Elements
All matter is made up of a number of basic chemical substances called elements. These cannot be split up into smaller units by normal chemical or physical means, such as treatment with acid, heat, or electricity. Most elements will combine with other elements to form a compound. This compound has entirely different properties from those of its constituent elements. For example, common salt, or sodium chloride, is a compound formed when sodium combines with chlorine. It is totally different from sodium (a light metal) and chlorine (a poisonous gas).
A compound must be distinguished from a mixture, in which the component parts are uncombined. A mixture can be separated physically, whereas a compound must be separated chemically. Every element is made up of a number of identical tiny atoms. Each atom consists of a number of minute particles i.e. protons and neutrons, in a central nucleus, around which circle even smaller particles called electrons. It is the electrons which determine how the various elements combine together.
Most elements occur in nature as a mixture of two or more isotopes forms with the same number of protons but a different number of neutrons. The isotopes have the same chemical properties (which depend on the number of protons) but different weights. The number of protons in the nucleus is called the atomic number.
The elements can be arranged in order of their atomic numbers so that they fall into a fairly regular pattern, with breaks at certain intervals, or periods. Such an arrangement is called the periodic table. Elements in the same vertical group in the table have similar properties. For example, in the second group are the light metals beryllium, magnesium, calcium, strontium, barium and radium, which all have distinct similarities.

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