A chemical element, which is also known as an element, is any substance which cannot be decomposed into simpler substances with ordinary chemical processes. Elements are defined as the fundamental materials, where all matter is composed. There are many alkali metals present in nature, which are called elements. For example, chlorine is a chemical element, where chlorine’s atomic number is given as 17.
Atomic Nature of the Elements
Paralleling the elements concept development was an understanding of the nature of matter. At different times in history, the matter has been considered to be continuous or discontinuous. Continuous matter can be postulated to be homogeneous and divisible without the limit, where each part exhibits identical properties irrespective of size.
Essentially, this was the point of view taken by Aristotle, when he associated his elemental qualities with the continuous matter.
On the other hand, the discontinuous matter is conceived of as particulate. It means, it is divisible only up to a point, at which certain basic units, known as atoms are reached. As per this concept, which is also called the atomic hypothesis, a subdivision of the basic unit (otherwise called atom) could give rise only to the particles with profoundly various properties. Then, atoms would be the ultimate carriers of the properties which are associated with bulk matter.
Structure of Atoms
Atoms of the elemental substances are complex structures themselves composed of many fundamental particles, which are called protons, neutrons, electrons. The experimental evidence indicates within an atom, is a small nucleus that generally contains protons and neutrons, will be surrounded by cloud, a swarm, or electrons. These fundamental property’s subatomic particles are their electrical charge and weight.
Whereas protons carry a positive charge, electrons carry a negative charge, and neutrons are explained as electrically neutral. The diameter of an atom (up to 10−8 centimetre) is 10,000 times larger compared to its nucleus. Protons and Neutrons, which are collectively referred to as nucleons, contain relative weights of approximately one atomic mass unit, and an electron is only up to 1/2000 as heavy. This is because protons and neutrons take place in the nucleus, virtually all the atom’s mass is concentrated there.
The Existence of Isotopes
The careful experimental examination of naturally occurring samples of several pure elements represents that not all the atoms present contain a similar atomic weight, even though they all hold similar atomic numbers. Such a situation can take place only if the atoms contain various numbers of neutrons in their nuclei. Such groups of atoms having similar atomic numbers but with various relative weights are known as isotopes.
The number of isotopic forms, which a naturally occurring element possesses ranges from one (for example, fluorine) to many more as ten (for example, tin); many elements contain at least two isotopes. Usually, the atomic weight of an element can be determined from large number of atoms that contain the natural distribution of isotopes, and, thus, it represents the atoms’ average isotopic weight constituting the sample. Very recently, precision mass-spectrometric methods have been used in the determination of weights and distribution of isotopes in different naturally occurring samples of the elements.
Origin of The Elements
The fundamental reaction which produces the large amounts of energy radiated by the Sun and most of the other stars is given as the fusion of the lightest element - hydrogen. Its nucleus, which has a single proton, into helium, which is the second lightest and most abundant element, having a nucleus consisting of two protons and neutrons. In many stars, the helium production is followed by the fusion into heavier elements, up to iron. Still, the heavier elements cannot be made in the energy-releasing fusion reactions where energy input is required to produce them.
Cosmic Abundances of the Elements
Usually, the relative numbers of atoms of various elements can be described as the abundances of the elements. And, the chief sources of data from which the information is gained on present-day abundances of the elements are the observations of the chemical composition of gas clouds in the Galaxy and stars, that contains the solar system and part of which is visible to the human eye as the Milky Way; of neighbouring galaxies; of the Moon, meteorites, and Earth; and of the cosmic rays.
Stars and Gas Clouds
Emit light, atoms absorb, and the atoms of every element do so at particular and characteristic wavelengths. A spectroscope spreads out these particular wavelengths of light from any source into a bright-coloured line spectrum, a different pattern identifying every element. When the light from an unknown source is analyzed in a spectroscope, various patterns of bright lines present in the spectrum reveal which elements emit the light. Such a type of pattern is known as emission, spectrum, or bright-line. When the light passes via the cloud or gas at a lower temperature compared to the light source, the gas absorbs at the identifying wavelengths, and absorption, or dark-line, the spectrum will be formed.
Thus, emission and absorption lines in the spectrum of light from stars yield information concerning the source of light chemical composition and the chemical composition of clouds through which the light travels.