Chemists use chemical formulas to express the composition of molecules and ionic compounds by means of chemical symbols. Composition means not only the elements present, but also the ratio in which the atoms combine. It is necessary to become familiar with two types of formulas: molecular formulas and empirical formulas.
Molecular formulas
A molecular formula indicates the exact number of atoms of each element that are present in the smallest unit of a substance.
In the analysis of molecules, each example is presented with its molecular formula. Thus H2 is the molecular formula for hydrogen, O2 represents oxygen, O3 is ozone and H2O represents water. The numerical subscript indicates the number of atoms of each element that are present. In the case of H2O there is no subscript for O because there is only one oxygen atom in a water molecule; thus the subscript “one” is omitted from the formulas. Note that oxygen (O2) and ozone (O3) are allotropes of oxygen.
An allotrope is one of two or more different forms of an element.
Two allotropic forms of the element carbon-diamond and graphite-are completely different not only in their chemical properties, but also in their relative cost.
Molecular models
Molecules are too small to be observed directly. An effective way to visualize them is by using molecular models. Two types of molecular models are commonly used: sphere and rod models and spatial models.
In sphere and rod models the atoms are represented by wooden or plastic spheres with holes drilled in them. Rods or springs are used to represent chemical bonds. The angles formed between the atoms in the models approximate the actual bond angles of the molecules. With the exception of the H atom, all spheres are the same size and each type of atom is represented by a specific color.
In the space models the atoms are represented by truncated spheres that are held together under pressure so that the bonds are not visible. The size of the spheres is proportional to the size of the atoms. The first step in constructing a molecular model is to write the structural formula, which shows how the atoms of a molecule are bonded together. For example, it is known that in the water molecule each of the H atoms is bonded to an atom of O. Therefore, the structural formula for water is H-O-H.
A line joining two atomic symbols represents a chemical bond.
The sphere and rod models clearly show the three-dimensional distribution of atoms and are relatively easy to construct. However, the size of the spheres is not proportional to the size of the atoms. As a consequence, rods usually exaggerate the distance between atoms in a molecule. Spatial models are more accurate because they show the difference in the size of the atoms. The drawback is that they are more time-consuming to build and do not show the three-dimensional position of the atoms well.
Empirical formulas
The molecular formula of hydrogen peroxide, a substance used as an antiseptic and as a bleaching agent for textile fibers and hair bleach, is H2O2 This formula indicates that each molecule of hydrogen peroxide contains two hydrogen atoms and two oxygen atoms. The ratio of hydrogen atoms to oxygen atoms in this molecule is 2 : 2 or 1 : 1. The empirical formula of hydrogen peroxide is HO.
Consequently, the empirical formula indicates which elements are present and the minimum ratio, in whole number, between their atoms, but does not necessarily indicate the actual number of atoms in a given molecule.
As another example, consider the compound hydrazine (N2H4), which is used as rocket fuel. The empirical formula for hydrazine is NH2 The ratio of nitrogen to hydrogen is 1 : 2 in both the molecular formula (N2H4) and the empirical formula (NH2); only the molecular formula indicates the actual number of N (two) and H(four) atoms present in a hydrazine molecule.
The empirical formulas are the simplest chemical formulas, written in such a way that the subscripts of the molecular formulas are reduced to the smallest possible integers.
Molecular formulas are the true formulas of molecules. Their empirical formula.
For many molecules, the molecular formula and the empirical formula are the same.
Examples are water (H2O), ammonia (NH3), carbon dioxide (CO2) and methane (CH4).
