Chemistry » Classification of Matter » Names And Formulae Of Substances

# Names and Formulae of Substances

## Names and formulae of substances

Think about what you call your friends. Some of your friends might have full names (long names) and a nickname (short name). These are the words we use to tell others who or what we are referring to. Their full name is like the substances name and their nickname is like the substances formulae. Without these names your friends would have no idea which of them you are referring to. Chemical substances have names, just like people have names. This helps scientists to communicate efficiently.

It is easy to describe elements and mixtures. We simply use the names that we find on the periodic table for elements and we use words to describe mixtures. But how are compounds named? In the example of iron sulfide that was used earlier, the compound name is a combination of the names of the elements but slightly changed.

The following are some guidelines for naming compounds:

1. The compound name will always include the names of the elements that are part of it.

1. A compound of iron ($$\text{Fe}$$) and sulfur ($$\text{S}$$) is iron sulfide ($$\text{FeS}$$)

2. A compound of potassium ($$\text{K}$$) and bromine ($$\text{Br}$$) is potassiumbromide ($$\text{KBr}$$)

3. A compound of sodium ($$\text{Na}$$) and chlorine ($$\text{Cl}$$) is sodiumchloride ($$\text{NaCl}$$)

2. In a compound, the element that is on the left of the Periodic Table, is used first when naming the compound. In the example of $$\text{NaCl}$$, sodium is a group 1 element on the left hand side of the table, while chlorine is in group 17 on the right of the table. Sodium therefore comes first in the compound name. The same is true for $$\text{FeS}$$ and $$\text{KBr}$$.

3. The symbols of the elements can be used to represent compounds e.g. $$\text{FeS}$$, $$\text{NaCl}$$, $$\text{KBr}$$ and $$\text{H}_{2}\text{O}$$. These are called chemical formulae. In the first three examples, the ratio of the elements in each compound is 1:1. So, for $$\text{FeS}$$, there is one atom of iron for every atom of sulfur in the compound. In the last example ($$\text{H}_{2}\text{O}$$) there are two atoms of hydrogen for every atom of oxygen in the compound.

4. A compound may contain ions (an ion is an atom that has lost or gained electrons). These ions can either be simple (consist of only one element) or compound (consist of several elements). Some of the more common ions and their formulae are given in the tables below. You should know all these ions.

 Compound ion Formula Compound ion Formula Compound ion Formula Hydrogen $$\text{H}^{+}$$ Lithium $$\text{Li}^{+}$$ Sodium $$\text{Na}^{+}$$ Potassium $$\text{K}^{+}$$ Silver $$\text{Ag}^{+}$$ Mercury (I) $$\text{Hg}^{+}$$ Copper (I) $$\text{Cu}^{+}$$ Ammonium $$\text{NH}_{4}^{+}$$ Beryllium $$\text{Be}^{2+}$$ Magnesium $$\text{Mg}^{2+}$$ Calcium $$\text{Ca}^{2+}$$ Barium $$\text{Ba}^{2+}$$ Tin (II) $$\text{Sn}^{2+}$$ Lead (II) $$\text{Pb}^{2+}$$ Chromium (II) $$\text{Cr}^{2+}$$ Manganese (II) $$\text{Mn}^{2+}$$ Iron (II) $$\text{Fe}^{2+}$$ Cobalt (II) $$\text{Co}^{2+}$$ Nickel $$\text{Ni}^{2+}$$ Copper (II) $$\text{Cu}^{2+}$$ Zinc $$\text{Zn}^{2+}$$ Aluminium $$\text{Al}^{3+}$$ Chromium (III) $$\text{Cr}^{3+}$$ Iron (III) $$\text{Fe}^{3+}$$ Cobalt (III) $$\text{Co}^{3+}$$ Chromium (VI) $$\text{Cr}^{6+}$$ Manganese (VII) $$\text{Mn}^{7+}$$

