In chemistry, pure substance refers to those substances which are made up of the same kind of constituent particles(atoms or molecules). Elements are pure substances that are made up of the same kind of atoms. They cannot be broken down into simpler particles by the means of physical or chemical changes. They are grouped into metals, nonmetals and metalloids on the periodic table of elements. The metals are known to possess certain differentiating properties which the non-metals are devoid of. Metalloids, on the other hand, are the borderline of metals and non-metals and possess properties from both groups.
All the known elements are arranged in a little cheat-sheet specifically designed for this purpose, known as the periodic table. Currently, there are close to 118 elements known to man. The periodic table is divided into rows and columns. The rows of the periodic table are called periods and determine the number of orbitals for that particular period. The vertical columns of the periodic tables are known as groups. All the elements in a particular group will have the same number of valence electrons.
The important characteristics of an element in the periodic table are its atomic symbol, atomic number and mass number.
The credit for creating the blueprint for the modern periodic table goes to Dmitri Mendeleev, a Russian chemist. When he designed his periodic table, only 63 elements were known. Currently, about 118 elements are known to humanity, which is listed on the periodic table.
An element possesses several characteristics. These characteristics define the element and are the points of difference between different elements and between elements and other substances. Some of the characteristics of an element are as follows:
Before the discovery of subatomic particles, it was widely regarded that atoms are indivisible entities. However, after the discovery of subatomic particles in the late nineteenth century, this field of thought was abandoned.
Atoms consist of protons, neutrons and electrons. According to Bohr’s Atomic Model, the protons and neutrons reside in the nucleus of the atom. The electrons revolve around the nucleus in fixed energy levels. The number of electrons in the outermost shell is known as valence electrons. Atoms donate, gain or share electrons when they react to gain an octet in the outermost shell. The number of electrons that an element gains or loses to gain an octet is called its valency.
The electronic configuration of an element refers to the arrangement of electrons in the orbits of a particular element. According to Bohr’s Atomic Model, the maximum number of electrons that can be accommodated in a given shell is calculated by the formula 2n2, where n equals the shell number. According to this formula, the first shell can accommodate 2 electrons, the second shell can accommodate 8 electrons, the third shell can fill up to 18 electrons and so on. According to the octet rule, the maximum number of electrons that can be accommodated in the outermost/valence shell of an element is 8.
The electrons are filled in the increasing order of energy levels. This property is referred to as the Aufbau principle. The orbitals with the lowest energy levels are filled first, followed by the higher energy levels. The first shell (n=1) consists of only one orbital (s). The second shell (n=2) consists of two orbitals (s,p). The third shell (n=3) consists of three orbitals (s,p,d). The fourth shell (n=4), consists of four orbitals (s,p,d,f).
The electronic configuration of the first 20 elements is given as follows:
Elements are pure substances that are made up of the same kind of constituent particles and cannot be broken down into simpler substances by physical or chemical processes. They are divided into metals, nonmetals and metalloids. All the known elements are listed on the periodic table. The arrangement of electrons in various shells of an element is called the electronic configuration of that element.
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