3.2: Chemische Formeln
A chemical formula presents information about the proportions of atoms constituting a particular chemical compound or molecule, mainly using symbols of elements and numbers. At times other symbols, such as dashes, parentheses, brackets, commas, plus, and minus signs, are also used. A chemical formula can be one of three types – molecular, empirical, and structural.
The molecular formula represents a molecule or a chemical compound using chemical symbols (to indicate the types of atoms) and subscript-numbers (to show the number of atoms of each type in the molecule). Molecular formulas are also used as abbreviations for the names of compounds.
Some elements consist of discrete, individual atoms, like the noble gases helium, neon, and argon. Such elements are called monatomic gases and have molecular formulas He, Ne and Ar, respectively. Few elements exist as molecules made up of two or more chemically bonded atoms of the same element. For example, elements such as hydrogen, oxygen, and nitrogen exist as diatomic molecules containing two atoms each and thus have the molecular formulas H2, O2, and N2, respectively. Similarly, elements such as phosphorus and sulfur exist as polyatomic molecules with molecular formulas P4 and S8, respectively.
It is important to note that a subscript following a symbol and a number in front of a symbol does not represent the same thing; for example, H2 and 2H represent distinctly different species. H2 is a molecular formula; it represents a diatomic molecule of hydrogen, consisting of two atoms of the element that are chemically bonded together. The expression 2H, on the other hand, indicates two separate hydrogen atoms that are not combined as a unit. The expression 2H2 represents two molecules of diatomic hydrogen.
Compounds are formed when two or more elements chemically combine, resulting in the formation of bonds. For example, hydrogen and oxygen can react to form water, and sodium and chlorine can react to form sodium chloride or table salt. We sometimes describe the composition of these compounds with an empirical formula, which indicates the types of atoms present and the simplest whole-number ratio of the number of atoms (or ions) in the compound. For example, titanium dioxide (used as a pigment in white paint and in the thick, white, blocking type of sunscreen) has an empirical formula of TiO2. This identifies the elements titanium (Ti) and oxygen (O) as the constituents of titanium dioxide and indicates the presence of twice as many atoms of the element oxygen as atoms of the element titanium.
In many cases, the molecular formula of a substance is derived from an experimental determination of both its empirical formula and its molecular mass (the sum of atomic masses for all atoms composing the molecule). For example, it can be determined experimentally that benzene contains two elements, carbon (C) and hydrogen (H), and that for every carbon atom in benzene, there is one hydrogen atom. Thus, the empirical formula is CH. An experimental determination of the molecular mass reveals that a molecule of benzene contains six carbon atoms and six hydrogen atoms, so the molecular formula for benzene is C6H6.
The structural formula for a compound gives the same information as its molecular formula (the types and numbers of atoms in the molecule) but also shows how the atoms are connected in the molecule. In other words, the structural formula represents the molecular structure of a compound graphically. It shows the probable arrangement of the atoms and how the atoms are connected in real three-dimensional space, giving more information about the molecular geometry. Lines are used to represent bonds present between the atoms. The formula may also depict the approximate bond angles, giving a sense of the molecule’s shape.
This text is adapted from Openstax Chemistry 2e, Section 2.4: Chemical Formulas.