Lesson Explainer: Chemical Formulas Chemistry

In this explainer, we will learn how to write and interpret the notations used in chemical formulas.

Molecules and compounds consist of various atoms and ions. Chemists have developed ways to talk about these molecules and compounds to distinguish them from one another and indicate their composition. Let us consider a molecule of water.

Water molecules consist of two hydrogen atoms and one oxygen atom. We can represent the composition of a water molecule by writing chemical symbols for the hydrogen and oxygen atoms and using subscript numerals to indicate the number of each atom type: HO2.

This is the chemical formula of water. The hydrogen and oxygen atoms are represented with the chemical symbols H and O and the number of each atom type is shown with a subscript numeral. Subscript values that are larger than one are written out explicitly but the subscript value of one is only implied and not written out explicitly. This is why the chemical formula of water is written as HO2 rather than HO21.

Definition: Chemical Formula

An expression of chemical symbols and numerical subscripts that represents the composition of one unit of a compound.

Let us consider a sample that contains three water molecules.

We know that each water molecule has the chemical formula HO2. We can place a three in front of this chemical formula to indicate that there are three water molecules: 3HO2. The numeral three is a coefficient. Coefficients indicate the number of discrete units (molecules or formula units). It is important to distinguish between subscript values and coefficients. Subscript values always indicate the number of atoms or ions in a single unit of a compound, while coefficients indicate the number of discrete units.

Example 1: Calculating the Total Number of Atoms of an Element in a Complex Formula

How many atoms of oxygen are present in the formula 10SO2?

Answer

Looking at the formula, we can see that we have two different types of numerals: coefficients and subscripts. Coefficients are numerals which appear in front of a chemical formula. The coefficient terms indicate the number of discrete units. Therefore, the value ten in the 10SO2 chemical formula indicates that there are ten separate SO2 molecules.

Subscripts are numerals which appear after a chemical symbol in a chemical formula. Subscripts indicate the number of atoms or ions in a single unit. Subscript values of one are assumed to be on the right-hand side of the chemical symbol if there is no other number written in this space. Therefore, one molecule of SO2 contains one sulfur atom and two oxygen atoms.

With this information in mind, we could answer this question in two ways. If we recognize that one molecule of SO2 contains two oxygen atoms, then ten molecules must contain ten times as many oxygen atoms as given by the following equation: 21×10=20.oxygenatomsmoleculeofSOmoleculesofSOoxygenatoms22

Furthermore, the total number of atoms present in a formula can be calculated by distributing the coefficient to the subscripts.

Thus, the formula 10SO2 will contain ten times one sulfur atoms and ten times two oxygen atoms. There are 20 oxygen atoms present in the formula 10SO2.

In addition to chemical formulas, chemists can refer to molecules and compounds by their name. Various naming rules and conventions must be followed so that a chemical name refers only to one specific compound. We will focus on naming binary molecular compounds and simple ionic compounds.

A binary molecular compound consists of atoms of two different elements which are covalently bonded together. The elements in question are typically nonmetals but may also be metalloids.

The chemical formula of a binary molecular compound will generally be written with the less electronegative element’s chemical symbol first followed by the more electronegative element’s chemical symbol.

There are exceptions to this general rule. The following is the order of several common nonmetals when written as a part of a binary molecular chemical formula: C,P,N,H,S,I,Br,Cl,O,F.

Binary molecular compounds that do not consist solely of carbon and hydrogen are easy to name by following a few simple rules.

How To: Naming a Binary Molecular Compound That Is Not a Hydrocarbon

  1. Write the name of the element which appears first in the chemical formula.
  2. Write the root of the name of the element which appears second in the chemical formula.
  3. Add the suffix “-ide” to the root.
  4. Add a prefix in front of each species name to indicate the number of atoms of that element in the chemical formula. If there is only one of the first element, do not write a prefix in front of its name.
    Greek Prefixes Used When Naming Binary Molecular Compounds
    Number of AtomsPrefix
    1Mono- (never used in front of the first element’s name)
    2Di-
    3Tri-
    4Tetra-
    5Penta-
    6Hexa-
    7Hepta-
    8Octa-
    9Nona-
    10Deca-
  5. If the prefix in front of oxygen or oxide ends in a- or o-, the last letter of the prefix may be dropped.

By following the rules for naming binary molecular compounds, we can identify NO24 as dinitrogen tetroxide.

There are several binary molecular compounds that are not typically named following the conventions and instead are often referred to by their common names. Examples are shown in the table below.

Binary Molecular Compounds with Common Names
Chemical FormulaCommon Name
HO2Water
NH3Ammonia
NO2Laughing gas
CH4Methane
HS2Hydrogen sulfide
PH3Phosphine

Example 2: Naming a Binary Molecular Compound

What would the name of a compound containing one atom of chlorine and two atoms of oxygen be?

