Naming Molecules

We now shift our focus from ionic compounds, which involve the transfer of electrons between metals and nonmetals, to covalent (or molecular) compounds. These compounds form when two or more nonmetal atoms bond by sharing electrons.

The rules for naming covalent compounds are fundamentally different from those for ionic compounds. The primary reason for this is that two nonmetals can often combine in multiple proportions to form a variety of different, stable molecules. For example, carbon and oxygen can form both CO (carbon monoxide) and CO2 (carbon dioxide). Because of this, the naming system cannot rely on deducing charges; instead, it must use a system of prefixes to explicitly state the number of atoms of each element present in a single molecule.

Elemental Molecules

The simplest type of molecule to name is an elemental molecule, which is a molecule composed of two or more atoms of a single element.

Naming Rule

The name for these elemental molecules is simply the name of the element itself.

The seven diatomic elements exist as two-atom molecules in their natural state. When we refer to “hydrogen gas” or “liquid bromine,” we are referring to H2 and Br2, not individual atoms.

The seven diatomic elements:

  • H2: hydrogen
  • N2: nitrogen
  • O2: oxygen
  • F2: fluorine
  • Cl2: chlorine
  • Br2: bromine
  • I2: iodine

Other common elemental molecules:

  • P4: phosphorus
  • S8: sulfur

Binary Compounds

We now move from molecules of a single element to the most common type of covalent compound: a binary compound, which is formed from two different nonmetal elements.

For ionic compounds, the cation always comes first. But with two nonmetals, how do we decide the order? Chemists have agreed upon a standard convention for ordering the elements in a binary molecular compound.

Before we can name a compound, we must first know which element to write first in the chemical formula. This order also dictates which element is named first.

Order of Elements in a Formula

The convention is to list the element that is further to the left or further down on the periodic table first. The element that comes second is therefore the one further to the right or higher up. There are a few exceptions to this rule, but it serves as an excellent guide for most compounds you will encounter.

The image below gives the official sequence as provided by IUPAC. This figure shows which element is given last in the molecular formula. Notice the position of hydrogen (H). While it is located in the top-left corner of the periodic table, it is positioned before nitrogen in the sequence.

Figure 1: Element sequence as provided by IUPAC

Compounds with Common Names

While the systematic IUPAC nomenclature provides a clear method for naming most covalent compounds, a small group of molecules are almost exclusively known by their historical or common names.

These names do not follow the prefix rules but have been used for centuries and are officially accepted by IUPAC due to their widespread use. For example, while H2O could systematically be named “dihydrogen monoxide,” it is universally known simply as water. Memorize the names and formulas of these common compounds, as they appear frequently in chemistry.

Examples:

  • H2O: water
  • H2O2: hydrogen peroxide
  • NH3: ammonia
  • N2H4: hydrazine
  • PH3: phosphine
  • NO: nitric oxide
  • N2O: nitrous oxide

Compounds Containing Hydrogen

Hydrogen is often treated as a special case. When hydrogen forms a binary compound with a nonmetal from Group 16 (like sulfur) or Group 17 (the halogens), hydrogen is always written and named first. These specific compounds are an important group to recognize, as we will see them again when we discuss acids.

Naming Rules

Once the formula is written correctly, the naming process is:

  1. The first element in the formula is named first, using its full element name.
  2. The second element is named as if it were an anion, by taking the root of its name and changing the suffix to –ide.

Examples:

  • HF: hydrogen fluoride
  • HCl: hydrogen chloride
  • HI: hydrogen iodide
  • H2S: hydrogen sulfide

Multiplicative Prefixes

The element + element-ide format works for some hydrogen compounds, but most binary covalent compounds need more specificity. Two nonmetals can often combine in multiple, distinct ratios. For example, nitrogen and oxygen can form N2O, NO, N2O3, NO2, N2O4, and N2O5. To unambiguously name these different molecules, we must use a system of multiplicative prefixes.

