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electronegativity , symbol ? , is a chemical property that describes the tendency of atoms to pull a pair of electrons (or electron density) together toward the self. The electronegativity of the atoms is affected by both atomic numbers and the distance at which the valence electrons reside from the charged nuclei. The higher the corresponding electronegativity rate, the more elements or compounds that attract the electrons toward it.
The term "electronegativity" was introduced by JÃÆ'¶ns Jacob Berzelius in 1811, although the concept was known even before that and was studied by many chemists including Avogadro. Regardless of its long history, the accurate scale of electronegativity was not developed until 1932, when Linus Pauling proposed a scale of electronegativity, which relied on bonding energy, as the development of valence bond theory. It has been shown to correlate with a number of other chemical properties. Electronegativity can not be measured directly and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed, and although there may be slight differences in the numerical values ​​of electronegativity, all methods show the same periodic trends between elements.
The most commonly used method of calculation was originally proposed by Linus Pauling. This gives an un-dimensional quantity, commonly referred to as Pauling scale ( r ), on a relative scale running from about 0.7 to 3, 98 (hydrogen = 2.20). When other calculation methods are used, it is conventional (though not mandatory) to cite results on a scale that includes the same range of numerical values: this is known as electronegativity within Pauling units .
As is usually calculated, electronegativity does not belong to the atom alone, but belongs to the atom in a molecule. The properties of free atoms include ionisation energy and electron affinity. It is expected that the electronegativity of the elements will vary with their chemical environment, but is usually considered a transferable property, meaning that the same value will apply in various situations.
At the most basic level, electronegativity is determined by factors such as nuclear charges (the more protons the atom has, the more "pull" that will occur in the electron) and the number/location of other electrons in the atomic shell (the more electrons have atoms, the farther from the valence electron nucleus will be, and as a result the less positive charge they will experience - both because of the increased distance from the nucleus, and because other electrons in the lower energy core orbitals will act to protect the valence electrons from positively charged nuclei ).
The opposite of electronegativity is electropositivity : the size of an element's ability to donate electrons.
Cesium is the most electronegative element in the periodic table (= 0.79), whereas fluorine is the most electronegative (= 3.98). Francium and cesium were originally both given 0.7; The cesium grade was then refined to 0.79, but no experimental data allowed a similar improvement to franium. However, the energy of franium ionization is known to be slightly higher than cesium, corresponding to the relativistic stabilization of the 7s orbital, and this in turn implies that franium is actually more electronegative than cesium.
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Metode perhitungan
Pauling electronegativity
Pauling first proposed the concept of electronegativity in 1932 as an explanation of the fact that the covalent bonds between two different atoms (A-B) are stronger than expected by taking the average strength of the A-A and B-B bonds. According to the theory of valence bonds, where Pauling is a leading proponent, the "additional stabilization" of the heteronuclear bond is due to the contribution of the ionic canonic form to the bond.
They perbedaan electroonegativitas antara atom A dan B diberikan oleh:
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where the energy dissociation, E d , from the bonds AB, AA and BB are expressed in electronvolts, factor (eV) - 1 / 2 is included to ensure dimensionless results. Therefore, the difference in Pauling electronegativity between hydrogen and bromine is 0.73 (dissociation energy: H-Br, 3.79 eV; H-H, 4.52 eV; Br-Br 2.00 eV)
Since only differences in the electronegativity are defined, it is necessary to select a random reference point to build the scale. Hydrogen is chosen as a reference, because it forms a covalent bond with various large elements: its electronegativity remains first at 2.1, then revised to 2.20. It is also important to decide which of the two elements is more electronegative (equivalent to choosing one of two possible marks for the square root). This is usually done by using "chemical intuition": in the example above, hydrogen bromide dissolves in water to form H and Br - ions, so it can be assumed that bromine is more electronegative than hydrogen. However, in principle, since the same electronegativity must be obtained for any two bonding compounds, the data is actually overdetermined, and the signs are unique after the reference point is fixed (usually, for H or F).
To calculate Paul's electronegativity for an element, it is necessary to have data on the dissociation energy of at least two types of covalent bonds formed by that element. A. L. Allred updated Pauling's original values ​​in 1961 to account for the greater availability of thermodynamic data, and these are the "revised Paul" values ​​of the most frequently used electronegativities.
Titik penting would be electro-activated Pauling adalah bahwa ada, mendasari, cukup akurat, semi-empiris rumus untuk energi disosiasi, yaitu:
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Atau kadang-kadang, kecocokan yang lebih accurat
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This is an approximate equation, but applies with good accuracy. Pauling derives it by noting that a bond can be roughly represented as a superposition of quantum mechanics of covalent bonds and two ionic bonds. The covalent energy of bonding is approximately, by quantum mechanical calculations, the geometric mean of two covalent bond energies of the same molecule, and there is additional energy derived from the ionic factors, ie the polar character of the bond.
