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 used in international practice. The following examples will help you understand what the entries 3H, 2H2O, 5O2 mean and what information is obtained from this set of numbers and letters.
Numbers in expressions and their possible types
If you read 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) participating in the reaction. It is customary to write coefficients 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 ones (1/2, 1/3, 1/4 …).
For example, the numbers in the notation 3H, 2H2O and 5O2 in chemistry mean that we are given 3 hydrogen atoms H, 2 and 5 water molecules H2O and gaseous oxygen O 2respectively. 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 a molecule H2 or in solution as a positively charged ion H +.
If a particle or substance is involved in the equation in the singular, for example, the chlorine ion 1Cl- or the sulfuric acid molecule 1H2SO 4, coefficient "1" is omitted and written without it: Cl- and H2SO 4.
In the case of writing equations in a general form, characteristic of the reaction of polymerization, polycondensation, electrolysis and other chemical interactions, in the conditions of the problem, letters of the number of particles or substances can be used, such as n or x, y, z. The coefficient n usually denotes any natural number, it must be present in both parts of the equation (example 1), and any other letters of the English alphabet denote unknowns that must be found according to the condition of the problem (example 2).
Letter expressions and their informativeness
The characters of the English alphabet in the records 3H, 2H2O and 5O2 mean that substances contain certain chemical elements, as well as their compounds. Such an entry without numbers in front is called a chemical formula. So, the records 3H+, 2H2O and 5O2 mean that, let's say, 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 fromequations as a description of reactants or reaction products.
Subscript characters 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. To indicate the number of certain particles, subscript numbers or letters are used, which 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 in complex ions like [Cu(NH3) 4]2+
Letters are used in subscripts as designations for a certain number of atoms or units (n), as well as an unknown number of atoms in compounds when composing problems (a, b, x, y).