These prefixes, derived from Greek, explicitly state the number of atoms of each element in a single molecule. The table below lists the prefixes you will need to learn.

NoteTable of Some Multiplicative Prefixes

Note: The hemi- prefix is used for hydrates, not binary covalent compounds.

Naming Rules

The full name of a binary covalent compound:

  1. First Element: The first element in the formula is named using the appropriate prefix followed by the full element name.
    • Important Exception: The prefix mono– is never used for the first element. If there is only one atom of the first element, its name is used without a prefix.
  2. Second Element: The second element is named using the appropriate prefix followed by the root of the element’s name with the suffix –ide.
WarningA Note on Spelling

To make the names easier to pronounce, the final vowel of a prefix is often dropped when the element name begins with a vowel. Specifically, the ‘o’ from mono– and the ‘a’ from prefixes like tetra– or penta– are removed.

For example:

  • CO is named carbon monoxide, not carbon monooxide.
  • N2O5 is named dinitrogen pentoxide, not dinitrogen pentaoxide.

Examples:

  • CO: carbon monoxide
  • PBr3: phosphorus tribromide
  • CCl4: carbon tetrachloride
  • SiS2: silicon disulfide
  • NCl3: nitrogen trichloride
  • IF7: iodine heptafluoride
  • S4N4: tetrasulfur tetranitride
  • N2O5: dinitrogen pentoxide
  • I2O5: diiodine pentoxide

Simple Organic Compounds

This section introduces organic chemistry.

Broadly, an organic compound is any chemical compound that contains carbon. However, this modern definition has historical exceptions; simple carbon-containing compounds like carbonates (e.g., CO32−), cyanides (e.g., CN−), and carbon oxides (e.g., CO2) are typically classified as inorganic. For our purposes, we will focus on the most common feature of organic chemistry: compounds built upon a framework of carbon-carbon and carbon-hydrogen bonds. The simplest of these are the hydrocarbons.

Writing Organic Formulas: The Hill System

In the previous section, we established the convention for ordering elements in binary molecular compounds based on electronegativity (for example, SO2 or PCl5). For the vast world of organic compounds, however, chemists use a different and more rigid standard known as the Hill system.

The presence of a carbon backbone in an organic compound signals that we should switch from the electronegativity convention to this specific system. The rules:

  1. Carbon (C) is always listed first.
  2. Hydrogen (H) is listed second.
  3. All other elements are then listed in alphabetical order.

Think of it this way: if a compound is organic, use the Hill system. If it’s an inorganic molecule, use the electronegativity order.

Examples:

  • The molecular formula for ethane is C2H6.
  • The molecular formula for ethanol (which also contains oxygen) is C2H6O. (Oxygen comes after Hydrogen).
  • The molecular formula for methylamine (which contains nitrogen) is CH3NH2. (Nitrogen comes after Hydrogen).
  • The molecular formula for chloromethane is CH3Cl. (Chlorine, Cl, comes after Hydrogen).

While the Hill system gives us a standard way to write the molecular formula (the total count of atoms), the names of organic compounds, which we will begin to explore next, are derived from their specific structure and bonding.

NoteHill System and Molecular vs. Structural Formulas

Note that the Hill system is the standard for writing molecular formulas, which show only the total number and type of atoms in a molecule.

However, much of organic chemistry relies on structural formulas (or condensed structural formulas), which are written to show how atoms are connected. In a structural formula, the goal is to communicate the molecule’s structure. Therefore, the order of elements often deviates from the Hill system to group atoms into meaningful functional groups.

For example, the molecular formula for ethanol is written C2H6O according to the Hill system. Its structural formula, however, is written as CH3CH2OH. This second formula intentionally places the oxygen (O) and hydrogen (H) of the hydroxyl group together at the end to show how they are bonded, providing far more chemical information.

As you move forward, you will primarily see structural formulas. Remember that they follow the rules of connectivity, not the indexing rules of the Hill system.