Geometric mean is roughly equal to the arithmetic mean - applied in the first formula above - when energy has the same value, for example, except for highly electropositive elements, where there is a greater difference of the two dissociation energies; geometric mean is more accurate and almost always gives positive excess energy, because of ionic bonding. The square root of this excess energy, according to Pauling, is an additional approximation, and hence one can introduce electronegativity. So here's the semi-empirical formula for the bonding energy that underlies Pauling's electronegativity concept.
The formula is approximate, but this rough estimate is actually quite good and gives the right intuition, with the idea of ​​bond polarity and some theoretical foundations in quantum mechanics. The electronegativity is then determined to best suit the data.
In more complex compounds, there are additional errors because electronegativity depends on the environment of the atomic molecule. Also, energy estimates can only be used for singles, not for multiple bonds. The energy of molecular formation containing only a single bond can then be approximated from the electronegativity table, and depends on the constituents and the sum of the squares of the electronegativity differences of all bonded atomic pairs. Such a formula for estimating energy usually has a relative error of the order of 10%, but can be used to get rough qualitative ideas and an understanding of molecules.
Mulliken electronegativity
Robert S. Mulliken proposes that the arithmetic mean of the first ionisation energy (E i ) and the electron affinity (E ea ) must be a measure of the tendency of the atoms to attract electrons. Since this definition does not depend on an arbitrary relative scale, this definition is also called absolute electronegativity , with units of kilojoules per mole or electronvolts.
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Namun, lebih umum menggunakan transformasi linear untuk mengubah nilai-nilai absolut ini menjadi nilai-nilai yang menyerupai nilai-nilai Pauling yang lebih dikenal. Untuk energi ionisasi dan afinitas electron dalam elektrovolt,
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give one to power dalam kilojoule per mol,
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The Mulliken electronegative can only be calculated for elements known for its electron affinity, fifty-seven elements in 2006. Mulliken atomic electronegativity is sometimes said to be a negative of chemical potential. By incorporating the energetic definition of ionization potential and electron affinity into Mulliken electronegativity, it is possible to show that the Mulliken chemical potential is a different approach to the electronic energy in terms of the number of electrons. That is,
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Allred-Rochow elektronegativity
A. Louis Allred and Eugene G. Rochow assume that electronegativity must be related to the charge experienced by the electrons on the "surface" of atoms: The higher the charge per unit of the atomic surface area the greater the tendency of the atom to attract. electron. Effective nuclear charge, experienced by valence electrons can be estimated using the Slater rule, while the surface area of ​​atoms in the molecule can be considered proportional to the square of the covalent radius, Z Z eff > r cov . When r cov is expressed in picometres,
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Sanderson_electronegativity_equalization Sanderson electronegativity
R.T. Sanderson also noted the relationship between Mulliken electronegativity and atomic size, and has proposed recalcitrant calculation methods of atomic volumes. With knowledge of the bond length, the Sanderson model allows estimation of bond energy in various compounds. The Sanderson model has also been used to calculate molecular geometry, energy energy, spin-spin NMR constants and other parameters for organic compounds. This work underlies the concept of equalization of electronegativity , which shows that electrons distribute themselves around molecules to minimize or equate Mulliken electronegativity. This behavior is analogous to the equitable distribution of chemical potentials in macroscopic thermodynamics.
Allen electronegativity
Mungkin defined electronegativitas yang paling sederhana adalah Leland C. Allen, yang telah mengusulkan bahwa ia terkait dengan energi rata-rata elektron valensi dalam atom bebas ,,,
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Where? s, p is the energy of the electron-one electron s and p in the free atom and n s, p is the number s- and the p-electron valence shells. It is usual to apply a scale factor, 1.75ÃÆ' â € "10 -3 for the energy expressed in kilojoules per mole or 0.169 for the energy measured in electronvolts, to provide numerically similar values ​​to the electronegativity of Pauling.
The energy of one electron can be determined directly from the spectroscopic data, so the electronegativity calculated by this method is sometimes referred to as electronegativities spectroscopy . The required data is available for almost all elements, and this method allows estimation of electronegativity for elements that can not be treated by other methods, eg. francium, which has Allen electronegativity of 0.67. However, it is unclear what should be considered as valence electrons for d-and f-block elements, leading to ambiguity for their electronegativity calculated by the Allen method.
On this scale neon has the highest electronegativity of all elements, followed by fluorine, helium, and oxygen.
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Correlation of electronegativity with other properties
The various methods of calculating electronegativity, all of which give results that correlate well with one another, are one indication of the number of chemical properties that may be affected by electronegativity. The most obvious application of electronegativity is in the discussion of bond polarities, whose concepts are introduced by Pauling. In general, the greater the difference in electronegativity between two atoms the more polar bonds that will form between them, with atoms having higher electronegativity being at the negative end of the dipole. Pauling proposes an equation for connecting the "ionic character" of the bond with the difference in electronegativity of the two atoms, although this has become unused.
