Rules for Writing Formulas and Names of Inorganic Compound |
 Video tutorial |
 | To write the formula of a compound from its name, apply the following rules in the order written. |
The chemical formula shows the relative number of each type of atom present in a substance using elemental symbols to represent atoms and subscripted numbers to indicate the number of each type of atom.
For binary compounds (those containing two elements), the following rules should be applied in order:
For binary compounds (those containing two elements), the following rules should be applied in order:
- Apart from hydrogen, the element further to the left in the Periodic Table appears first in the formula. Examples include NaCl, MgO and Fe2O3.
- Hydrogen appears last except when the other element is from Group 16 or 17. Examples include LiH, NH3 and CH4 and H2O, H2O2 and HF.
- If both elements are from the same group, the lower one appears first. Examples include SiC, SO3 and ClF3.
- If the compound is ionic, the cation is written first and the anion is written second. Examples include NaCl, MgCl2 and AlI3.
Ionic compounds contain cations and anions:
Common polyatomic anions include sulfate SO42-, sulfite SO32-, nitrate NO3- and nitrite NO2-. Common polyatomic cations include the ammonium ion NH4+.
- The cation is written first, followed by the anion. Examples include NaCl and MgO.
- The total charge on an ionic compound is zero, so the numbers of cations and anions present must achieve this. As the sodium ion is Na+ and the chloride ion is Cl-, sodium chloride is NaCl. As the magnesium ion is Mg2+ however, two Cl- ions are required to balance its charge and so magnesium chloride is MgCl2.
- Msny common anions and a few cations contain are polyatomic (they contain a group of atoms). These are grouped together. The nitrate ion is NO3- and is always written in this way. As it has a charge of -1, sodium nitrate is NaNO3 and magnesium nitrate is Mg(NO3)2 to achieve charge balance. Note the use of brackets to show that Mg(NO3)2 contains two NO3- ions.
For a list of common ions, use our Common Cations and Anions Flashcards.
Covalent compounds contain atoms bonded through sharing of one or more pairs of electrons
- The order of the elements often follows the same rules as listed above for binary compounds.
- However, more commonly the formula is written to give an indication of the chemistry or the structure. Examples include the inorganic acids such as H2SO4, HNO3 and H3PO4 in which the replaceable hydrogen is written first. Similarly, chain compounds are written to show the connectivity. For example, cyanic acid is HOCN and isocyanic acid is HNCO to show that the hydrogen is bonded to O in the former and N in the latter.
- For organic compounds, the chemical formula are written with carbon first, followed by hydrogen and then the remaining elements in alphabetical order. More commonly structural formula are used which show more useful information including the functional groups present and the connectivity.
| To write the name of a compound from its formula, apply the following rules in the order written. |
Nonmetallic binary compounds:
For compounds made up of two nonmetallic elements:
For compounds made up of two nonmetallic elements:
- The element closer to the left of the Periodic Table appears first in the name. If both elements are in the same group, the lower one appears first in the name. This element retains its name in the compound name.
- The name of the second element is written with a root derived from its name with the suffix "ide" added.
Common roots:
element full name root As arsenic arsen Br bromine brom C carbon carb Cl chlorine chlor F fluorine fluor H hydrogen hydr I iodine iod N nitrogen nitr P phosphorus phosph S sulfur sulf
- Numerical prefixes based on the Greek, di-, tri-, tetra-, penta-, hexa- are used to indicate the presence of more than one of any given element. For example, CO is carbon monoxide and CO2 is carbon dioxide. Note that in the former (and whenever the prefix ends in "o" or "a" and the element begins with a vowel), the last letter of the prefix is dropped.
- By convention, compound names are written in lower case except where the name begins a sentence.
- There is a single space between the two parts of the name.
Binary compounds of hydrogen:
- Hydrogen appears last (and is written as "hydride") in the name except when the other element is from group 16 or 17. Examples include LiH is lithium hydride and HCl is hydrogen chloride.
- Many hydrogen compounds of group 13, 14, 15 and 16 elements have commonly used, unsystematic names. Examples include water (H2O) and hydrogen peroxide (H2O2) for oxygen, diborane (B2H6) and many others for boron, methane (CH4) and many others for carbon, silane (SiH4), ammonia (NH3) and hydrazine (N2H4) for nitrogen and phosphine (PH3) for phosphorus.
- By convention, compound names are written in lower case except where the name begins a sentence.
- There should be a single space between the two parts of the name.
Ionic compounds:
Binary compounds containing a metal and a non-metal are commonly ionic.
Binary compounds containing a metal and a non-metal are commonly ionic.
- The cation is written first and the anion is written second.
- The name of the anion is written with a root derived from its name with the suffix "ide" added.
Common roots:
element full name root As arsenic arsen Br bromine brom C carbon carb Cl chlorine chlor F fluorine fluor H hydrogen hydr I iodine iod N nitrogen nitr P phosphorus phosph S sulfur sulf
- For metals like those of groups 1 and 2 that only form one oxidation state, the number of cations and anions required for charge balance can be derived without it being stated in the name. Hence, the Latin prefixes used in other non-ionic binary compounds are not required. For example, magnesium iodide must be MgI2 since magnesium only forms Mg2+ so has to combine with two I- anions.
- For other metals, several oxidation states are possible. This is very common for the transition metals but also occurs for elements like tin (Sn2+ and Sn4+) and lead (Pb2+ and Pb4+). In this case, the name of the cation is the element name followed by the oxidation number in Roman numerals in brackets. There is no space between the element name and its oxidation number: both together constitute the name of the cation. Examples include iron(II) and iron(III) for Fe2+ and Fe3+ respectively and manganese(II), manganese(III), manganese(IV), manganese(V) and manganese(VII) for Mn2+, Mn3+, Mn4+, Mn5+ and Mn7+ respectively.
