Electronegatativity?

Question

I got a chem final 2morrow And this is 1 of the concepts
How do You determine this?

For example
my study guide says
example:H2O.

Electronegativity O - Electronegativity H
3.44-2.20=1.24

The electronegativity between H and O is 1.24.

Therefore, H2O is polar covalent.

How do you know
O is 3.44 AND H is 2.20??

And another question.
Any good websites for polar & non polar bonds
like just a good site for explaining them well.

any Answer to these 2 questions.
THANKS

Answer 1

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to caesium and francium which are the least electronegative at 0.7.

Defining polar bonds

This is described as a polar bond. A polar bond is a covalent bond in which there is a separation of charge between one end and the other - in other words in which one end is slightly positive and the other slightly negative. Examples include most covalent bonds. The hydrogen-chlorine bond in HCl or the hydrogen-oxygen bonds in water are typical.

What happens if B is a lot more electronegative than A?

In this case, the electron pair is dragged right over to B's end of the bond. To all intents and purposes, A has lost control of its electron, and B has complete control over both electrons. Ions have been formed.

Summary

No electronegativity difference between two atoms leads to a pure non-polar covalent bond.

A small electronegativity difference leads to a polar covalent bond.

A large electronegativity difference leads to an ionic bond.



Polar bonds and polar molecules

In a simple molecule like HCl, if the bond is polar, so also is the whole molecule. What about more complicated molecules?

In CCl4, each bond is polar.

Methods of calculation

] Pauling electronegativity
Pauling first proposed 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 proponent, this "additional stabilization" of the heteronuclear bond is due to the contribution of ionic canonical forms to the bonding.

The difference in electronegativity between atoms A and B is given by:


where the dissociation energies, Ed, of the A–B, A–A and B–B bonds are expressed in electronvolts, the factor (eV)−½ being included to ensure a dimensionless result. Hence, 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 availability of thermodynamic data, and it is these "revised Pauling" values of the electronegativity 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 tendency 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 resemble the more familiar Pauling values. For ionization energies and electron affinities in electronvolts,
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[10] 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 ångströms,
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. Also Sanderson electronegativities were used for different investigations in organic chemistry.


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.[16] 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.


Correlation of electronegativity with other properties

The variation of the isomer shift (y-axis, in mm/s) of [SnX6]2− anions, as measured by 119Sn Mössbauer spectroscopy, against the sum of the Pauling electronegativities of the halide substituents (x-axis).The wide variety of methods of calculation of electronegativities, which all give results which correlate well with one another, is one indication of the number of chemical properties which might be affected by electronegativity. The most obvious application of electronegativities is in the discussion of bond polarity, for which the concept was introduced by Pauling. In general, the greater the difference in electronegativity between two atoms, the more polar the bond that will be formed between them, with the atom having the higher electronegativity being at the negative end of the dipole. Pauling proposed an equation to relate "ionic character" of a bond to the difference in electronegativity of the two atoms, although this has fallen somewhat into disuse.

Several correlations have been shown between infrared stretching frequencies of certain bonds and the electronegativities of the atoms involved: however, this is not surprising as such stretching frequencies depend in part on bond strength, which enters into the calculation of Pauling electronegativities. More convincing are the correlations between electronegativity and chemical shifts in NMR spectroscopy or isomer shifts in Mössbauer spectroscopy (see figure). Both these measurements depend on the s-electron density at the nucleus, and so are a good indication that the different measures of electronegativity really are describing "the ability of an atom in a molecule to attract electrons to itself".

Answer 2

Linus Pauling calculated electronegativities from experimental data many years ago. Really, the best values are O=3.5 and H=2.2. So you have to accept his values. Your teacher may have told you how Pauling got them. In that case, you'll know.

I don't know of such web sites. The only thing is to compare electronegativities. If they are slightly different, then the bond is slightly polar. If they are equal, then the bond is nonpolar. For example, the electronegativities of carbon and sulfur are each 2.5. So the bonds in CS2 are nonpolar.

The next question is whether molecules are polar. Water is a polar molecule, because it is shaped like a boomerang. The slightly postively charged H atoms are in the wings, while the slightly negatively charged O is in the center. CO2 is anonpolar molcule, because it islinear, O=C=O. The O's are 3.5, and the C is 2.5, but the symmetry of the moecule means that there is no net separation of charge.

Answer 3

The electronegativity values are given in tables. Unless your teacher is obsessed with details you shouldn't have to know them. However, it is good to know that the largest values are found at the top right of the table and the smallest on the bottom left. The wiki is a good place to find basic information about polar and non polar bonds. Basic chemistry textbooks generally have more thorough explanations though.

http://en.wikipedia.org/wiki/Chemical_polarity

Answer 4

I don't know but logical reasoning is that why H is 2.20 is because hydrogen's mass is 1.1 and you have 2 molecules of H in H20 and therefore you do 2 times 1.1 which is 2.2.

Answer 5

simply
I can say that E.N is the tendency of an atom to attract an electron pair
and you said the EN of O and H.
It is referred from EN scales.
The popular EN scales are pauling scale,Mullicken scale,
Alfred Rouschow scale.
YOU will get more on this site-------wikipedia
good luck

Answer 6

Eah of the answer is to the point.I would only be repeating it.

Answer 7

read this
http://en.wikipedia.org/wiki/Chemical_polarity
http://en.wikipedia.org/wiki/Electronegativity