Alkanes

The simplest family of hydrocarbons is the alkanes. Alkanes are known as saturated hydrocarbons. This means they consist of chains of carbon atoms linked only by single bonds, with each carbon atom bonded to the maximum possible number of hydrogen atoms.

The names for alkanes are systematic and based on the number of carbon atoms in the longest continuous chain.

NotePrefixes for Carbon Chains

The prefix of an alkane’s name indicates the number of carbon atoms in its main chain. While some of these are the same as the Greek prefixes we have already learned (penta–, hexa–, etc.), the first four are unique to organic chemistry and must be memorized.

The names for simple, unbranched alkanes (also called straight-chain or normal alkanes) are constructed by combining the appropriate prefix with the suffix –ane. Sometimes, the prefix n– (for normal) is added to the name to specify a straight chain, but it is often omitted.

Examples:

  • CH4: methane
  • C2H6: ethane
  • C3H8: propane
  • C4H10: butane
  • C5H12: pentane
  • C6H14: hexane
TipBeyond The Basics: Structural Isomers

In organic chemistry, a single molecular formula does not always correspond to a single, unique molecule. Molecules that share the same molecular formula but have a different arrangement (or connectivity) of their atoms are called structural isomers.

Each isomer is a distinct compound with its own physical and chemical properties, such as boiling point, melting point, and density.

The alkane with the formula C5H12 is a perfect example, as it exists as three different structural isomers. While they all have 5 carbons and 12 hydrogens, their carbon “skeletons” are fundamentally different. These are commonly known as n–pentane (a straight chain), isopentane (a single branch), and neopentane (a central carbon bonded to four methyl groups), as shown in the structures below. This is why organic compounds need a more detailed naming system: to give each isomer its own unique name.

n–Pentane

Isopentane

Neopentane

Alcohols

The next family of organic compounds we will introduce is the alcohols. An alcohol is an organic compound that contains one or more hydroxyl (–OH) functional groups. A functional group is a specific arrangement of atoms responsible for the characteristic chemical reactions of a molecule.

The nomenclature for simple, straight-chain alcohols is directly derived from the names of the alkanes.

Naming Rules

To name a simple alcohol, modify the name of the parent alkane (the alkane with the same number of carbon atoms):

  1. Take the name of the parent alkane.
  2. Drop the final -e from the alkane name.
  3. Add the suffix -ol.

For example, the two-carbon alkane is ethane. To name the two-carbon alcohol, simply drop the “–e” and add “–ol” to get ethanol.

Examples:

  • CH3OH: methanol
  • C2H5OH: ethanol
  • C3H7OH: propanol
  • C4H9OH: butanol
NoteWhat do the numbers in alcohol names mean?

As carbon chains get longer, the hydroxyl (–OH) group can be attached to different carbon atoms, creating structural isomers. To specify the location of the –OH group, a number (called a locant) is used in the name. For example, propan-2-ol means the –OH group is on the second carbon of a three-carbon chain.

For the simple, straight-chain alcohols we are discussing here, the –OH group is always on the first carbon at the end of the chain (position 1). While the formal IUPAC names are therefore methanol, ethan-1-ol, propan-1-ol, and butan-1-ol, the “1-” is often omitted for these simple primary alcohols when there is no ambiguity.

Carboxylic Acids

Carboxylic acids are a class of organic compounds distinguished by the presence of a carboxyl functional group (–COOH). This group consists of a carbonyl group (C=O) bonded to a hydroxyl group (–OH).

Naming Rules

To name a straight-chain carboxylic acid:

  1. Count the total number of carbon atoms, including the carboxyl carbon.
  2. Take the name of the alkane with that number of carbons.
  3. Drop the final -e from the alkane name.
  4. Add the suffix -oic acid.

For example, a two-carbon carboxylic acid is derived from “ethane,” which becomes “ethanoic acid.” The table below lists the first twelve straight-chain carboxylic acids, along with their systematic and common names.