Table: Table of cations

 Compound ion Formula Compound ion Formula Fluoride $$\text{F}^{-}$$ Oxide $$\text{O}^{2-}$$ Chloride $$\text{Cl}^{-}$$ Peroxide $$\text{O}_{2}^{2-}$$ Bromide $$\text{Br}^{-}$$ Carbonate $$\text{CO}_{3}^{2-}$$ Iodide $$\text{I}^{-}$$ sulfide $$\text{S}^{2-}$$ Hydroxide $$\text{OH}^{-}$$ Sulfite $$\text{SO}_{3}^{2-}$$ Nitrite $$\text{NO}_{2}^{-}$$ Sulfate $$\text{SO}_{4}^{2-}$$ Nitrate $$\text{NO}_{3}^{-}$$ Thiosulfate $$\text{S}_{2}\text{O}_{3}^{2-}$$ Hydrogen carbonate $$\text{HCO}_{3}^{-}$$ Chromate $$\text{CrO}_{4}^{2-}$$ Hydrogen sulfite $$\text{HSO}_{3}^{-}$$ Dichromate $$\text{Cr}_{2}\text{O}_{7}^{2-}$$ Hydrogen sulfate $$\text{HSO}_{4}^{-}$$ Manganate $$\text{MnO}_{4}^{2-}$$ Dihydrogen phosphate $$\text{H}_{2}\text{PO}_{4}^{-}$$ Oxalate $$\text{COO}_{2}^{2-}$$/$$\text{C}_{2}\text{O}_{4}^{2-}$$ Hypochlorite $$\text{ClO}^{-}$$ Hydrogen phosphate $$\text{HPO}_{4}^{2-}$$ Chlorate $$\text{ClO}_{3}^{-}$$ Nitride $$\text{N}^{3-}$$ Permanganate $$\text{MnO}_{4}^{-}$$ Phosphate $$\text{PO}_{4}^{3-}$$ Acetate (ethanoate) $$\text{CH}_{3}\text{COO}^{-}$$ Phosphide $$\text{P}^{3-}$$

Table: Table of anions

1. Prefixes can be used to describe the ratio of the elements that are in the compound. This is used for non-metals. For metals, we add a roman number (I, II, III, IV) in brackets after the metal ion to indicate the ratio. You should know the following prefixes: “mono” (one), “di” (two) and “tri” (three).

• $$\text{CO}$$ (carbon monoxide) – There is one atom of oxygen for every one atom of carbon

• $$\text{NO}_{2}$$ (nitrogen dioxide) – There are two atoms of oxygen for every one atom of nitrogen

• $$\text{SO}_{3}$$ (sulfur trioxide) – There are three atoms of oxygen for every one atom of sulfur

### Tip:

When numbers are written as “subscripts” in compounds (i.e. they are written below and to the right of the element symbol), this tells us how many atoms of that element there are in relation to other elements in the compound. For example in nitrogen dioxide ($$\text{NO}_{2}$$) there are two oxygen atoms for every one atom of nitrogen. Later, when we start looking at chemical equations, you will notice that sometimes there are numbers before the compound name. For example, $$2\text{H}_{2}\text{O}$$ means that there are two molecules of water, and that in each molecule there are two hydrogen atoms for every one oxygen atom.

The above guidelines also help us to work out the formula of a compound from the name of the compound. The following worked examples will look at names and formulae in detail.

We can use these rules to help us name both ionic compounds and covalent compounds. However, covalent compounds are often given other names by scientists to simplify the name (or because the molecule was named long before its formula was discovered). For example, if we have 2 hydrogen atoms and one oxygen atom the above naming rules would tell us that the substance is dihydrogen monoxide. But this compound is better known as water!

Some common covalent compounds are given in the table below.

 Name Formula Name Formula water $$\text{H}_{2}\text{O}$$ hydrochloric acid $$\text{HCl}$$ sulfuric acid $$\text{H}_{2}\text{SO}_{4}$$ methane $$\text{CH}_{4}$$ ethane $$\text{C}_{2}\text{H}_{6}$$ ammonia $$\text{NH}_{3}$$ nitric acid $$\text{HNO}_{3}$$

Table: Names of common covalent compounds

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