Periodic table

The periodic table of the chemical elements, also called the Mendeleev periodic table, is a tabular display of the known chemical elements. First created by Dmitri Mendeleev, the elements are arranged by electron configuration so that many chemical properties follow a regular pattern across the table. Each element is listed by its atomic number and chemical symbol. Mendeleev's ordering of the periodic table was one of the greatest developments in modern chemistry. Chemists were able to quantitatively explain the behavior of the elements, and to predict the existence of yet undiscovered ones. There are 116 chemical elements whose discoveries has been confirmed. Ninety four can be found naturally on Earth, and the rest have been produced in laboratories.

Explanation of the structure of the periodic table

The primary determinant of an element's chemical properties is its electron configuration, particularly the valence shell electrons. For instance, all atoms whose four valence electrons are found on the p shell will behave similarly, regardless of which energy level that last p shell is on. The shell in which the atom's outermost electrons reside determines the "block" to which it belongs. The number of valence shell electrons determines which family, or group, the element belongs.

Related Topics:
Electron configuration - Valence shell

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The total number of electron shells an atom has, determines to which period it belongs. Each shell is divided into different subshells, which as atomic number increases are filled in roughly this order:

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1s

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2s 2p

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3s 3p

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4s 3d 4p

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5s 4d 5p

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6s 5d 6p

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7s 6d 7p

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4f

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5f

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...

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Hence the structure of the table. Since the outermost electrons determine chemical properties, those with the same number of valence electrons are grouped together..

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Progressing through a group from lightest element to heaviest element, the outer-shell electrons (those most readily accessible for participation in chemical reactions) are all in the same type of orbital, with a similar shape, but with increasingly higher energy and average distance from the nucleus. For instance, the outer-shell (or "valence") electrons of the first group, headed by hydrogen all have one electron in an s orbital. In hydrogen, that s orbital is in the lowest possible energy state of any atom, the first-shell orbital (and represented by hydrogen's position in the first period of the table). In francium, the heaviest element of the group, the outer-shell electron is in the seventh-shell orbital, significantly further out on average from the nucleus than those electrons filling all the shells below it in energy. As another example, both carbon and lead have four electrons in their outer shell orbitals.

Related Topics:
Hydrogen - Francium

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Because of the importance of the outermost shell, the different regions of the periodic table are sometimes referred to as periodic table blocks, named according to the sub-shell in which the "last" electron resides, e.g. the s-block, the p-block, the d-block, etc.

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