What is Periodic table?
{{no sources|date=August 2023}} {{TopicTOC-Chemistry}} The '''periodic table''' is a table that puts all known [[chemical element]]s in a specific order. Elements that have similar characteristics are often put near each other. In the [[Table (information)|table]], the elements are placed in the order of their [[atomic number]]s starting with the lowest number of one, hydrogen. The atomic number of an element is the same as the number of [[protons]] in that particular nucleus of an [[atom]]. In the table the elements are arranged into ''periods'' and ''group.'' A row of elements across the table is called a ''period''. Each period has a number; from 1 to 8. Period 1 has only 2 elements in it: [[hydrogen]] and [[helium]]. Period 2 and Period 3 both have 8 elements. Other periods are longer. Elements in a period have consecutive atomic numbers. [[File:Periodic_table_large.svg|alt=Periodic_table_large|center|thumb|507x507px|A standard periodic table]] A column of elements down the table is called a ''group''. There are 18 groups in the standard periodic table. Each group has a number: from 1 to 18. Elements in a group have electrons arranged in similar ways, according to the number of valency electrons, which gives them similar [[chemical property|chemical properties]] (they behave in similar ways). For example, group 18 is known as the [[noble gas]]es because they are all gases and they do not combine with other atoms. There are two systems of group numbers; one using Arabic numerals (1, 2, 3) and the other using [[Roman numeral]]s (I, II, III). The Roman numeral names were used in most of the 20th century. In 1990 the [[International Union of Pure and Applied Chemistry]] (IUPAC) decided to use the new system with Arabic numerals, to replace the two old group systems that used Roman numerals. The periodic table has been used by [[chemist]]s to observe patterns and relationships between elements. There are 3 main groups in the Periodic Table; [[metal]]s, [[metalloid]]s, and [[nonmetal]]s. For example, element