Electron shell

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In atomic chemistry and physics, an electron shell , or the primary energy level , can be thought of as an orbit followed by electrons around the atomic nucleus. The closest shell to the nucleus is called " 1 shell" (also called "K shell"), followed by " 2 shell" (or "L shell"), then " 3 shell "(or" M shell "), and so farther and farther from the nucleus. The shells correspond to the major quantum numbers ( n = 1, 2, 3, 4Ã,...) or are labeled alphabetically by the letters used in X-ray notation (K, L, M,...).

Each shell can only contain a fixed number of electrons: The first shell can store up to two electrons, the second shell can hold up to eight (2 6) electrons, the third shell can hold up to 18 (2 6 10) and so on. The general formula is that the shell n can principally store up to 2 ( n ² ) electrons. Because electrons are electrically attracted to the nucleus, the atomic electrons will generally occupy the outer shell only if the deeper shell has been completely filled by other electrons. However, this is not a strict requirement: the atom may have two or even three outer shells that are incomplete. (See Madelung's rule for more details.) For an explanation of why electrons present in these shells see the electron configuration.

Electrons in the outer shell (or outermost shell) determine the chemical properties of atoms; it's called valence skin .

Each shell consists of one or more subshells , and each subshell consists of one or more atomic orbitals.

Video Electron shell

History

The shell terminology comes from Arnold Sommerfeld's modification of the Bohr model. Sommerfeld maintains the Bohr planet model, but adds a light elliptical orbit (marked with additional quantum numbers l and m ) to explain the fine spectroscopic structure of some elements. Many electrons with the same primary quantum number ( n ) have close orbits that form "shells" with positive thickness rather than the infinite circular orbit of the Bohr model.

The existence of electron shells was first observed experimentally in the study of X-ray absorption of Charles Barkla and Henry Moseley. Barkla labeled them with the letters K, L, M, N, O, P, and Q. The origin of the term is the alphabet. A series of "J" is also suspected, although later experiments show that the K uptake line is generated by the innermost electron. These letters are then found in accordance with n values ​​1, 2, 3, etc. They are used in Siegbahn spectroscopic notation.

The physical chemist Gilbert Lewis was responsible for much of the early development of the theory of electron participation of shell valence in chemical bonds. Linus Pauling then generalized and expanded the theory while applying insights from quantum mechanics.

Maps Electron shell

Shells

The electron shell is labeled K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7; going from the outer shell to the outside. Electrons in the outer shell have higher average energies and travel farther from the nucleus than those in the inner shell. This makes them more important in determining how the atoms react chemically and behave as conductors, because the pull of the atomic nuclei on them is weaker and more easily broken. In this way, the reactivity of a particular element depends heavily on its electronic configuration.

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Subcuten

Each shell consists of one or more subshells, which themselves consist of atomic orbitals. For example, the first shell (K) has one subshell, called 1s ; the second shell (L) has two subshells, called 2s and 2p ; the third shell has 3s , 3p , and 3d ; the fourth shell has 4s , 4p , 4d and 4f ; the fifth shell has 5s , 5p , 5d , and 5f and could theoretically hold more but Section 5f , although partially occupied in the actinide, is not filled in any naturally occurring element. Various possible subshells are shown in the following table:

• The first column is a "subshell label", a lowercase label for the type of subshell. For example, the subkey " 4s " is a subshell of the fourth shell (N), with the type ( s ) described in the first line.
• The second column is the azimuthal quantum number (l) of the subshell. The exact definition involves quantum mechanics, but it is a number that characterizes subshell.
• The third column is the maximum number of electrons that can be inserted into the subshell of that type. For example, the top line says that every subshell of s -type ( 1s , 2s , etc.) can have at most two electrons in it. In each case the number is 4 larger than the one above.
• The fourth column says which shell has that type of subshell. For example, looking at the top two rows, each shell has a subset of s , while only the second and higher shells have a subset of p (that is, there is no "1p" subshell).
• The last column gives the historical origin of the s , p , d , and f labels. They are from the initial study of the atomic spectral lines. Other labels, namely g , h and i , are the alphabetical continuation following the last historical label of f .

Although it is generally stated that all electrons in the shell have the same energy, this is an estimate. However, the electrons in one subshell do have the exact same energy levels, with the subcell then having more energy per electron than the previous one. This effect is large enough that the energy range associated with the shell can overlap (see valence skin and the principle of Aufbau ).

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Number of electrons in each shell

Each subshell is limited to hold 4 l 2 most electrons, that is:

• Each subshell s holds at most 2 electrons
• Each subplot p holds at most 6 electrons
• Each subshell d holds at most 10 electrons
• Each subshell f
• Each subshell g holds at most 18 electrons

Therefore, the K shell, which contains only s subshells, can store up to 2 electrons; shell L, containing s and p , can store up to 2 6 = 8 electrons, and so on; in general, the shell n can store up to 2 n ² electrons.

Although the formula gives the maximum principle, in fact that the maximum is only achieved (by known elements) for the first four shells (K, L, M, N). No element is known to have more than 32 electrons in a single shell. This is because the subshell is filled according to the principle of Aufbau. The first elements that have more than 32 electrons in a single shell will belong to the 8-g block period of the periodic table. These elements will have multiple electrons within their 5g subshell and thus have more than 32 electrons in the O shell (the fifth major shell).

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Valence shell

The valence shell is the outermost shell of the atom. Valence electrons in non-transition metal elements reside in this shell. These elements with perfect valence shells (noble gases) are the most non-reactive chemists, while those with only one electron in their valence shell (alkali metal) or just losing an electron from having a complete shell (halogen) are the most reactive.

However, this terminology is somewhat misleading in the case of transition metal. In these elements, valence electrons can also be in the inner skin. Thus, the electrons that determine how the atoms react chemically are the electrons that travel the farthest away from the nucleus, those with the highest energy, and not necessarily in the valence shell.

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List of elements with electrons per shell

The list below gives the elements arranged by increasing the atomic number and showing the number of electrons per shell. At a glance, one can see that the subsets of the list show a clear pattern. In particular, the seven elements (in blue blue span) before the noble gas (group 18, in yellow) are higher than helium having the number of electrons in the valence shell in the arithmetic progression. (However, this pattern can break down in the seventh period because of the relativistic effect.)

Sorting tables by chemical group shows additional patterns, especially with respect to the last two outermost shells. (Elements 57-71 include lanthanides, while 89 to 103 are actinides.)

The list below is particularly consistent with the Aufbau principle. However, there are a number of exceptions to the rule; for example palladium (atomic number 46) has no electrons in the fifth shell, unlike other atoms with lower atomic numbers . Some entries in the table are uncertain, when experiment data is unavailable. (For example, elements passing through 108 have a very short half-life whose electron configuration has not been measured.)

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See also

• Periodic table (electron configuration)
• Calculation of electrons
• 18-Electron Rule
• Core billing

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References

Source of the article : Wikipedia