how can one compare electronegativity of elements?

Question

how can one compare electronegativity of elements?

Answer 1

Electronegativity, symbol χ, is an chemical property which describes the power of an atom (or, more rarely, a functional group) to attract electrons towards itself. First proposed by Linus Pauling in 1932 as a development of valence bond theory, it has been shown to correlate with a number of other chemical properties. Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed and, although there may be small differences in the numerical values of the electronegativity, all methods show the same periodic trends between elements.

Electronegativity, as it is usually calculated, is not strictly an atomic property, but rather a property of an atom in a molecule: the equivalent property of a free atom is its electron affinity. It is to be expected that the electronegativity of an element will vary with its chemical environment,but it is usually considered to be a transferable property, that is to say that similar value values will be valid in a variety of situations.





Methods of calculation:





Pauling electronegativity

Pauling first proposed[2] the concept of electronegativity in 1932 as an explanation of the fact that the covalent bond between two different atoms (A–B) is stronger than would be expected by taking the average of the strengths of the A–A and B–B bonds. According to valence bond theory, of which Pauling was a notable proponant,this "additional stabilization" of the heteronuclear bond is due to the contribution of ionic canonical forms to the bonding.



The difference in Pauling electronegativity between hydrogen and bromine is 0.73 (dissociation energies: H–Br, 3.79 eV; H–H, 4.52 eV; Br–Br 2.00 eV)



As only differences in electronegativity are defined, it is necessary to choose an arbitrary reference point in order to construct a scale. Hydrogen was chosen as the reference, as it forms covalent bonds with a large variety of elements: its electronegativity was fixed first at 2.1, later revised[5] to 2.20. It is also necessary to decide which of the two elements is the more electronegative (equivalent to choosing one of the two possible signs for the square root). This is done by "chemical intuition": in the above example, hydrogen bromide dissolves in water to form H+ and Br− ions, so it may be assumed that bromine is more electronegative than hydrogen.

To calculate a Pauling electronegativity for an element, it is necessary to have data on the dissociation energies of at least two types of covalent bond formed by that element. Allred updated Pauling's original values in 1961 to take account of the greater availablity of thermodynamic data,[5] and it is these "revised Pauling" values of the ronegativity which are most usually used.



Mulliken electronegativity



The correlation between Mulliken electronegativities (x-axis, in kJ/mol) and Pauling electronegativities (y-axis).Mulliken proposed that the arithmetic mean of the first ionization energy and the electron affinity should be a measure of the tendancy of an atom to attract electrons. As this definition is not dependent on an arbitrary relative scale, it has also been termed absolute electronegativity, with the units of kilojoules per mole or electronvolts.



However, it is more usual to make use of a linear transformation to transform these absolute values into values which ressemble the more familiar Pauling values. For ionization energies and electron affinities in electronvolts,



χ = 0.187(Ei + Eea) + 0.17

and for energies in kilojoules per mole,

The Mulliken electronegativity can only be calculated for an element for which the electron affinity is known, fifty-seven elements as of 2006.



Allred-Rochow electronegativity

The correlation between Allred-Rochow electronegativities (x-axis, in Å−2) and Pauling electronegativities (y-axis).Allred and Rochow considered that electronegativity should be related to the charge experienced by an electron on the "surface" of an atom: the higher the charge per unit area of atomic surface, the greater the tendency of that atom to attract electrons. The effective nuclear charge, Z* experienced by valence electrons can be estimated using Slater's rules, while the surface area of an atom in a molecule can be taken to be proportional to the square of the covalent radius, rcov. When rcov is expressed in angstroms,

Sanderson electronegativity

The correlation between Sanderson electronegativities (x-axis, arbitrary units) and Pauling electronegativities (y-axis).Sanderson has also noted the relationship between electronegativity and atomic size, and has proposed a method of calculation based on the reciprocal of the atomic volume. With a knowledge of bond lengths, Sanderson electronegativities allow the estimation of bond energies in a wide range of compounds.

Allen electronegativity

The correlation between Allen electronegativities (x-axis, in kJ/mol) and Pauling electronegativities (y-axis).Perhaps the simplest definition of electronegativity is that of Allen, who has proposed that it is related to the average energy of the valence electrons in a free atom.

where εs,p are the one-electron energies of s- and p-electrons in the free atom and ns,p are the number of s- and p-electrons in the valence shell. It is usual to apply a scaling factor, 1.75×10−3 for energies expressed in kilojoules per mole or 0.169 for energies measured in electronvolts, to give values which are numerically similar to Pauling electronegativities.

The one-electron energies can be determined directly from spectroscopic data, and so electronegativities calculated by this method are sometimes referred to as spectroscopic electronegativities. The necessary data are available for almost all elements, and this method allows the estimation of electronegativities for elements which cannot be treated by the other methods, e.g. francium, which has an Allen electronegativity of 0.67. However, it is not clear what should be considered to be valence electrons for the d- and f-block elements, which leads to an ambiguity for their electronegativities calculated by the Allen method.

