The periodic table is now ubiquitous within the academic discipline of chemistry, providing a useful framework to classify, systematize, and compare all of the many different forms of chemical behavior. It has many applications in chemistry, physics, biology, and engineering, especially chemical engineering.
Other alternative periodic tables exist.
The layout of the periodic table demonstrates recurring ("periodic") chemical properties. Elements are listed in order of increasing the number of protons in the atomic nucleus (atomic number). Rows are arranged so that elements with similar properties fall into the same columns. Each row (period) in the table corresponded to the filling of a quantum shell of electrons. There are progressively longer periods further down the table, grouping the elements into s-, p-, d- and f-blocks to reflect their electron configuration.
As of 2010, the table contains 118 chemical elements. Ninety-four are found naturally on Earth, and the rest are synthetic elements that have been produced artificially in particle accelerators. Not all elements have stable isotopes. However also some unstable isotopes have since been discovered in trace amounts on Earth as products of natural radioactive decay processes. In some cases these amounts are as small as 20 - 30 g per all crust of the Earth (francium, astatine).
A group or family is a vertical column in the table. Groups are considered the most important method of classifying the elements. In some groups, the elements have very similar properties and exhibit a clear trend in properties down the group. These groups frequently have trivial (unsystematic) names, e.g., the alkali metals, alkaline earth metals, halogens, noble gases, etc. Some other groups in the periodic table display fewer similarities and/or vertical trends (for example Group 14), and these have no trivial names. They are referred to simply by group numbers.
A period is a horizontal row in the table. Although groups are the most common way of classifying elements, there are some regions of the periodic table where the horizontal trends and similarities in properties are more significant than vertical group trends. This is mostly true in the d-block (or "transition metals"), and for the f-block, where the lanthanides and actinides form two substantial horizontal series of elements.
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