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In chemistry and atomic physics, the main group is the group of elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, and fluorine as arranged in the periodic table of the elements.
Schematic illustration of idealized fiber arrays and their corresponding unit cells. In the theory of composite materials, the representative elementary volume (REV) (also called the representative volume element (RVE) or the unit cell) is the smallest volume over which a measurement can be made that will yield a value representative of the whole. [1]
Each chemical element has a unique atomic number (Z— for "Zahl", German for "number") representing the number of protons in its nucleus. [4] Each distinct atomic number therefore corresponds to a class of atom: these classes are called the chemical elements. [5] The chemical elements are what the periodic table classifies and organizes.
A chemical element, often simply called an element, is a type of atom which has a specific number of protons in its atomic nucleus (i.e., a specific atomic number, or Z). [ 1 ] The definitive visualisation of all 118 elements is the periodic table of the elements , whose history along the principles of the periodic law was one of the founding ...
The representation theory of groups divides into subtheories depending on the kind of group being represented. The various theories are quite different in detail, though some basic definitions and concepts are similar. The most important divisions are: Finite groups — Group representations are a very important tool in the study of finite groups.
When such an element is chosen, it is called a representative of the class. The choice of a representative in each class defines an injection from X / R {\displaystyle X/R} to X . Since its composition with the canonical surjection is the identity of X / R , {\displaystyle X/R,} such an injection is called a section , when using the terminology ...
Elements in groups 1–2 or 13–18, excluding hydrogen * Transition elements are sometimes referred to as transition metals † Although the heavier elements of groups 15 (Mc), 16 (Lv), 17 (Ts) and 18 (Og) have been notionally assigned to the indicated groups their chemical properties have not yet been experimentally confirmed.
Representative democracy, type of democracy in which elected officials represent a group of people; House of Representatives, legislative body in various countries or sub-national entities; Legislator, someone who is a member of a legislature