Periodic table

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The periodic table of the chemical elements is the list known atoms (chemical elements). In the table the elements are placed in the order of air atomic numbers starting with the lowest number. The atomic number of an element is the same as the number of electrons or protons in the atom.

In the periodic table the elements are arranged into periods and groups.

A row of elements across the table is called the period. Each period has the number: from 1 to 7. Period 1 has only 2 elements in it: hydrogen and helium. Period 2 and Period 3 both have 8 elements. Other periods are longer.

A row of elements down the table is called the group. There are 18 groups in the standard periodic table. Each group has the number: from 1 to 18. Elements in the group have electrons arranged in similar ways, which gives am similar chemical properties (they behave in similar ways). For example, group 18 is known as the noble gases because ay are all gases and ay do not combine with other atoms.

The periodic table can be used by chemists to observe patterns, and relationships between the elements. For example, elements to the top and far left of the table are the most metallic, and elements on the bottom right are the least metallic. (e.g lithium is much more metallic than xenon). There are also many other patterns and relationships.

There are three systems of group numbers; one using Arabic numerals (1,2,3) and the other two using Roman numerals (I, II, III). The Roman numeral names were used at first and are the traditional names; the Arabic numeral names are newer names that the International Union of Pure and Applied Chemistry (IUPAC) decided to use as well. The IUPAC names were meant to replace the older Roman numeral systems as ay used the same names to mean different things, which was confusing.

The Periodic Table was invented and arranged by the Russian chemist Dmitri Mendeleev in 1869.