0.2 Introduction - Concept of a Chemical Element

About eighty years ago Clemens Winkler, the German chemist who discovered germanium which had been predicted by D. Mendeleev under the name of "eka-silicon", likened the world of the elements to the theatre stage where scene after scene is played out with elements, as characters. Each element, Winkler said, plays its own role. Sometimes it is a subsidiary role, sometimes it is a leading role.

In this way the scientist characterized the significance of the elements already discovered and known to man.

Clemens Winkler

Therefore, it is up to us to decide in what sequence the history of the discovery of the elements should be presented.From the standpoint of the history of discovery, there can be neither leading nor subsidiary elements. All elements can lay equal claim to our attention.

We can describe elements in the order of increasing atomic numbers: hydrogen, helium, lithium ... up to element No. 107, which is still unnamed. Or we may describe the history of the discovery of the elements that compose the successive groups of the periodic system. Or we may deal with the elements in an alphabetical order.

We believe that all these ways of presentation are not very successful since they distort the chronology of discoveries. And it is exactly the chronology that we want to make the basis of presentation here.

But at first let us try to understand clearly what is meant by the term "a chemical element".

Concept of a "Chemical Element"

An element is the totality of atoms of a certain type. An atom consists of a nucleus and electrons surrounding it. A nucleus has an integral positive charge denoted by the Latin letter Z. The charge, in its turn, is determined by the number of elementary particles (protons) contained in the nucleus. The charge of the proton (positive) is equal in magnitude to that of the electron (negative). This means that the number of protons (Z) in the nucleus determines the number of electrons in electron shells of the atom. The chemical properties and behaviour of the elements depend on how the electrons are distributed in the shells. Consequently, the nuclear charge Z determines the properties of the chemical element. In addition, Z coincides with the atomic number of the element in the periodic table. For instance, the nucleus of the oxygen atom (atomic number 8) has a positive charge equal to 8, i.e. it contains 8 protons.

Thus, an element is a set of atoms with the same nuclear charge Z which determines the position of the element in the periodic system.

Can atoms of the same element differ from one another? The answer proves to be "yes". In addition to protons, a nucleus contains neutrons. As regards their mass, neutrons differ only slightly from protons, but, in contrast to protons, they carry no charge: they are neutral. There are no nuclei without neutrons (the only exception is the nucleus of the lightest element, hydrogen, which is just a single proton; however, there are different types of hydrogen atoms whose nuclei contain neutrons as well). The total mass of protons and neutrons in a nucleus determines the mass of the atom since the masses of electrons are negligibly small (an electron is 1840 times lighter than a proton). The varieties of the atoms of this or that element whose nuclei contain a different number of neutrons are called isotopic atoms or isotopes. The word "isotope" originates from the Greek isos, "the same", and topos, "place". This means that all the isotopes of the same element occupy the same position in the periodic table. About three-fourths of the naturally occurring elements have isotopes or, as is said, represent a pleiad of isotopes. The remaining elements have no isotopes, i.e. they exist only in one variety of atoms.

Even though the concept of "a chemical element" seems to be quite definite, in reality it is a rather abstract term denoting only a group of atoms with a given nuclear charge. In practice we deal with elements either as constituents of various chemical compounds or as simple substances. A simple substance is a free form of an element which makes it possible to see what the element looks like. Some elements occur in nature only as simple substances, others—as simple substances or as constituents of compounds, and still others exclusively in combinations with other elements. The representatives of the last group are especially numerous. The forms of existence of elements in nature played an important role in the history of their discovery.

Read more at the Science Pole