Answer 2

One method of setting up the scale of electronegativities involves the use of bond energies. Bond energies is defined as the energy required to break a bond so as to from neutral atoms. It can be determined experimentally by measuring the heat involved in the decomposition reaction or by measuring spectroscopically the energy difference between the molecules in its lowest vibrational state and in the completely dissociated state. The relation between bond energy and electronegativity can be seen from the following example: It is found that 431 kJ of heat is required to break the Avogadro number of H2 molecules into individual atom. Thus the bond energy of H2 is 431 kJ per Avogadro number of bonds, or 7.16 × 10^ (–22) kJ per bond. Because the shearing of the electron pair is equal between two H atoms, it would be reasonable to assume that each bonded atom contributes half the bond energy, or 3.58× 10^(–22)kJ. Similarly the bond energy found for Cl 2,239 kJ per Avogadro number of bonds, we deduce that a Cl atom should contribute 1.99 ×10^(–22) to any bond in which the sharing of an electron pair is equal.



Suppose we now consider the bond in HCl. The bond is polar, but for the moment let us imagine that it is nonpolar; i.e. the electron pair is shared equally. This amounts to picturing H in HCl to be the same as in H2, and Cl to be same as in CL2. If H contributes 3.58× 10^(–22) kJ and if Cl contributes 1.99× 10^(–22) kJ, the expected bond energy of HCl should be the same of these contributions, or 5.57×10^(–22) kJ. Actually the bond energy of HCl found by experiment is 7.09×10^ (–22) kJ. This is significantly greater than the calculated value, which means that HCl is more stable than our model predicts.



The enhanced stability of HCl can be attributed to unequal sharing of the electron pair. If the electron pair spent more time On Cl, that end of the molecule would become negative, and the H end positive. Since the positive and negative ends would attract each other, there would be additional binding energy. The amount of additional energy would depend on the relative electron-pulling ability of the bonded atoms, since the greater the charge difference between the ends of the molecules, the greater the additional binding energy. Thus, it should be possible to estimate relative electronegativities from the difference between experimental bond energies and those calculated assuming equal sharing.



Specific numerical values of electronegativity have been selected by a complex procedure, and support for such assignment of elecrtonegativity values comes from measurements of dipole moments. Value of 4.0 has been assigned to fluorine and the electronegativities of other elements have been calculated against this standard by the application of this formula:



Xa – Xb = 0.208[E(a–b) – {E(a–a) × E(b–b)}^(1/2)]^(1/2)



Xa → electronegativity of element a

Xb → electronegativity of element b

E(a–b) → bond energy of the molecule (a–b)

E ( a–a) → bond energy of the molecule (a–a)

E (b–b) → bond energy of the molecule (b–b)

Answer 3

Indeed, electronegativity of an element is the tendency to attract shared pair of electrons toward its side. The larger then anion, the greater the electonegativity it would possess. Flourine is the most electronegative element in present periodic table.

For instance, in H-Cl bond cleavage, Cl is more electronegative than hydrogen, thus, the shared pair of electrons is shifted towards Cl. Thus, in this instance, Cl is more electronegative than H. There are certain other examples regarding this. Thus, the elemnts can be easily compared on th ebasus of electronegativity an dhence can be ''scaled''. I hope your doubt is now clear.

Answer 4

Yes, electronegativity is the tendency of an atom (element) to attract the electrons to his nucleus. When we are talking of a molecule (2 or more elements), There´s a fight for who will keep for more the electrons. Obviously, the most electronegative will have the electrons for a more time.



There is a Scale, called The Pauling electronegativity Scale, in which the Fluorine is the most electronegative, whit a value of 4.



There are some rules:



when the difference of electronegativy between the 2 atoms is Zero, the bond will be Covalent. So, both atoms will have the electrons in the same way.



When the difference is 1.7 or higher, it´s a ionic bond; and a Cation and an Anion will be generated, like in the sodium chloride; as happens between Oxigen or Fluorine with the elements of the I and II group.



Nevertheless, when the difference is among 0 to 1.7,. a covalent character will predominate. as in the C-H bond.



So, Depending on the difference of electronegativity, the covalent bond can be classified in polar covalent and; pure covalent or non polar.



If the difference of electronegativity is between 0.4 and 1.7 is a covalent bond, and if is lower than 0.4 is non polar covalent bond.

Answer 5

BY STUDYING STRUCTURE OF THE ATOM

Answer 6

Electronegativity of elements in the periodic table have a trend. It increases as you go up and right of the table. That is how you compare electronegativities of elements. However, you are right when you said that it cannot be directly measured.

Answer 7

Yes you are right in saying that electronegativity cannot be scaled, but just to compare their tendencies to draw a shared pair of electrons towards themselves, a formal scale has been defind based on some thermodynamical values by quite a lot of people.

Among the best one has been given by Linus Pauling and is called Pauling Scale