Atoms can form bonds in two ways: ionic and covalent. Ionic compounds consist of oppositely charged ions held together by electrical attraction, while covalent compounds involve the sharing of electrons. Formula units show the smallest ratio of atoms in a compound. Empirical formulas show the simplest ratio of elements, while molecular formulas show the actual number of atoms. The determination of formula units involves knowing the oxidation number of the elements involved. Some elements can have multiple oxidation numbers, leading to complications in determining formula units.
Atoms of different elements can join in two main ways. In an ionic bond, a metal donates one or more electrons to a nonmetal, forming oppositely charged ions which are held together by electrical attraction. Covalent bonds are formed by two or more nonmetals sharing electrons. Ionic compounds do not form molecules as such, but instead, in solid form, consist of three-dimensional crystal lattices containing large numbers of atoms. Some covalent compounds can form similar crystalline networks. A formula unit is the smallest ratio of atoms of different elements in this type of structure that can be expressed as whole numbers.
Ionic compounds
A simple example is sodium chloride, or common salt, a compound of the elements sodium and chlorine. A salt crystal consists of positively charged sodium ions bonded to negatively charged chloride ions: negative ions formed from non-metals end in “–ide”. The crystal contains huge numbers of sodium and chloride ions, but there is one chloride ion for every sodium ion, so the unit of the formula shows one of each. The chemical symbols for sodium and chlorine are Na and Cl respectively, so the unit of the formula is written as NaCl.
Many ionic compounds are slightly more complex, such as aluminum oxide. Here, oxygen is looking for two electrons and aluminum is looking for three. They can therefore form a stable compound with the formula unit Al2O3. The number of atoms of an element in any type of chemical formula appears in subscript and to the right of the symbol for that element. If there is only one atom, the subscript is omitted.
Covalent compounds
Although covalent compounds and substances often form distinct, self-contained molecules, they can also form crystalline structures. For example, silicon dioxide, also known as silica, can form crystals. These are commonly known as quartz and, like salt, are made up of a huge number of two different atoms, in this case silicon and oxygen, but held together by covalent bonds. Since the ratio of oxygen to silicon atoms is 2:1, quartz has the formula unit SiO2.
Related terms
There are a couple of other related terms that might cause confusion. Empirical formula is a more general term for the simplest ratio of elements in a compound, ionic or covalent, crystalline or not. In a crystalline compound, it is the same as the formula unit, but the term also applies to self-contained, non-crystalline covalent molecules. The molecular formula is the actual number of atoms of each element in a self-contained covalent molecule and does not apply to ionic compounds, as these do not form distinct molecules.
In ionic compounds, the formula unit tends to be used to show the simplest ratio of atoms, whereas in non-crystalline covalent compounds, the usual term for this is the empirical formula. For example, the carbon-hydrogen compounds acetylene and benzene both contain the same number of carbons as hydrogen atoms, and therefore both have the empirical formula CH. The molecular formula of acetylene, on the other hand, is C2H2, while that of benzene is C6H6. These are very different compounds, with different properties.
In many covalent compounds, the empirical formula and the molecular formula are the same. In water, for example, they are both H2O. However, this is rare in the case of organic compounds, which can be very complex. In these compounds, there is often more than one possibility for the same ratio of elements, as already noted with acetylene and benzene. Sometimes there are many different variations.
In many cases, even the molecular formula doesn’t tell the whole story. For example, glucose and fructose, two different types of sugar, have the same molecular formula, C6H12O6. The hydrogen and oxygen atoms are, however, arranged slightly differently, giving the two compounds slightly different properties. The empirical formula for glucose and fructose is CH2O.
Determination of the formula unit for a compound
In many cases, all that is needed to find the formula unit for an ionic compound, and for some simple crystalline covalent compounds, is knowledge of how many single bonds the elements are capable of forming. In the case of metals it is the number of electrons they can supply, while in non-metals it is the number of electrons they can accept or, in the case of covalent compounds, share. This is known as the oxidation number. It is usually positive in metals – which lose negatively charged electrons in forming compounds – and negative in nonmetals – which gain electrons, at least when combining with metals.
Returning to the example of aluminum oxide, aluminum has an oxidation number of +3, while oxygen has an oxidation number of -2. To find the unit of the formula for aluminum oxide, these numbers are simply swapped to obtain a compound with three oxygen atoms for every two of aluminum: Al2O3. The same procedure works for many other ionic compounds and some simple covalent compounds. There are, however, complications, because some elements can have more than one oxidation number, depending on the circumstances. Iron, for example, can be +2 or +3, and many nonmetals can have multiple oxidation numbers, which can be positive in some covalent compounds.
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