In this explainer, we will certainly learn just how to write and interpret the notations provided in chemistry formulas.

You are watching: Use subscripts to represent the number of each type of atom or ion in a molecule.

Molecules and compounds consists of various atoms and also ions. Chemistry have arisen ways come talk about these molecules and compounds to distinguish them from one another and indicate your composition. Let us consider a molecule of water.


Water molecule consist of 2 hydrogen atoms and one oxygen atom. We have the right to represent the ingredient of a water molecule by writing chemical symbols for the hydrogen and oxygen atoms and also using subscript numerals to suggest the variety of each atom type: HO2.

This is the chemistry formula of water. The hydrogen and also oxygen atom are stood for with the chemical icons H and O and the variety of each atom kind is shown with a subscript numeral. Subscript worths that are bigger than one space written the end explicitly however the subscript worth of one is only implied and also not composed out explicitly. This is why the chemistry formula of water is created as HO2 rather than HO21.

Definition: chemical Formula

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

Let us consider a sample that consists of three water molecules.


We recognize that each water molecule has the chemical formula HO2. We have the right to place a three in former of this chemistry formula to show that there room three water molecules: 3HO2. The character three is a coefficient. Coefficients suggest the variety of discrete systems (molecules or formula units). The is important to distinguish in between subscript values and also coefficients. Subscript values constantly indicate the variety of atoms or ions in a solitary unit that a compound, when coefficients suggest the variety of discrete units.

Example 1: Calculating the Total number of Atoms the an aspect in a complicated Formula

How numerous atoms the oxygen are present in the formula 10SO2?


Looking at the formula, we have the right to see the we have actually two different types of numerals: coefficients and also subscripts. Coefficients are numerals which show up in former of a chemical formula. The coefficient terms indicate the number of discrete units. Therefore, the worth ten in the 10SO2 chemical formula indicates that there room ten separate SO2 molecules.

Subscripts room numerals which show up after a chemistry symbol in a chemical formula. Subscripts indicate the variety of atoms or ion in a solitary unit. Subscript values of one are assumed to it is in on the right-hand side of the chemical symbol if there is no various other number written in this space. Therefore, one molecule of SO2 consists of one sulfur atom and also two oxygen atoms.

With this details in mind, we can answer this question in 2 ways. If we recognize that one molecule of SO2 consists of two oxygen atoms, climate ten molecules have to contain ten times as many oxygen atoms as offered by the adhering to equation: 21×10=20.oxygenatomsmoleculeofSOmoleculesofSOoxygenatoms22

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


Thus, the formula 10SO2 will contain ten time one sulfur atoms and ten times 2 oxygen atoms. There room 20 oxygen atoms present in the formula 10SO2.

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

A binary molecule compound consists of atoms of 2 different aspects which are covalently external inspection together. The aspects in concern are frequently nonmetals yet may also be metalloids.


The chemistry formula that a binary molecular link will typically be written through the much less electronegative element’s chemistry symbol an initial followed by the much more electronegative element’s chemistry symbol.


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

Binary molecular compounds that perform not consist solely of carbon and also hydrogen are basic to name by complying with a few simple rules.

How To: naming a Binary molecule Compound the Is no a Hydrocarbon

Write the surname of the aspect which appears first in the chemical formula.Write the source of the surname of the aspect which appears second in the chemical formula.Add the suffix “-ide” to the root.Add a prefix in former of each types name to suggest the variety of atoms that that facet in the chemical formula. If over there is only among the an initial element, perform not create a prefix in front of the name.
Greek Prefixes supplied When specify name Binary molecular CompoundsNumber of AtomsPrefix
1Mono- (never provided in front of the very first element’s name)

If the prefix in front of oxygen or oxide end in a- or o-, the critical letter of the prefix might be dropped.

By complying with the rules because that naming binary molecular compounds, we deserve to identify NO24 as dinitrogen tetroxide.

There are number of binary molecular compounds that are not typically named complying with the conventions and instead are frequently referred come by their common names. Examples are shown in the table below.

