In nature, there are simple and complex substances. The main difference between them is in their composition. So, simple substances include atoms of one element. Their (simple substances) crystals can be synthesized in the laboratory, and sometimes at home. However, it is often necessary to create certain conditions for storing the obtained crystals.
There are five classes into which simple substances are divided: metals, semimetals, non-metals, intermetallics and halogens (not found in nature). They can be represented by atomic (Ar, He) or molecular (O2, H2, O3) gases.
As an example, you can take a simple substance, oxygen. It includes molecules consisting of two atoms of the oxygen element. Or, for example, the substance iron consists of crystals that include only atoms of the element Iron. Historically, it is customary to call a simple substance by the name of an element whose atoms are part of it. The structure of these compounds can be molecular and non-molecular.
Complex substances include atoms of various types and upon decomposition can form two (or more) compounds. For example, water splits oxygen and hydrogen. However, not every compound can be decomposed into simple substances. For example, iron sulfide formed by sulfur and iron atoms cannot be broken down. In this case, in order to prove that the compound is complex and includes heterogeneous atoms, the principle of the reverse reaction is applied. In other words, iron sulfide is obtained using the starting components.
Simple substances are free-form chemical elements. Today, science knows more than four hundred types of these elements.
Unlike complex substances, simple cannot be obtained from other simple substances. They also cannot be decomposed into other compounds.
One chemical element can form different types of simple substances. (For example, the Oxygen element forms triatomic ozone and diatomic oxygen, and Carbon is able to form diamond and graphite). This property is called allotropy. Allotropic modifications differ in the structure and method of placing molecules in crystals or in the composition of the element’s molecules (atoms) themselves. The ability to form several types of simple substances is due to the atomic structure, which determines the type of chemical bond, as well as structural features of the molecules and crystals.
All allotropic modifications have the property of passing into each other. Different types of simple substances formed by one chemical element can have different physical properties and different levels of chemical activity. So, for example, oxygen exhibits less activity than ozone, and the melting point of fullerene, for example, is lower than that of diamond.
Under normal conditions, for eleven elements simple substances will be gases (Ar, Xe, Rn, N, H, Ne, O, F, Kr, Cl, He,), for two liquids (Br, Hg), and for other elements - solids.
At a temperature close to room temperature, five metals will assume a liquid or semi-liquid state. This is due to the fact that their melting point is almost equal to room temperature. So, mercury and rubidium melt at 39 degrees, France - at 27, cesium - at 28, and gallium at 30 degrees.
It should be noted that the concepts of "chemical element", "atom", "simple substance" should not be mixed. So, for example, an atom has a specific, concrete meaning and really exists. The definition of "chemical element" is generally abstract, collective. In nature, elements are present in the form of free or chemically bonded atoms. Moreover, the characteristics of simple substances (a set of particles) and chemical elements (isolated atoms of a particular type) have their own characteristics.