Answer

Using a periodic table, we can identify chlorine (Cl) and oxygen (O) as nonmetals. Compounds that contain two nonmetals are binary molecular compounds. The formula and name of a binary molecular compound is usually written with the less electronegative element first, followed by the more electronegative element. However, when chlorine, bromine, or iodine is combined with oxygen, the halogen is always written first. Thus, in both the formula and name, chlorine will appear before oxygen.

To name a binary molecular compound, we begin by writing the name of the first element, in this case, chlorine: compoundname:chlorine.

Then, we write the root of the second element’s name followed by the suffix “-ide”: compoundname:chlorineoxide.

Next, we need to indicate how many atoms of each element are in the compound by adding a Greek prefix in front of the corresponding element name. There is one atom of chlorine and two atoms of oxygen in the compound. The Greek prefix for one is “mono-” and the Greek prefix for two is “di-.” The prefix “mono-” is never placed in front of the first element. Therefore, we will only add the prefix “di-” in front of oxide: compoundname:chlorinedioxide.

The name of a compound containing one atom of chlorine and two atoms of oxygen will be chlorine dioxide.

Example 3: Converting a Complex Chemical Name with Prefixes into a Chemical Formula

What is the chemical formula of dichlorine heptoxide?

Answer

A chemical formula is an expression of chemical symbols and numerical subscripts that represents the composition of one unit of a compound. Before we can write a chemical formula, we must first identify the type of compound, as different types of compounds have different rules for naming and formula writing.

The Greek prefixes “di-” and “hept-” indicate that this is the name of a molecular compound. To write the chemical formula of a molecular compound, we can begin by separating the name into prefixes, root words, and suffixes.

The root words can be used to identify the elements in the compound. The first element is chlorine, which has the chemical symbol Cl. The second element has the root “ox.” Using the periodic table, we can identify the element that shares this root as oxygen, which has the chemical symbol O. We can write these chemical symbols in the same order in which the elements appear in the name: chemicalformula:ClO.

The prefixes indicate the number of atoms of each element. The Greek prefix “di-” means two and the prefix “hept-” means seven. We represent the number of atoms of each element in the chemical formula with a subscript numeral that appears immediately after the corresponding element. Thus, we can place a subscript two after cl and a subscript seven after O: chemicalformula:ClO27.

The chemical formula of dichlorine heptoxide is ClO27.

Ionic compounds consist of positively charged cations and negatively charged anions that are bound together through electrostatic interactions. The cations are generally metal ions and the anions are generally nonmetal ions. The following figure shows the charges of several common ions.

Elements in the d block of the periodic table are frequently able to form ions with multiple charges. For example, an atom of iron may form the Fe2+ or Fe3+ ion.

Ionic compounds exist as a three-dimensional network of many cations and anions rather than as discrete molecular units. It is impossible to indicate the total number of cations and anions in an ionic compound with a chemical formula. Instead, the chemical formula represents the simplest ratio of cations and anions with the cation term written first. For example, table salt consists of many sodium ions (Na+) and chloride ions (Cl), but the ratio of sodium ions and chloride ions in the compound is 11. Thus, the chemical formula of table salt is NaCl.

Likewise, an ionic compound consisting of magnesium ions, Mg2+, and bromide ions would have the chemical formula MgBr2.

Ionic compounds may contain polyatomic ions. Polyatomic ions are cations or anions that are made up of at least two different types of atoms. For example, phosphate is a polyatomic ion that has a charge of 3 and consists of one phosphorus atom and four oxygen atoms.

Chemical formulas are written similarly for ionic compounds that contain either simple monatomic ions or more complex polyatomic ions. Let us consider a compound composed of phosphate ions and zinc ions, Zn2+.

Three zinc ions and two phosphate ions are needed for the combination to be electrically neutral. Thus, the chemical formula for this compound will be Zn(PO)342.

Notice that the polyatomic ion is placed in parentheses with a subscript two outside of the parentheses. This subscript indicates that there are two phosphate ions.

The table below lists several common polyatomic ions.

Polyatomic Ions
ChargeIon NameIon Formula
1+AmmoniumNH4+
1BicarbonateHCO3
HydroxideOH
NitrateNO3
NitriteNO2
2CarbonateCO32
SulfateSO42
SulfiteSO32
3PhosphatePO43

It is easier to determine the chemical formula of ionic compounds using a series of simple steps instead of complex drawings.

How To: Writing the Chemical Formula of an Ionic Compound

  1. Write the chemical symbol and charge of the cation followed by the chemical symbol and charge of the anion.
  2. Drop the signs (+/).
  3. Rewrite the superscript values as subscripts following the opposite chemical symbol. If a subscript is placed after a polyatomic ion, place the polyatomic ion in parentheses followed by the subscript.
  4. If the newly written subscripts share a common factor, simplify them.