Binary molecular Compounds with usual NamesChemical FormulaCommon Name
NO2Laughing gas
HS2Hydrogen sulfide

Example 2: specify name a Binary molecule Compound

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


Using a routine table, we have the right to identify chlorine (Cl) and oxygen (O) together nonmetals. Compounds that contain two nonmetals room binary molecular compounds. The formula and name of a binary molecular compound is typically written v the less electronegative aspect first, complied with by the more electronegative element. However, when chlorine, bromine, or iodine is an unified with oxygen, the halogen is always written first. Thus, in both the formula and name, chlorine will show up before oxygen.

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

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

Next, we require to suggest how plenty of atoms the each facet are in the compound by adding a Greek prefix in prior of the corresponding facet name. Over there is one atom the chlorine and also two atom of oxygen in the compound. The Greek prefix because that one is “mono-” and the Greek prefix for 2 is “di-.” The prefix “mono-” is never inserted in prior of the first element. Therefore, we will certainly only add the prefix “di-” in former of oxide: compoundname:chlorinedioxide.

The name of a link containing one atom the chlorine and also two atom of oxygen will certainly be chlorine dioxide.

Example 3: convert a complicated Chemical Name v Prefixes into a chemical Formula

What is the chemical formula of dichlorine heptoxide?


A chemical formula is one expression of chemistry symbols and also numerical subscripts the represents the composition of one unit that a compound. Prior to we can write a chemistry formula, we must an initial identify the form of compound, together different types of compound have different rules because that naming and formula writing.

The Greek prefixes “di-” and “hept-” show that this is the name of a molecular compound. To write the chemistry formula the a molecule compound, us can begin by separating the name into prefixes, source words, and also suffixes.

The root words can be provided to identify the elements in the compound. The an initial element is chlorine, which has the chemical symbol Cl. The second element has the root “ox.” using the routine table, we deserve to identify the element that shares this root as oxygen, which has actually the chemical symbol O. We can write these chemical signs in the exact same order in i m sorry the elements show up in the name: chemicalformula:ClO.

The prefixes indicate the variety of atoms of every element. The Greek prefix “di-” way two and also the prefix “hept-” way seven. We represent the number of atoms of each element in the chemical formula v a subscript character that appears immediately ~ the corresponding element. Thus, we deserve to place a subscript 2 after cl and also a subscript seven after O: chemicalformula:ClO27.

The chemistry formula the dichlorine heptoxide is ClO27.

Ionic compounds consist of positively charged cations and negatively fee anions that space bound together through electrostatic interactions. The cations are normally metal ions and the anions are usually nonmetal ions. The following figure shows the dues of several typical ions.

Elements in the d block of the periodic table are typically able to type ions through multiple charges. Because that example, one atom of stole may kind the Fe2+ or Fe3+ ion.

Ionic compounds exist as a three-dimensional network of plenty of cations and also anions quite than as discrete molecular units. That is impossible to indicate the total number of cations and also anions in one ionic compound through a chemistry formula. Instead, the chemical formula represents the simplest ratio of cations and anions through the cation term composed first. Because that example, table salt consists of plenty of sodium ions (Na+) and also chloride ion (Cl–), yet the ratio of salt ions and also chloride ions in the compound is 1∶1. Thus, the chemistry formula the table salt is NaCl.

Likewise, one ionic link consisting of magnesium ions, Mg2+, and bromide ion would have actually the chemical formula MgBr2.

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

Chemical formulas space written likewise for ionic compounds the contain either simple monatomic ion or more complicated polyatomic ions. Permit us think about a compound composed of phosphate ions and zinc ions, Zn2+.

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

Notice the the polyatomic ion is inserted in parentheses with a subscript two outside of the parentheses. This subscript shows that there room two phosphate ions.

The table below lists several typical polyatomic ions.

Polyatomic IonsChargeIon NameIon Formula1+1−2−3−

It is simpler to identify the chemical formula that ionic compounds utilizing a series of basic steps instead of facility drawings.

How To: creating the chemical Formula of one Ionic Compound

Write the chemical symbol and also charge that the cation complied with by the chemical symbol and also charge the the anion.Drop the indicators (+/−).Rewrite the superscript values as subscripts adhering to the opposite chemistry symbol. If a subscript is put after a polyatomic ion, place the polyatomic ion in parentheses followed by the subscript.If the newly written subscripts share a typical factor, leveling them.

