We all breathe air, which mainly consists of nitrogen and oxygen molecules with a minor addition of other elements. Thus, oxygen is one of the most important chemical elements. In addition, its molecules exist in a huge variety of chemical compounds used in everyday life. A hundred pages are not enough to describe all the properties of this element, so we will limit ourselves to the main facts from the history, as well as the basic characteristics of the element - valence and oxidation state of oxygen, specific gravity, application, basic physical properties.
History of the discovery of a chemical element
The official date for the discovery of the chemical element "oxygen" is August 1, 1774. It was on this day that the British chemist J. Priestley completed his experiment on the decomposition of mercury oxide in a hermetically sealed vessel. At the end of the experiment, the scientist received a gas that supported combustion. However, this discovery went unnoticed even by the scientists themselves. Mr. Priestley thought that he had succeeded in isolating not a new element, butconstituent of air. Joseph Priestley shared his results with the famous French scientist and chemist Antoine Lavoisier, who was able to understand what the Englishman could not do. In 1775, Lavoisier was able to establish that the resulting "component of air" was in fact an independent chemical element, and he named it oxygen, which in Greek means "forming acids." Lavoisier then believed that oxygen is found in all acids. Subsequently, the formulas of acids that did not contain oxygen atoms were derived, but the name stuck.
Oxygen - structural features of the molecule
This chemical element is a colorless gas, odorless and tasteless. The chemical formula is O2. Chemists refer to ordinary diatomic oxygen as either "atmospheric oxygen" or "dioxygen".
A substance molecule consists of two bonded oxygen atoms. There is also a molecule consisting of three atoms - O3. This substance is called ozone, more details about it will be written below. A molecule with two atoms has an oxygen oxidation state of -2, since it has two unpaired electrons capable of forming a covalent bond. The energy that is released during the decomposition (dissociation) of an oxygen molecule into atoms is 493.57 kJ / mol. This is quite a big value.
Valence and oxidation state of oxygen
Under the valence of a chemical element, they mean its ability to attach a certain number of atoms to itselfanother chemical element. The valency of the oxygen atom is two. The valence of an oxygen molecule is also equal to two, since two atoms are connected to each other and have the ability to attach one more atom of another compound to their structure, that is, to form a covalent bond with it. For example, the water molecule H2O resulted from the formation of a covalent bond between one oxygen atom and two hydrogen atoms.
Oxygen is found in many of the known chemical compounds. There is even a separate type of chemical compounds - oxides. These are substances obtained by combining almost any chemical element with oxygen. The oxidation state of oxygen in oxides is -2. However, in some compounds, this indicator may be different. This will be discussed in more detail below.
Physical properties of oxygen
Ordinary diatomic oxygen is a gas that is colorless, odorless and tasteless. In its normal state, its density is 1.42897 kg/m3. The weight of 1 liter of a substance is slightly less than 1.5 grams, that is, in its pure form, oxygen is heavier than air. When heated, the molecule dissociates into atoms.
When the medium temperature drops to -189.2 оС oxygen changes its structure from gaseous to liquid. This is where boiling occurs. When the temperature decreases to -218, 35 оС, the structure changes from liquid to crystalline. At this temperature, oxygen has the form of bluish crystals.
At room temperature, oxygen is slightly soluble in water - 31 milliliters of oxygen per liter of it. Solubility with other substances: 220 ml per 1 liter of ethanol, 231 ml per 1 liter of acetone.
Chemical properties of oxygen
A whole Talmud can be written about the chemical properties of oxygen. The most important property of oxygen is oxidation. This substance is a very strong oxidizing agent. Oxygen is able to interact with almost all known elements from the periodic table. As a result of this interaction, oxides are formed, as mentioned earlier. The oxidation states of oxygen in compounds with other elements are basically -2. An example of such compounds is water (H2O), carbon dioxide (CO2), calcium oxide, lithium oxide, etc. But there is a certain category oxides, called peroxides or peroxides. Their feature is that in these compounds there is a peroxide group "-О-О-". This group reduces the oxidizing properties of O2, so the oxidation state of oxygen in peroxide is -1.
In combination with active alkali metals, oxygen forms superoxides or superoxides. An example of such formations is:
- potassium superoxide (KO2);
- rubidium superoxide (RbO2).
Their feature is that the oxidation state of oxygen in superoxides is -1/2.
In combination with the most active chemical element - fluorine, fluorides are obtained. About them willdescribed below.
The highest oxidation state of oxygen in compounds
Depending on what substance oxygen interacts with, there are seven oxidation states of oxygen:
- -2 - in oxides and organic compounds.
- -1 - in peroxides.
- -1/2 - in superoxides.
- -1/3 - in inorganic ozonides (true for triatomic oxygen - ozone).
- +1/2 - in s alts of the oxygen cation.
- +1 – in oxygen monofluoride.
- +2 – in oxygen difluoride.
As you can see, the highest degree of oxygen oxidation is achieved in oxides and organic compounds, and in fluorides it even has a positive degree. Not all types of interactions can be done naturally. Some compounds require special conditions to form, for example: high pressure, high temperature, exposure to rare compounds that are almost never found in nature. Consider the main compounds of oxygen with other chemical elements: oxides, peroxides and fluorides.
Classification of oxides by acid-base properties
There are four types of oxides:
The oxidation states of oxygen in compounds of these species are -2.
- Basic oxides are compounds with metals with low oxidation states. Usually, when reacted with acids, the corresponding s alt and water are obtained.
- Acid oxides - oxides of non-metals with a high degree of oxidation. When added towater forms acid.
- Neutral oxides are compounds that do not react with either acids or bases.
- Amphoteric oxides are compounds with metals that have a low electronegativity value. They, depending on the circumstances, exhibit the properties of both acidic and basic oxides.
Peroxides, the oxidation state of oxygen in hydrogen peroxide and other compounds
Peroxides are compounds of oxygen with alkali metals. They are obtained by burning these metals in oxygen. Peroxides of organic compounds are extremely explosive. They can also be obtained by absorption by oxygen oxides. Examples of peroxides:
- hydrogen peroxide (H2O2);
- barium peroxide (BaO2);
- sodium peroxide (Na2O2).
All of them are united by the fact that they contain an oxygen group -O-O-. As a result, the oxidation state of oxygen in peroxides is -1.
One of the most famous compounds with the -O-O- group is hydrogen peroxide. Under normal conditions, this compound is a pale blue liquid. According to its chemical properties, hydrogen peroxide is closer to a weak acid. Since the -O-O- bond in the compound is weakly stable, even at room temperature, a hydrogen peroxide solution can be decomposed into water and oxygen. It is the strongest oxidizing agent, however, when interacting with stronger oxidizing propertiesthe reducing agent possesses just hydrogen peroxide. The oxidation state of oxygen in hydrogen peroxide, as in other peroxides, is -1.
Other types of peroxides are:
- superoxides (superoxides in which oxygen has an oxidation of -1/2);
- inorganic ozonides (extremely unstable compounds having an ozone anion in their structure);
- organic ozonides (compounds having an -O-O-O- bond in their structure).