Acids and Bases

An important distinction in chemical naming arises when a substance becomes an acid upon being dissolved in a solvent, typically water.

Binary Acids

When hydrogen forms a covalent compound with a nonmetal from Group 16 (such as sulfur) or Group 17 (the halogens), these compounds have different names depending on their physical state.

Naming Rules

  1. As a pure gas or liquid: Use the standard binary covalent naming convention: “hydrogen” + element name + “-ide”
  2. When dissolved in water (aqueous): Use the acid naming convention: “hydro-” + root of element name + “-ic acid”

For example, the compound HCl has two names depending on its state:

  • HCl(g): hydrogen chloride (the pure gas)
  • HCl(aq): hydrochloric acid (dissolved in water)

Examples of binary acids:

Oxyacids

While binary acids contain only hydrogen and one other nonmetal, oxyacids are acids that contain hydrogen, oxygen, and another element (usually a nonmetal). These are sometimes called ternary acids since they contain three different elements. Common examples include sulfuric acid (H2SO4), nitric acid (HNO3), and phosphoric acid (H3PO4).

Naming Rules

The name of an oxyacid is directly related to the name of its corresponding polyatomic anion, the ion that remains when the acidic hydrogen ions are removed. If you know the name of the polyatomic anion, you can determine the name of the acid:

  1. If the anion name ends in -ate: Change the ending to -ic acid
    • Example: NO3− is nitrate → HNO3 is nitric acid
    • Example: SO42− is sulfate → H2SO4 is sulfuric acid
  2. If the anion name ends in -ite: Change the ending to -ous acid
    • Example: NO2− is nitrite → HNO2 is nitrous acid
    • Example: SO32− is sulfite → H2SO3 is sulfurous acid

Additional Prefixes for Multiple Oxidation States

Some elements (particularly the halogens) form multiple oxyacids with different numbers of oxygen atoms. For these, we use additional prefixes:

  • per-…ic acid: One more oxygen than the -ic acid (anion ends in per-…ate)
  • -ic acid: The reference acid (anion ends in -ate)
  • -ous acid: One fewer oxygen than the -ic acid (anion ends in -ite)
  • hypo-…ous acid: One fewer oxygen than the -ous acid (anion ends in hypo-…ite)

Examples of chlorine oxyacids (in order of decreasing oxygen content):

  • HClO4: perchloric acid (from perchlorate, ClO4−)
  • HClO3: chloric acid (from chlorate, ClO3−)
  • HClO2: chlorous acid (from chlorite, ClO2−)
  • HClO: hypochlorous acid (from hypochlorite, ClO−)

Common oxyacids and their corresponding anions:

TipConnection to Polyatomic Ions

Notice the systematic relationship between oxyacids and polyatomic anions. When you memorize the common polyatomic ions (like sulfate, nitrate, phosphate), you automatically know the names of their corresponding acids. Simply apply the rule: -ate → -ic acid and -ite → -ous acid.

Common Acids and Bases

The acid naming conventions we have covered apply systematically to binary acids and oxyacids. However, some acids (particularly organic acids like carboxylic acids) are called acids even in their pure, anhydrous (water-free) form. For example, pure acetic acid is commonly known as “glacial acetic acid.”

Bases, which we will study in detail in the next chapter, also appear frequently in chemistry. From a nomenclature perspective, most common bases are either ionic compounds containing a metal cation and the hydroxide anion (OH−), such as sodium hydroxide (NaOH), or simple molecular compounds like ammonia (NH3). These follow the naming rules you have already learned for ionic and molecular compounds.

Below are tables listing common examples of strong and weak acids and bases you will encounter throughout general chemistry. Strong acids and bases ionize completely in water, whereas weak ones ionize only partially. The chemical behavior of these substances will be covered in later chapters.

Summary: Molecular Compound Nomenclature

The following table summarizes the key naming patterns for molecular compounds covered in this chapter: