Understanding formulae
Reading Chemical Formulas
A compound is a substance of two or more chemically bonded elements. To read a chemical formula, it is necessary to note the number after the chemical symbol in the subscript (below the line). This number denotes the number of atoms of that element in the compound. If a symbol has no number in the subscript, only one atom of that element is present in that compound.
The chemical formula for water is H2O, where the '2' after the 'H' denotes that there are two atoms of hydrogen. The 'O' has no subscript number, so there is one atom of oxygen.
The way that compounds form, and the proportions in which they form, depend on the element's valency. Therefore, the elements of a compound need to exist in fixed proportions to each other to belong to that specific compound.
Consider water (H2O) and hydrogen peroxide (H2O2), which both contain the elements of hydrogen and oxygen, but in different proportions. In water, there are two hydrogen atoms to one oxygen atom, giving a 2:1 ratio. In hydrogen peroxide there are two hydrogen atoms to two oxygen atoms, giving a ratio of 1:1. See image 1 and image 2.
The properties of hydrogen and oxygen, which are both gases at room temperature, compared to their combination as a water molecule, which is a liquid at room temperature, shows how compounds possess different characteristics from the elements of which they are made. Similarly, the properties of water compared with hydrogen peroxide also differ, which means that the nature and behaviour of compounds relies on the proportion of elements in the compound.
Numbers in chemistry
Reading a chemical formula is reasonably straightforward but it is more difficult to write a formula from the chemical term. This is where an understanding of valency applies. Generally it will be possible to tell the ratio of one element to another by their combining power. From the chemical term 'sodium chloride' it is possible to deduce that the compound contains sodium and chlorine. Sodium has a combining power of +1 (one 'spare' electron) and chlorine has a combining power of -1 (one 'gap' for an electron). Sodium chloride, therefore, should only need one atom of sodium to one atom of chlorine. This ratio would be represented as NaCl.
In ionic compounds, the charge of the ions is written in superscript (above the line) after the chemical symbol. To do this for sodium chloride, it would look like this:
Na+ Cl-
It is possible, however, to have more than one ion of either charge. For example, when oxygen forms an ionic compound it has two anions, written like this:
O2-
Note that when there is only one ion the number is not included, but when there is more than one ion, the number comes after the chemical symbol but before the charge, in superscript.
It is simple to deduce the valency of any atom by its position on the periodic table in relation to the noble gases, which have a stable octet of electrons on their outer shell. Since noble gases do not have a combining power, consider group VIII as zero and then group I as +1 (one cation) and group VII as -1 (one anion) and so forth.
It must be remembered that the transition metals are an exception. Although they generally form two cations in a chemical reaction, their charge cannot always be predicted. For example, iron is Fe2+ or Fe3+ and copper is Cu+ or Cu2+ depending on the compound. Another common element to note is lead, which is Pb2+ or Pb4+.
| Group | Assumed Valency |
|---|---|
|
I |
+1 |
|
II |
+2 |
|
III |
+3 |
|
IV |
±4 |
|
V |
-3 |
|
VI |
-2 |
|
VII |
-1 |
|
VIII |
0 |
Radicals
Radicals are groups of atoms that have a charge. These polyatomic ions, ions containing more than one atom, usually combine to form compounds with elements or other radicals with an equally opposite charge.
For example, sulfate is a radical represented by the formula SO42-, where the five atoms of the sulfate radical usually remain together during chemical reactions. Sulfate has one sulfur atom to four oxygen atoms, giving it a negative charge of two anions. Copper sulfate combines copper (Cu2+) with the sulfate radical to make CuSO4, where the two positive charges of the copper balance the two negative charges of the sulfate radical.
Occasionally more than one radical or atom is needed to balance the equation. To create copper nitrate, for example, where nitrate has the formula NO3-, it would be necessary to balance copper's two positive charges with two nitrate radicals. The formula would look like this:
Cu(NO3)2
This represents a ratio of one copper atom to two nitrate radicals where each nitrate radical contains one nitrogen atom to three oxygen atoms. Note that the subscript number following the parentheses applies to every element in the molecule contained within the parentheses. The ingredients of the entire reaction are one copper atom, two nitrogen atoms and six oxygen atoms.
Clarifying numbers
It is common practice to indicate multiple polyatomic ions in parentheses with the subscript number after the parentheses. This is why copper nitrate has the formula Cu(NO3)2, rather than CuN2O6, to indicate that the elements in the parentheses, NO3, behaves as an ionic group rather than individual atoms. This information can be used to read formulae and identify radicals in a reaction.
Earlier it was mentioned that some elements have a variable valency, such as copper with Cu+ or Cu2+, depending on the compound. When copper reacts with oxygen, it can form one of two different compounds by losing two electrons or by losing one electron. Both of these are 'copper oxide' so Roman numerals are used to indicate which compound of copper oxide.
Copper (I) oxide for Cu2O, where copper becomes a Cu+ ion with a valency of one.
Copper (II) oxide for CuO, where copper becomes a Cu2+ ion with a valency of two.
See animation 1.
