Those who are just starting to get acquainted with chemistry and its inherent types of designation of various substances and equations need to know certain rules that are used in international practice. The following examples will help you understand what 3H, 2H2O, 5O2 meanings and what information is obtained from this set of numbers and letters.
Numbers in expressions and their possible types
If you read the expressions from left to right, the so-called coefficients are always written first. These indicators characterize the number of particles or substances (atoms, molecules, ions, moles) involved in the reaction. The coefficients are usually written in Arabic numerals: usually these are all positive natural numbers (1, 2, 3 ...), unless we are talking about equivalents, where they can be written as fractional (1/2, 1/3, 1/4 ...).
For example, the numbers in the records of 3H, 2H2O and 5O2 in chemistry mean that we are given 3 hydrogen atoms H, 2 and 5 water molecules H 2 O and gaseous oxygen O 2, respectively. It should be noted that the first example is incorrect, since atomic H does not exist in nature, but is present only in the form of an H 2 molecule or in solution as a positively charged H + ion .
If a particle or substance participates in the singular in the equation, for example, a 1Cl chlorine ion or 1H 2 SO 4 sulfuric acid molecule, the coefficient “1” is omitted and written without it: Cl - and H 2 SO 4 .
If the equations are written in a general form, which is characteristic of the polymerization, polycondensation, electrolysis, and other chemical interactions, in the conditions of the problem, lettering of the number of particles or substances, such as n or x, y, z, can be used. The coefficient n usually denotes any natural number, it must be present in both sides of the equation (example 1), and any other letters of the English alphabet indicate unknowns that must be found by the condition of the problem (example 2).
Literal expressions and their information content
The characters of the English alphabet in the entries 3H, 2H2O and 5O2 mean that the substances contain certain chemical elements, as well as their compounds. Such a record without numbers in front is called the chemical formula. Thus, the records 3H + , 2H2O and 5O2 mean that, for example, 3 hydrogen ions, 2 and 5 water and oxygen molecules, respectively, participate in the reaction. But such expressions can also be indicated in the text and in a separate form from the equations as a description of the reactants or reaction products.
Subscripts and their designations
The composition of substances can include many atoms of individual elements, as well as the compounds themselves have repeating units depending on their nature and structure. Subscript numbers or letters are used to indicate the number of particles. They have the same designation as the main coefficients. For example, the lower numbers in the entries 3H + , 2H2O and 5O2 mean that such symbols can only be found in complex ions of the type [Cu (NH 3 ) 4 ] 2+ , and in the molecules of substances such as water and oxygen, there are 2 atoms each hydrogen and oxygen.
The letters are used in subscripts as designations of a certain number of atoms or units (n), as well as an unknown number of atoms in compounds in the preparation of tasks (a, b, x, y).