- Many common anions and a few cations contain are polyatomic (they contain a group of atoms). These are treated in the same way as other cations and anions and the name is written with the cation first and the anion second. Common polyatomic anions include sulfate SO42-, sulfite SO32-, nitrate NO3- and nitrite NO2-. Common polyatomic cations include the ammonium ion NH4+.
For a list of common ions, use our Common Cations and Anions Flashcards. - There is a single space between the cation and the anion.
- By convention, compound names are written in lower case except where the name begins a sentence.
| Work through these examples carefully to see how the rules are applied and then test yourself with the self test quiz. |
1. The formula of the compound calcium nitrate is Ca(NO3)2:
- The cation and anion are written as separate words with a space inbetween: the cation is calcium and the anion is nitrate.
- Calcium, like the other group 2 metals, always forms a +2 cation. The nitrate anion is the polyatomic NO3- ion. As two NO3- ions are needed to balance the charge of one Ca2+ ion, the formula is Ca(NO3)2.
- The polyatomic nitrate ion is written inside brackets to show that that the subscript that follows it means that there are two NO3- ions per one Ca2+.
2. The name of the ionic compound Al2(CO3)3 is aluminium carbonate:
- The cation is aluminium which always has an oxidation number of +3 in ionic compounds so its oxidation number is not included.
- The anion is written in brackets and can be recognized to be the polyatomic carbonate anion CO32-.
- The name is written in lower case with the cation first and the anion second. There is a single space between them.
3. The formula of bromine trifluoride is BrF3:
- As in the name, Br appears first in the formula as it is in the same group but lower in the Periodic Table than F.
- As suggested by the Latin prefix of "tri", there are three fluorine atoms present for every bromine in the molecule so the formula is BrF3.
4. The name of SF4 is sulfur tetrafluoride:
- Sulfur and fluorine are both nonmetallic. Note that the spelling of sulfur - "sulphur" is incorrect.
- S is to the left of F in the Periodic Table so sulfur is written first in the name, just as it is in the formula.
- The second element is therefore fluorine. This is convert to fluoride by adding the suffix "ide" to the root "fluor".
- There are four F atoms per sulfur in the formula. This is indicated in the name using the Latin prefx "tetra".
- Sulfur is to the left of fluorine in the Periodic Table so appears before it in both the formula and the name.
- The name is written in lower case with a single space between the two parts.
5. The formula of iron(III) sulfate is Fe2(SO4)3:
- The first word gives the name of the cation. The cation is therefore iron(III). This is an iron atom with a +3 oxidation number.
- The second word gives the name of the anion. The anion is therefore sulfate. This is the polyatomic SO42- ion.
- As the cation has a charge of +3 and the anion has a charge of -2, the simplest ratio which leads to charge balance is two cations : three anions.
- The formula is therefore written, with cation followed by anion, as Fe2(SO4)3. Note that brackets are written around the anion to show that it is polyatomic and that the subscript following it refers to their being three SO42- for every two Fe3+.
6. The name of N2O5 is dinitrogen pentoxide
- Nitrogen and oxygen are both nonmetallic.
- N is to the left of O in the Periodic Table so nitrogen is written first in the name, just as it is in the formula.
- The second element is therefore oxygen. This is convert to oxide by adding the suffix "ide" to the root "ox".
- There are two N atoms and five O atoms so the prefixes "di" and "penta" are added to these names to give dinitrogen and pentoxide respectively. Note that as the Latin prefix "penta" ends in "a" and "oxide" begins with the vowel "o", the last letter of the prefix is dropped.
- The formula is written in lower case with a single space between the two parts.
7. The formula of magnesium phosphate is Mg3(PO4)2:
- The first word gives the name of the cation. The cation is therefore magnesium: Mg2+
- The second word gives the name of the anion. The anion is therefore phosphate. This is the polyatomic PO43- ion.
- As the cation has a charge of +2 and the anion has a charge of -3, the simplest ratio which leads to charge balance is three cations : two anions.
- The formula is therefore written, with cation followed by anion, as Mg3(PO4)2. Note that brackets are written around the anion to show that it is polyatomic and that the subscript following it refers to their being two PO43- for every three Mg2+.
8. The name of SrSO3 is strontium sulfite
- The cation is strontium which, being a group 2 metal, always has an oxidation number of +2.
- The anion is the polyatomic SO32- ion. This is the sulfite ion. Note that this is different to the SO42- ("sulfate") and S2- ("sulfide").
- As there is only one SO32- for every one Sr2+ and so no bracket was used in the formula for the polyatomic anion. This can make the polyatomic anion hard to spot!
- The formula is written in lower case with the cation written before tha anion and a single space between them.
9. The formula of ammonium nitrite is NH4NO2:
- The first word gives the name of the cation. The cation is the polyatomic ammonium ion: NH4+
- The second word gives the name of the anion. The anion is therefore nitrite. This is the polyatomic NO2- ion.
- As the cation has a charge of +1 and the anion has a charge of +1, the simplest ratio which leads to charge balance is one cation : one anion.
- The formula is therefore written, with cation followed by anion, as NH4NO2. Note that no brackets are written around the polyatomic cation as there is only one in the formula. Similarly, no brackets are written around the the polyatomic anion for the same reason. It is important to be able to spot and recognize polyatomic ions when written in formula and compound names!
10. The name of Ca(HCO3)2 is calcium hydrogencarbonate
- The cation is calcium which, being a group 2 metal, always has an oxidation number of +2.
- The anion is the polyatomic HCO3- ion. This is the hydrogencarbonate ion. Note that this is written as one word - it is not "hydrogen carbonate"!.
- The formula is written in lower case with the cation written before tha anion and a single space between them.