2.4 Chemical Formulas

Chemical formulas are how chemists represent the composition of molecules and compounds on paper. By combining element symbols with subscripts, a formula tells you exactly what atoms are present and how many of each. Understanding how to read and write these formulas is foundational for everything else in chemistry, from balancing equations to calculating molar masses.

Chemical Formulas

Chemical Formula Notation

Every chemical formula is built from element symbols pulled from the periodic table, combined with subscripts that indicate atom counts. A few rules govern how they're written:

• Element symbols are one or two letters. The first letter is always capitalized; if there's a second letter, it's lowercase. This matters: $Co$ is cobalt, but $CO$ is carbon monoxide.
• Subscripts go right after the element symbol to show how many atoms of that element are present. If there's only one atom, you skip the subscript. So hydrogen chloride is $HCl$ (one hydrogen, one chlorine), while water is $H_2O$ (two hydrogens, one oxygen).
• Element order follows convention. In binary compounds (two elements), the less electronegative element is written first: $NaCl$, not $ClNa$. For acids, hydrogen comes first, followed by the anion: $H_2SO_4$ for sulfuric acid.

Some examples to get comfortable with:

• Methane: $CH_4$ (one carbon, four hydrogens)
• Iron(III) oxide: $Fe_2O_3$ (two iron atoms, three oxygens)
• Phosphoric acid: $H_3PO_4$ (three hydrogens, one phosphorus, four oxygens)

Types of Chemical Formulas

There are three main types of formulas, and each one tells you something different about a compound.

Molecular formulas show the actual number of each type of atom in one molecule. For example, ethanol's molecular formula is $C_2H_6O$, meaning each molecule contains 2 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom. This is the formula you'd use to calculate molar mass or set up stoichiometry problems.

Empirical formulas give the simplest whole-number ratio of atoms. You get them by reducing all the subscripts in a molecular formula by their greatest common factor. Ethanol ($C_2H_6O$) has an empirical formula of $C_2H_6O$ itself, since 2:6:1 is already in simplest terms. A better example: glucose has the molecular formula $C_6H_{12}O_6$, but its empirical formula is $CH_2O$ (a 1:2:1 ratio). Multiple compounds can share the same empirical formula while having different molecular formulas.

Structural formulas show how atoms are actually connected to each other. These come in a few styles:

• Lewis structures use lines for bonds and dots for lone-pair electrons. Water looks like $H-O-H$.
• Condensed structural formulas group atoms together without showing every bond. Ethanol can be written as $CH_3CH_2OH$.
• Skeletal formulas (used more in organic chemistry) show just the carbon backbone as zigzag lines, with hydrogen atoms on carbon implied rather than drawn.

Structure and Properties of Isomers

Isomers are compounds that share the same molecular formula but have different arrangements of atoms. Same ingredients, different structure. This distinction matters because structure affects properties.

Structural isomers have atoms connected in a different order. Pentane ($C_5H_{12}$) is a classic example with three structural isomers:

• n-pentane: $CH_3CH_2CH_2CH_2CH_3$ (a straight chain)
• Isopentane: $CH_3CH(CH_3)CH_2CH_3$ (one branch)
• Neopentane: $C(CH_3)_4$ (a central carbon with four methyl groups)

All three have the formula $C_5H_{12}$, but their different structures give them different boiling points: n-pentane boils at 36.1 °C, while the more compact neopentane boils at just 9.5 °C. More branching generally means weaker intermolecular forces and a lower boiling point.

Stereoisomers have atoms connected in the same order but arranged differently in three-dimensional space. Lactic acid ($C_3H_6O_3$) exists as two stereoisomers, L-lactic acid and D-lactic acid, which are mirror images of each other. Even though their formulas and connectivity are identical, they behave differently in biological systems: L-lactic acid is what your muscles produce during intense exercise, while D-lactic acid is far less common in the human body.

Chemical Bonding and Electron Configuration

Chemical formulas don't just list atoms; they imply something about how those atoms are held together. Two major types of bonding show up repeatedly:

• Covalent bonds form when atoms share valence electrons. This typically happens between two nonmetals. Both atoms get closer to a full outer shell by sharing.
• Ionic bonds form when one atom transfers electrons to another, creating oppositely charged ions that attract each other. This usually happens between a metal and a nonmetal with a large electronegativity difference.

Valence electrons, the electrons in an atom's outermost shell, drive bonding behavior. The number of valence electrons determines how many bonds an atom tends to form. Carbon has 4 valence electrons and typically forms 4 bonds; oxygen has 6 and typically forms 2.

Oxidation states represent the hypothetical charge an atom would carry if all its bonds were treated as purely ionic. These are useful for tracking electron transfer in reactions and for naming compounds (like distinguishing iron(II) from iron(III)).

Polyatomic ions are groups of covalently bonded atoms that carry an overall charge. You'll encounter these frequently in chemical formulas:

• Ammonium: $NH_4^+$
• Carbonate: $CO_3^{2-}$
• Sulfate: $SO_4^{2-}$

When polyatomic ions appear in formulas, parentheses group them: $Ca(OH)_2$ means one calcium ion paired with two hydroxide ions.