The perception of the idea electronegativity will open up a whole new aspect of life. Electronegative can be regarded as the back bone of chemistry; and the major reason is not farfetched. We hear of bonding in any chemical reaction before we can think of any desired product, be it a cure, the fuels we use every day in our vehicle, or cosmetics or anything one can think of. The reason we get our products to precision is because we can predict how elements react; the nature of bonds that are most likely going to take place, which tells us state in which the product will exist, their properties and characteristics too. For us to get all accurate, electronegativity is inevitable.
In 1809, Amedeo Avogadro an Italian scientist published a research he carried out that shows a relationship between neutralization that occurs between acids and bases, interestingly enough the same reaction occurs between positive and negative charges. So this led him to suggest that the neutralization reactions could be applied to all sorts of chemical interaction and this led him to propose what he called ‘oxygenicity scale’. This scale is on which any element could be placed. Its position is determined by the element’s tendency to react with other elements. This was done so that comparison can be made between properties of elements tested and those yet to be tested. Therefore, this was what led to the discovery of modern electronegativity scale.
However, Avogadro’s method of determining oxygencity turned out to be easily affected by outside factors such as moisture and impurities. As a result, most of his experiments were inconsistent and inaccurate. It was in 1811, Jacob Berzelius published an article ; a modification of Avogadro’s work which he termed ‘electronegativity’ instead of ‘oxygenicity’. His publication was short lived as his theory failed to account for half of all possible chemical reaction and consequently it was disregarded in favor of more modern views of electronegativity scales. Although Berzelius did provide an almost complete listing of his measured electronegative scales which remarkably corresponds to Pauling’s electronegative scales.
It was finally in 1932; Linus Pauling an American chemist came up with a development of an accurate electronegative scale which is still in use worldwide. According to Pauling, the concept of electronegative is measured along a relative scale that compares the degree to which atoms of different elements tend to attract electrons from their surrounding environment. Because electronegativity scale has no measurable constant value, the scale itself has been difficult to develop.
In making his scale, he used fluorine as a standard for calculation of other electronegative. Fluorine is the most electronegative element. It had a value of four on the electronegative scale while element francium is the least electronegative scale which has a value of zero.
DEFINITION OF ELECTRONEGATIVITY
According to Wikipedia, Electronegativity is basically defined as ‘Electronegativity is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself’ http://en.wikipedia.org/wiki/Electronegativity; however other definitions are as follows:
‘Electronegativity is a measure of the tendency of an element to attract electrons to itself.’ (Philips Matthews (1992). Advanced Chemistry. United Kingdom: Cambridge University Press. 107)
‘Electronegativity of an atom is the power of the atom in a molecule to attract electrons’.(Graham Hill and John Holman, Chemistry in Context, page114,1978)
‘Electronegativity is the relative ability of an atom to attract electrons. When bounded to another atom, the more electronegative element attracts a pair of electrons more strongly than dose the less electronegative element’. (Douglas C. Neckers and Michael P. Doyle, Organic chemistry, pg.6)
‘The ability of an atom in a given molecule to attract electrons to itself is called the relative electronegativity of an atom’. (Mensah, I. A et al, A-level chemistry volume I and II, pg 59)
‘The different affinities of atoms for the electrons in a bound are described by a property called electronegativity: the ability of an atom in a molecule to attract shared electrons to itself’. (Steven S. Zumdahl, 3rd ed. Chemistry, pg 345, 1993).
ELECTRONEGATIVITY TRENDS IN THE PERIODIC TABLE
Electronegativity of the elements exhibit trends just like other properties exhibited by element; ionization energy, atomic radius and so on.
In a period
Electronegativity increases on moving from left to right of a period. For example, fluorine is more electronegative than beryllium, chlorine is more electronegative than magnesium or sodium, bromine is more electronegative than cupper, iron or vanadium and so on. This is as a result of the increase in nuclear charge and therefore, the added electrons can be held more tightly.
In conclusion, the electronegativities increase with the increase in the number of outer electrons. An example of this has been shown using the second and third periods.
From analysis, the graphs that will be obtained will be straight line and will have a constant gradient.
In a group
In moving down a group of the periodic table; from top to bottom, a new shell is added and the nuclear charge increases. Since the nuclear charge increases from top to the bottom, this indicates that the electronegativity of the elements that are located in the bottom of the periodic table should be more than that of element in the top. This is not the case though.
The reason is because of the increase in the atomic radii as we move down the group. The increase in atomic radii increases the electron screening effect, and it is much more than the increase in the nuclear charge. As such, the elements at the bottom are less electronegative than the elements located at the top. Thus, as we move down a group, electronegativity decreases.
In summary, by knowing the electronegativity of two atoms in a molecule, we can predict the type of bond:
Electronegativity difference is higher than 1.7 (sometimes 1.9), it should be an ionic bond
Electronegativity difference is between 0.5-1.7, it should be a polar covalent bond (explained in the next chapter)
Electronegativity difference is under 0.5, the bond should be pure covalent bond
Miroslav Erdelyi. (). Electrogativity. Available: http://chemistry-4end.blogspot.com/2012/02/electrogativity.html. Last accessed 9th november 2012.
Linus Pauling was the original scientist to describe the phenomena of electronegativity. The best way to descbribe his method is to look at a hypothetical molecule that we will call XY. By comparing the measured X-Y bond energy with the theoretical X-Y bond energy (computed as the average of the X-X bond energy and the Y-Y bond energy), we can describe the relative affinities of these two atoms with respect to each other.
Δ Bond Energies = (X-Y)measured – (X-Y)expected
If the electonegativities of X and y are the same, then we would expect the measured bond energy to equal the theoretical (expected) bond energy and therefore the Δ bond energies would be zero. If the electronegativities of these atoms are not the same, we would see a polar molecule where one atom would start to pull electron density toward itself, causing it to become partially negative.