Let us consider a link that is composed of Pb4+ and CO32– ions. We start by writing the cation adhered to by the anion: PbCO4+32–.

We then eliminate the + and also – signs and rewrite the numerals together subscripts on opposing species:

Notice that the polyatomic ion was inserted in parentheses through the subscript four outside of the parentheses.

The chemical formula have the right to be simplified by splitting the subscripts two and also four by their greatest usual factor: Pb(CO)32.

Example 4: Deducing the Ionic Formula of one Ionic Compound where Both Ions have a charge That Is better Than One

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


To determine the chemistry formula of an ionic compound, us can begin by composing the chemistry symbol and also charge that the positively fee cation adhered to by the chemistry symbol and also charge the the negatively charged anion. The chemistry symbol of steel is Fe. Oxide is the surname of the anion that oxygen and has the chemical symbol O. We therefore write FeO3+2–.

Then, we remove the + and also – signs and also rewrite the numerals as subscripts complying with the opposite species:

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

In enhancement to the chemistry formula, ionic link can likewise be established by your chemical name.

How To: specify name an Ionic Compound

Write the surname of the cation, the varieties that appears an initial in the chemical formula; monatomic cations have actually the very same name as the element.Write the name of the anion, the types that appears second in the chemistry formula. Monatomic anions are called by composing the root of the facet name adhered to by the suffix “-ide.”If the cation is an facet that can form ions that multiple charges (e.g., Fe2+ and Fe3+), the magnitude of the cation charge have to be written as a roman character in parentheses adhering to the cation name. If the charge of the cation is unknown, the size of the charge can be figured out using the following equation:


Let us take into consideration CuSO4. We begin by specify name the cation. The cation is the types which appears an initial in the chemical formula. Cu appears very first and is monatomic, containing only one atom. Thus, that is given the name copper: compoundname:copper.

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

Copper is an element that can kind ions of multiple charges. Us will require to show which copper ion is in the compound by writing the size of the ion’s charge as a roman character in parentheses following the word copper. We have the right to determine the size of the charge of the copper ion by utilizing the adhering to equation: magnitudeofcationchargechargeofanionnumberofanionsnumberofcations=|||(×)|||.

We need to be careful when counting polyatomic ion as the chemistry formula that a polyatomic ion generally contains subscripts. If the chemistry formula contains an ext than one unit that a single form of polyatomic ion, the ion will be written in parentheses with a subscript number ~ above the outside of the parentheses.

In the chemistry formula CuSO4, there is one copper cation and also one sulfate ion with a fee 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, an interpretation that every copper cation has actually a fee of 2+. We create the size of the cation fee 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 link of calcium and also fluorine be?


Calcium is a metal and also fluorine is a nonmetal. Metals and nonmetals typically form ionic compounds when they are combined together. The element calcium is discovered in group two that the periodic table. Atom of elements in this team can kind cations through a fee of 2+. The aspect fluorine is found in team 17. Atoms of facets in this team can kind anions through a charge of 1−.

When composing ionic chemical formulas and also names, the cation will be the first species complied with by the anion. The cation name is the exact same as the aspect name: compoundname:calcium.

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

Calcium only creates ions through a charge of 2+. Therefore, we carry out not need to specify the fee of the cation v roman numerals. The surname of a link of calcium and fluorine is calcium fluoride.

The following circulation charts have the right to be provided to help us name and also write chemistry formulas for basic ionic and covalent compounds. These flow charts can not be used for more complex ionic and covalent link nor deserve to they be provided for specify name hydrates, acids, or necessary compounds.

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Key Points

A chemical formula shows the kind and variety of atoms in a molecule or formula unit.Subscripts in a chemistry formula indicate the number of atoms or ions, while coefficients in front of a chemistry formula indicate the number of molecules or formula units.Names and chemical formulas of molecular compounds are usually written starting with the the very least electronegative aspect first.Greek prefixes are contained in the names of binary molecule compounds to show the variety of atoms that each facet in the compound.Names and chemical recipe of ionic compounds are written beginning with the cation first.Roman number are had in the name of one ionic compound once the cation aspect is capable of forming ions through multiple charges.