Let us consider a compound that consists of Pb4+ and CO32 ions. We begin by writing the cation followed by the anion: PbCO4+32.

We then remove the + and signs and rewrite the numerals as subscripts on the opposite species:

Notice that the polyatomic ion was placed in parentheses with the subscript four outside of the parentheses.

The chemical formula can be simplified by dividing the subscripts two and four by their greatest common factor: Pb(CO)32.

Example 4: Deducing the Ionic Formula of an Ionic Compound Where Both Ions Have a Charge That Is Greater Than One

Iron(III) ions have a charge of 3+ and oxide ions have a charge of 2. What is the formula of iron(III) oxide?

Answer

To determine the chemical formula of an ionic compound, we can begin by writing the chemical symbol and charge of the positively charged cation followed by the chemical symbol and charge of the negatively charged anion. The chemical symbol of iron is Fe. Oxide is the name of the anion of oxygen and has the chemical symbol O. We therefore write FeO3+2.

Then, we remove the + and signs and rewrite the numerals as subscripts following the opposite species:

We verify that the subscripts do not share a common factor and cannot be simplified. The formula of iron(III) oxide is FeO23.

In addition to the chemical formula, ionic compounds can also be identified by their chemical name.

How To: Naming an Ionic Compound

  1. Write the name of the cation, the species that appears first in the chemical formula; monatomic cations have the same name as the element.
  2. Write the name of the anion, the species that appears second in the chemical formula. Monatomic anions are named by writing the root of the element name followed by the suffix “-ide.”
  3. If the cation is an element that can form ions of multiple charges (e.g., Fe2+ and Fe3+), the magnitude of the cation charge should be written as a roman numeral in parentheses following the cation name. If the charge of the cation is unknown, the magnitude of the charge can be determined using the following equation: magnitudeofcationchargechargeofanionnumberofanionsnumberofcations=|||(×)|||.

Let us consider CuSO4. We begin by naming the cation. The cation is the species which appears first in the chemical formula. Cu appears first and is monatomic, containing only one atom. Thus, it is given the name copper: compoundname:copper.

The anion is SO4, a polyatomic ion called sulfate: compoundname:coppersulfate.

Copper is an element that can form ions of multiple charges. We will need to indicate which copper ion is in the compound by writing the magnitude of the ion’s charge as a roman numeral in parentheses following the word copper. We can determine the magnitude of the charge of the copper ion by using the following equation: magnitudeofcationchargechargeofanionnumberofanionsnumberofcations=|||(×)|||.

We need to be careful when counting polyatomic ions as the chemical formula of a polyatomic ion frequently contains subscripts. If the chemical formula contains more than one unit of a single type of polyatomic ion, the ion will be written in parentheses with a subscript number on the outside of the parentheses.

In the chemical formula CuSO4, there is one copper cation and one sulfate ion with a charge of 2. Substituting the values into the equation: magnitudeofcationchargemagnitudeofcationcharge=|||(2×1)1|||=2.

The magnitude of the cation charge of this copper ion is two, meaning that each copper cation has a charge of 2+. We write the magnitude of the cation charge as a roman numeral in parentheses following the cation name: compoundname:copper(II)sulfate.

Example 5: Naming a Binary Ionic Compound

What would the name of a compound of calcium and fluorine be?

Answer

Calcium is a metal and fluorine is a nonmetal. Metals and nonmetals typically form ionic compounds when they are combined together. The element calcium is found in group two of the periodic table. Atoms of elements in this group can form cations with a charge of 2+. The element fluorine is found in group 17. Atoms of elements in this group can form anions with a charge of 1.

When writing ionic chemical formulas and names, the cation will be the first species followed by the anion. The cation name is the same as the element name: compoundname:calcium.

The anion is monatomic, consisting of one atom. Monatomic anions are named by taking the root of the element name and adding the suffix “-ide”: compoundname:calciumuoride.

Calcium only forms ions with a charge of 2+. Therefore, we do not need to specify the charge of the cation with roman numerals. The name of a compound of calcium and fluorine is calcium fluoride.

The following flow charts can be used to help us name and write chemical formulas for simple ionic and covalent compounds. These flow charts cannot be used for more complex ionic and covalent compounds nor can they be used for naming hydrates, acids, or organic compounds.

Key Points

  • A chemical formula indicates the type and number of atoms in a molecule or formula unit.
  • Subscripts in a chemical formula indicate the number of atoms or ions, while coefficients in front of a chemical formula indicate the number of molecules of formula units.
  • Names and chemical formulas of molecular compounds are generally written starting with the least electronegative element first.
  • Greek prefixes are included in the names of binary molecular compounds to indicate the number of atoms of each element in the compound.
  • Names and chemical formulas of ionic compounds are written starting with the cation first.
  • Roman numbers are included in the name of an ionic compound when the cation element is capable of forming ions with multiple charges.

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