throughout the remainder of chemistry.
Key Points
Chemical formulas provide shorthand descriptions of substances.
Molecular formulas show the actual number of atoms in a molecule.
Structural formulas show how atoms are connected.
Molecules with identical molecular formulas may have different structures.
Such compounds are called isomers.
Space-filling models show three-dimensional molecular shape and atomic size.
Molecular shape influences chemical and biological behavior.
Empirical formulas show the simplest whole-number atom ratio.
Empirical formulas are obtained by reducing molecular formula subscripts.
Different compounds can share the same empirical formula.
The molecular formula is always a whole-number multiple of the empirical formula.
Knowing molar mass allows determination of molecular formulas from empirical formulas.
Different chemical formula representations provide different types of information.