Principles of Substance Classification
In chemistry, all substances are generally divided into groups based on two main criteria: their chemical composition and characteristic chemical properties. This results in two unequal categories:
- Simple substances. Their composition includes atoms of only one specific element. These are basic forms of matter that cannot be chemically decomposed into simpler components.
- Complex substances (also known as chemical compounds). They are built from atoms of two or more different elements. If a compound contains exactly two elements, it is called binary.
Metals and Non-metals: Key Differences
Globally, all simple substances are divided into two large classes with fundamentally different physical properties and internal structure.
Metals are distinguished by a unique set of physical characteristics. They are characterized by a specific metallic luster, high electrical and thermal conductivity, and good malleability. At the microscopic level, they form so-called metallic crystal structures. Visually, their lattices may resemble framework structures, but the nature of the chemical bonds in them is completely different.
Non-metals This group lacks the common physical properties characteristic of metals. Their structure is fundamentally different: the vast majority of non-metals consist of individual molecules. However, there are exceptions—some non-metals, such as carbon (C) and silicon (Si), form strong framework crystal structures.
Allotropy: The Many Faces of Elements
Nature is arranged such that there are significantly more simple substances than chemical elements themselves. This is explained by the phenomenon of allotropy—the ability of a single element to exist in the form of several different simple substances.
A classic example of allotropy is demonstrated by oxygen. It forms two different gases:
- Dioxygen ($O_2$) — the common, vital gas for us.
- Trioxygen ($O_3$) — ozone, which is characterized by instability and the ability to decompose rapidly.
Some elements possess a dual nature and are able to form both metallic and non-metallic forms. For example, arsenic (As) exists in metallic (gray) and molecular (yellow) modifications.
Position in the Periodic Table
To determine whether an element belongs to metals or non-metals, it is sufficient to look at its position in the D.I. Mendeleev Periodic Table.
A conditional rectangle including elements from IIIA to VIIIA groups is analyzed. If a diagonal line is drawn from boron (B) to astatine (At), the elements are distributed as follows:
- Non-metals are located strictly on this diagonal and in the area above it (hydrogen also belongs to this group).
- Metals include all other chemical elements of the table.
Search for New Simple Substances
Science does not stand still, and even for long- and well-studied elements, scientists continue to discover new forms of simple substances. A striking example is the discovery made in 1986, when a completely new form of carbon was synthesized — a substance with the formula $C_{60}$.
In this structure, carbon atoms are located at the vertices of a complex volumetric polyhedron resembling a football. An entire class of such molecules was named fullerenes.
Briefly: answers to topical questions
What allotropic modifications of carbon exist besides fullerenes?
Besides fullerenes, the following allotropic modifications of carbon exist: diamond, graphite, carbyne, nanotubes, and coal.
- Diamond — has a framework crystal structure with a tetrahedral coordination of atoms.
- Graphite — a crystalline form of a simple substance.
- Carbyne — consists of linear molecules where atoms are in the $sp$-hybridized state.
- Nanotubes — a structure in which carbon atoms are arranged along the surface of a cylinder.
- Coal — the most active allotropic modification.
Which specific chemical elements belong to noble gases?
Noble (inert) gases include six chemical elements: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). In nature, they exist as separate isolated atoms. The source also indicates that helium, neon, and argon do not form oxides; the statement about the inability of all six elements to form oxides is not confirmed in the cited sources.
Which biogenic organogen elements belong to the class of non-metals?
In the cited sources, carbon (O), oxygen (O), nitrogen (N), hydrogen (H), sulfur (S), and phosphorus (P) are classified as organogen and biogenic elements. Carbon, oxygen, nitrogen, and hydrogen make up 98% of the composition of living organisms. Sulfur is part of proteins in the form of sulfhydryl groups, and phosphorus is necessary for the synthesis of nucleic acids and ATP. The direct classification of all listed elements as non-metals is not indicated in the sources.
To easily find non-metals in the Periodic Table, mentally draw the "Boron – Astatine" diagonal. Everything lying on it and above-right (plus hydrogen) is non-metals. The rest of the realm of elements consists of metals.
Class and exam questions
Why does the number of simple substances not match the number of chemical elements?
This is due to allotropy. Many elements are capable of forming not one, but several simple substances at once, differing in structure and properties (like oxygen and ozone).
How does the structure of most non-metals differ from the structure of carbon?
Most non-metals have a molecular structure, whereas carbon (along with silicon) forms a strong framework crystal structure.
Are there elements that exist as gases consisting of single atoms?
Yes, noble gases (6 elements) exist as monatomic gases. Only at significantly lower temperatures are they able to transition into non-metallic liquid or solid states.
What else to study on this topic
- Crystal Lattices: Metallic vs. Framework
- Physicochemical properties of allotropic modifications of oxygen
- Dual nature of chemical elements using arsenic as an example
- Spatial structure and applications of fullerenes
- Binary compounds: nomenclature and formation features
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Other topics in section «Chapter 3. Classification of Substances»
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