By doing some careful experiments and calculations, Pauling came up with a slightly more sophisticated equation for the relative electronegativities of two atoms in a molecule: EN(X) – EN(Y) = 0.102 (Δ1/2).1 In that equation, the factor 0.102 is simply a conversion factor between kJ and eV to keep the units consistant with bond energies.
By assigning a value of 4.0 to Fluorine (the most electronegative element), Pauling was able to set up relative values for all of the elements. This was when he first noticed the trend that the electronegativity of an atom was determined by it’s position on the periodic table and that the electronegativity tended to increase as you moved left to right and bottom to top along the table. The range of values for Pauling’s scale of electronegativity ranges from Fluorine (most electronegative = 4.0) to Francium (least electronegative = 0.7). 2 Furthermore, if the electronegativity difference between two atoms is very large, then the bond type tends to be more ionic, however if the difference in electronegativity is small then it is a nonpolar covalent bond. (Matthew Salem (UC Davis). (). Pauling Electronegativity. Available: http://chemwiki.ucdavis.edu/Physical_Chemistry/Atomic_Theory/Pauling_Electronegativity. Last accessed 10th November, 2012.)
Unlike Pauling’s method of comparison, Robert S. Mulliken however, proposed a scale which is similar to that of Pauling by relating to the electron affinity EA and ionization potential EP. His values obtained were absolute so he needed convert them to normal values.
Mulliken scale= (3.48X)-0.602
Allen and Rochow, established their electronegativity scale by putting into consideration the charge on an atom. To calculate the electronegativity, the charge is measured per unit area of the atomic surface of that atom using. Here, Slater’s rules is very crucial, it is used determine nuclear charge experienced by valence electrons. However, surface area of an atom in a molecule is needed so that we can calculate the covalent radius.
http://upload.wikimedia.org/wikipedia/commons/thumb/d/d1/Pauling_and_Allred-Rochow_electronegativities.png/300px-Pauling_and_Allred-Rochow_electronegativities.png The correlation between Allred-Rochow electronegativities (x-axis, in Å-2) and Pauling electronegativities (y-axis). The correlation between Allred-Rochow electronegativities (x-axis, in Å-2) and Pauling electronegativities (y-axis).
Sanderson’s electronegativity scale is based on idea on how atoms tend to attract other atoms or molecules to themselves taking into consideration the charge experienced by the valency electron from its nucleus
This charge is what determines how the atom could attract other atoms in a chemical reaction. The size on an atom is inversely proportional to the nuclear charge. Therefore, if the nuclear charge is high, the atom becomes compacted and the atom becomes small. This was what built the grounds of Sanderson’s electronegativity scale. He proposed his theory which was that the tendency of an atom to attract or repel other atoms should be as a result of how compacted or compressed an atom is. He prove this mathematically,
Sanderson’s major contribution to chemistry was the principle of electronegativity equalization, which states that when atoms of differently electronegative combine chemically, they adjust their electronegativities so that equal values of the combined electronegativity is maintained.
In the absence of metals to donate electrons, the atoms of non-metals usually attain a stable configuration by donating equal number of electrons for sharing. This type of bonding is known as covalent bonding.
Kasimir Fajans proposed some rules that predict the type of bond that is likely to occur between ions. He explained that when two oppositely charges ions come together to form a bond, the positively charged ion attracts the negatively charged one. But due to the positive charge in the nucleus, the bonding positively charged ion is being repelled and attracted at the same time. This continuous repulsion and attraction results to distortion of the negatively charged ion as result covalent bonding occurs whereby the positively charged ions share the negatively charged ions in order to attain stability
An increase in the degree of polarization of anion is favoured by:
Large charge and little size of the cation.
Large charge and large size of the anion.
The intensity of distortion of an anion is related to its softness
INTERMOLECULAR FORCES OF ATTRACTION
¤+CL F¤- ¤+H Cl¤-
¤+CL F¤- ¤+H Cl¤-
¤+CL F¤- ¤+H Cl¤-
O¤- O¤- O¤-
¤+H ¤+H ¤+H ¤+H ¤+H ¤+H
A dipole first, can be defined as the process of separating charge from a molecule. A dipole-dipole attraction however is defined as an intermolecular force of attraction developed between opposite partially charged ends of molecules. This can be explain best as; when the electrons shared between two atoms in a covalent bond which have high difference in electronegativity. The electrons be will be pulled more towards the electronegative element. This will make a more electronegative partial negative charge and hence creates a dipole. The chargers are mostly found between the ends two polar molecules. Dipole- dipole occurs when there is an unequal sharing of particles (electrons) between the atoms of a molecule. Dipoles are most prominent in polar molecules such as water and hydrogen fluoride. The bond between the partial positive charged walls of the polar molecule and the partial negative charged walls of one polar molecule is a very weak bond formed and it is termed as asymmetric distribution of charge.
This type of dipole-dipole attraction involving hydrogen in one bond and highly electronegative bond in the other liquid contains hydrogen bond tends to have an usually higher boiling point than expected. Example is in water where the boiling point is suppose to be below 60oC it rises up to 100oC. this is as a result of hydrogen bond.
VANDER WAALS FORCES
¤+H H ¤-
H¤+H H ¤-
¤+H H ¤-
Vander Waals suggested that in the absence of difference in electronegativity between two atoms in a bond, a dipole does not develop. In such cases, effects of external forces can cause the molecule to become asymmetrical. The attraction will exist between molecules is instantaneous and weak. It was named after the founder; Vander Waals forces of attraction.