Some correlations have been shown between the frequency of infrared stretching of a particular bond and the electronegativity of the atoms involved: however, this is not surprising since such frequency of stretching depends on the bond strength, which goes into Pauling electronegativity calculations. More convincing is the correlation between electronegativity and chemical shifts in NMR spectroscopy or isomer shift in MÃÆ'¶ssbauer spectroscopy (see figure). Both of these measurements depend on the electron-sectron density of the nucleus, and so is a good indication that different electronegativity measures really represent "the ability of atoms in a molecule to attract electrons to itself."
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Trends in electronegativity
Periodic trends
In general, electronegativity increases as it moves from left to right over the period, and decreases in the decreasing group. Therefore, fluorine is the most electronegative of the elements (excluding noble gases), whereas cesium is the most electronegative, at least of the elements for which substantial data is available. This will lead people to believe that cesium fluoride is the most ionic compound.
There are some exceptions to this general rule. Gallium and germanium have higher electronegativity than aluminum and silicon, respectively, due to d-block contraction. The elements of the fourth period immediately after the first line of the transition metal have very small atomic radius because 3d electrons are not effective in protecting the increased nuclear charge, and the smaller atomic size is correlated with higher electronegativity (see Allred-Rochow electronegativity , Sanderson electronegativity above). Very high lead electronegativity, especially when compared to thallium and bismuth, appears to be an artifact of data selection (and data availability) - methods of calculation other than Pauling's method show a normal periodic trend for these elements.
Variation of electronegativity with oxidation number
In inorganic chemistry it is common to consider a value of electronegativity that applies to most "normal" situations. While this approach has the advantage of simplicity, it is clear that the electronegativity of the element is not the property of a mutable atom and, in particular, increases with the oxidation state of the element.
Allred uses the Pauling method to calculate separate electronegativities for different oxidation states of several elements (including lead and lead) for which data are available. However, for most elements, there are not enough different covalent compounds, so the bonding dissociation energy is known to make this approach feasible. This is especially true for transition elements, where the quoted electronegativity values ​​are usually, of necessity, averaged over several different oxidation states and where trends in electronegativity are more difficult to see as a result.
The chemical effects of this increase in electronegativity can be seen in both the oxide and halide structures and in the oxide and oxoacidal acids. Therefore CrO 3 and Mn 2 O 7 are acidic oxides with low melting point, while Cr 2 O 3 is amphoter and Mn 2 O 3 is a completely basic oxide.
The effect can also be clearly seen in the dissociation constants of chlorine oxoacids. The effect is much greater than that which can be explained by the negative charge divided among the large number of oxygen atoms, which will cause the difference in p K a of the log 10 ( 1 / 4 ) Ã, = -0.6 between hypochlorite acid and perchloric acid. When the oxidation state of central chlorine atoms increases, more electron density is taken from the oxygen atoms to the chlorine, reducing the partial negative charge on the oxygen atoms and increasing the acidity.
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Group electronegativity
In organic chemistry, electronegativity is associated more with different functional groups than with individual atoms. The terms electronegativity group and substituent electronegativity are used synonymously. However, it is common to distinguish between inductive effects and resonance effects, which can be described as? - and? electron-interference, respectively. There are a number of linear free energy relationships that have been used to measure these effects, in which the Hammet equation is the best known. Kabachnik parameters are group electronegativity for use in organophosphorous chemistry.
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Electropositivity
Electropositivity is a measure of the ability of an element to donate electrons, and therefore to form a positive ion; thus, it is opposed to electronegativity.
Particularly, this is an attribute of metal, which means that, in general, the greater the metal character of an element the greater the electropocyte. Therefore, most alkali metals are electropositive. This is because they have one electron in its outer shell and, since it is relatively far from the nucleus, it is easily lost; in other words, these metals have low ionisation energies.
While electronegativity increases over periods in the periodic table, and decreases the group, the electropositivity decreases over the period (from left to right) and increases in the lower group.
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See also
- The electronegativity element (data page)
- Chemical polarity
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References
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Bibliography
- Jolly, William L. (1991). Modern Inorganic Chemistry (2nd ed.). New York: McGraw-Hill. pp.Ã, 71-76. ISBN: 0-07-112651-1.
- Mullay, J. (1987). "Estimated electronegativity of atoms and groups". Structure and Bond . Structure and Bonding. 66 : 1-25. doi: 10.1007/BFb0029834. ISBN: 3-540-17740-X. Ã,
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External links
- WebElements, lists the values ​​of electronegativity with a number of different calculation methods
- The video explains electronegativity
- Electronegativity Chart, summary list of each element of electronegativity together with interactive periodic table
Source of the article : Wikipedia
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