All chemicals are composed of molecules that are the smallest particles. They are electrically neutral and are composed of atoms that are linked by covalent bonds. The ability of atoms of chemical elements to form a certain number of bonds in molecules is called valency. In connection with the emergence and development of the theory of chemical bonds, this concept has undergone significant changes and currently does not have a clear scientific interpretation, is used mainly for methodological purposes and is often identified with the degree of oxidation. If a molecule is the smallest particle of a substance, then an atom is the smallest indivisible particle of a chemical element. An atom is also an electrically neutral particle, in its center there is a nucleus consisting of positively charged protons, and around it is a cloud of electrons (negatively charged). Therefore, the positive charge of the nucleus is balanced by the negative charge of electrons. To understand how to determine the degree of oxidation and what it is, you need to consider the structure of the molecules of specific substances.
In a molecule of hydrogen chloride (HCl), one common pair of electrons exists between hydrogen and chlorine. The hydrogen atom has 1 proton and 1 electron. The chlorine atom has 17 protons and 17 electrons, one of 7 electrons in the outer shell of the electron density combined with the electron of the hydrogen atom, thus forming an electron pair: H: Cl. In the H2S molecule, the sulfur atom formed 2 common electron pairs with both hydrogen atoms: H: S: H. How to determine the oxidation state of an element, for example, chlorine and hydrogen in the HCl molecule and hydrogen and sulfur in the H2S molecule? Valence gives an idea of the number of bonds or electron pairs formed in the molecule, but does not show their displacement.
It is assumed that the electrons of the atoms in the molecule are combined into electron pairs that are shifted to the most electronegative atom. Hydrogen and chlorine in HCl are able to enter into one bond, which is shifted to the chlorine atom, since it is the most electronegative. Thus, in hydrogen, the oxidation state is plus 1, and in chlorine it is minus 1. Each hydrogen atom in the H2S molecule enters into one bond with a sulfur atom, which, in turn, has two bonds. In this case, both electron pairs are shifted to the sulfur atom. Therefore, the hydrogen oxidation state is plus 1, and the sulfur atom is minus 2. How to determine the oxidation state of a substance, for example, HCl, H2S, O2, N2, Na, Ag, Si? The oxidation state of a simple or complex substance is always 0, since the substance consists of molecules or atoms, and they are electrically neutral. Based on this, find the degree of oxidation of the element in the molecule.
Using the hypochlorous acid example, we can consider how to determine the oxidation state for the Cl atom. In the HClO molecule, as in the molecule of any other substance, hydrogen is always (except hydrides) characterized by the oxidation state plus 1, and oxygen is always characterized by the oxidation state minus 2 (except for barium peroxide and oxygen fluoride). Since the molecule itself has no charge, we can write the equation: (+1) + x + (- 2) = 0. This equation needs to be solved: x = 0 - (+1) - (-2) = (+1) Thus, it turns out that the oxidation state of Cl in the hypochlorous acid molecule is plus 1. This example shows how to determine the oxidation state of a particular element in any molecule. First of all, the known values of those elements whose valency or oxidation state do not change are placed.
Thus, a conditional quantity (the formal charge of an atom in a molecule) is used to record the equations of reactions associated with the oxidation or reduction of various substances. This value is equal to the number of electron pairs that are completely shifted towards the electronegative chemical elements of the molecule. The oxidation state is written over the atom’s symbol. Such a record indicates how to determine the oxidation state of a particular element in the molecule of any substance. For example, the entry (Na + 1Cl-1) should be understood that in the salt molecule the oxidation state of sodium is plus 1, and chlorine minus 1, thus: (+1) + (-1) = 0. In the molecule of potassium permanganate (K + 1Mn + 7O-24) for potassium plus 1, for manganese plus 7, for oxygen minus 2, that is: (+1) + (+7) + 2 • (-2) = 0. In the molecule of nitric acid (H + 1N + 5O-23) for hydrogen plus 1, for nitrogen plus 5, for oxygen minus 2, and the molecule as a whole is electrically neutral: (+1) + (+5) + 3 • (-2) = 0. In the nitrogen molecule acid (H + 1N + 3O-22), for hydrogen plus 1, for nitrogen plus 3, for oxygen minus 2, since the charge of the molecule is zero, then: (+1) + (+3) + 2 • (-2) = 0. Using the example of nitrogen, it can be seen that this atom can have different oxidation states in different substances from plus 5 to minus 3 (with an interval of one unit).