Historical and modern approaches to classification
Historically, chemists separated substances based on their natural source. Inorganic substances were extracted from the minerals of the earth's crust, while organic ones were obtained from plants, animal tissues, or their decomposition products (for example, coal and peat). The Swedish chemist J.J. Berzelius was the first to establish organic chemistry as an entirely independent discipline.
Today, the main classification criterion is the presence of carbon atoms:
- Organic compounds: Include the vast majority of carbon-containing substances (with the exception of hydrocarbons, which, although consisting of two elements, belong to organics).
- Inorganic compounds: All substances without carbon, as well as a small list of carbon exceptions: binary oxides ($CO, CO_2$), carbides, and carbonic acid salts (carbonates).
Binary compounds: rules and nomenclature
Binary compounds are those consisting of exactly two chemical elements. Almost all of them are inorganic. As a rule, they contain one or two nonmetals, based on the nature of which further classification is carried out.
Principles of systematic nomenclature:
- The name is read from the end of the chemical formula.
- First, the name of the nonmetal is pronounced with the addition of the suffix -ide.
- This is followed by the name of the second element (metal or less active nonmetal) in the genitive case.
- If the first element (cation) has a variable valence, it is mandatory to indicate it with a Roman numeral in parentheses without a space (for example, iron(III) chloride).
If a compound consists of two nonmetals, the suffix -ide is given to the element that is located higher and/or to the right in the Periodic Table. At the same time, this element is always placed second in the chemical formula.
Families of binary compounds
| Defining element | Group name | Compound example |
|---|---|---|
| Hydrogen | Hydrides | Calcium hydride ($CaH_2$) |
| Carbon | Carbides | Calcium carbide ($CaC_2$) |
| Oxygen | Oxides | Nitrogen dioxide ($NO_2$) |
| Sulfur | Sulfides | Aluminum sulfide ($Al_2S_3$) |
| Chlorine | Chlorides | Gold(III) chloride ($AuCl_3$) |
Properties of binary compounds
Binary compounds exist in all states of aggregation: solid, liquid, and gaseous. They encompass various classes of substances, including salts (for example, metal halides and sulfides), which are of immense practical importance.
Their interaction with water varies greatly:
- Most metal halides are readily soluble.
- The remaining binary substances are either inert to water or undergo hydrolysis (are decomposed by its action).
Such compounds are most often obtained by the reaction of simple substances, varying conditions from simple mixing to high heating and high pressure. For example, highly reactive sodium reacts with hydrogen only when heated to 300 °C, forming a white layer of sodium hydride on the surface: $2Na + H_2 = 2NaH$.
Multi-element compounds
Multi-element (including ternary) compounds are usually formed during the interaction of binary substances. A typical example is the reaction between oxides resulting in the formation of salts, oxygen-containing acids, or bases (for example, the synthesis of phosphoric acid: $P_2O_5 + 3H_2O = 2H_3PO_4$).
Structural features: The recording of chemical formulas for such substances is traditional and often does not reflect the actual geometry of the molecule. The most important structural feature is that the first element in the formula (hydrogen or metal) is almost never directly attached to the central atom. The bond is realized through an oxygen bridge.
Most natural minerals also belong to multi-element chemical compounds. A characteristic example is common mica with the complex chemical formula $KAl_3Si_3O_{10}(OH)_2$.
Briefly: answers to topical questions
What families of binary compounds are distinguished by the defining element?
According to the defining (electronegative) element, the following families of binary compounds are distinguished:
- Hydrogen — hydrides;
- Carbon — carbides;
- Silicon — silicides;
- Nitrogen — nitrides;
- Oxygen — oxides;
- Sulfur — sulfides;
- Fluorine — fluorides;
- Chlorine — chlorides;
- Bromine — bromides;
- Iodine — iodides.
Compounds of fluorine, chlorine, bromine, and iodine are also united by the common name — halides.
What types of hydrolysis of binary compounds exist?
The following types of hydrolysis of binary compounds exist:
- Strong hydrolysis — typical for soluble sulfides, with solutions acquiring the odor of hydrogen sulfide.
- Complete (irreversible) hydrolysis — complete decomposition of a substance by water. This type undergoes compounds with bonds of aluminum with chlorine, sulfur, carbon (for example, aluminum chloride and sulfide), as well as nitrides of active metals.
By what methods are binary compounds obtained?
Binary compounds are obtained mainly by reactions between simple substances. The conditions of such reactions can vary: mixing under normal conditions, heating, or high pressure.
Oxides are also formed during chemical transformations of complex substances:
- combustion of complex substances (for example, wood, methane, alcohol, propane);
- decomposition reactions;
- substitution reactions;
- exchange reactions.
Which elements can act as the central atom in multi-element oxygen-containing compounds?
Sources indicate that in most multi-element oxygen-containing compounds, atoms of both metals and non-metals are bonded to oxygen. The specific list of elements acting as central atoms is not provided in the sources. In the formulas of such compounds, the first element—usually hydrogen or a metal—is typically not bonded directly to the central atom; the bond is established through an oxygen bridge.
What trivial names of binary compounds are used in chemical practice instead of systematic ones?
The sources directly give an example of a traditional name for a binary compound:
- Water — instead of the systematic name "hydrogen oxide" ($H_2O$).
Sources also mention the names ammonia ($NH_3$), phosphine ($PH_3$), arsine ($AsH_3$), hydrazine ($N_2H_4$), quartz ($SiO_2$), and carbon dioxide ($CO_2$), but explicitly designate only "water" as a traditional name used instead of the systematic one.
To avoid confusing the word order in binary compounds of two non-metals, remember the "Right-Higher-Ide" rule: the element located further to the right and/or higher in the periodic table takes the "-ide" suffix and is placed at the end of the formula.
Class and exam questions
Why is carbon monoxide (CO) classified as inorganic?
This is a historically established nomenclature exception. Despite containing carbon, carbon oxides, carbides, and carbonates are closer in their properties and origin to inorganic substances.
Are traditional names used today instead of systematic ones?
Yes, they are used everywhere. For example, the systematic name "hydrogen oxide" is almost never heard in chemical practice—abandoning the trivial name "water" is impractical.
Does the formula of a ternary substance always show the bonding order?
No. The formula is written according to traditional rules. In practice, terminal elements (metals or hydrogen) in oxygen-containing compounds are bonded to the central atom via oxygen.
What else to study on this topic
- Structural formulas of oxygen-containing acids
- Hydrolysis processes of binary compounds
- Chemical properties and applications of halides
- Reactions for obtaining hydrides of active metals
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Classification of Complex Substances by Composition" and 3,009 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in section «Chapter 3. Classification of Substances»
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.