HISTORICAL BACKGOUND
In 1809, Amedeo Avogadro an Italian scientist shared a research he completed that presents a romance between neutralization occurring between acids and bases, oddly enough enough the same response occurs between negative and positive charges. Which means this led him to claim that the neutralization reactions could be applied to all types of chemical interaction which led him to propose what he called 'oxygenicity level'. This size is on which any component could be put. Its position depends upon the element's tendency to react with other elements. This is done so that comparison can be made between properties of elements analyzed and the ones yet to be examined. Therefore, this is what resulted in the breakthrough of modern electronegativity range.
However, Avogadro's method of determining oxygencity turned out to be easily damaged by outside the house factors such as wetness and impurities. Because of this, most of his tests were inconsistent and inaccurate. It had been in 1811, Jacob Berzelius published an article ; an adjustment of Avogadro's work which he termed 'electronegativity' instead of 'oxygenicity'. His publication was temporary as his theory failed to account for half of most possible chemical effect and consequently it was disregarded and only more modern views of electronegativity scales. Although Berzelius have provide an almost complete report on his measured electronegative scales which extremely corresponds to Pauling's electronegative scales.
It was finally in 1932; Linus Pauling an American chemist came up with a development of a precise electronegative level which is still used worldwide. Matching to Pauling, the concept of electronegative is measured along a relative level that compares the degree to which atoms of varying elements tend to draw in electrons from their surrounding environment. Because electronegativity level does not have any measurable continuous value, the size itself has been difficult to build up.
In making his level, he used fluorine as a typical for calculation of other electronegative. Fluorine is the most electronegative element. It got a value of four on the electronegative range while aspect francium is minimal electronegative scale that includes a value of zero.
DEFINITION OF ELECTRONEGATIVITY
According to Wikipedia, Electronegativity is actually thought as 'Electronegativity is a chemical property that represents the tendency associated with an atom or an operating group to get electrons towards itself' http://en. wikipedia. org/wiki/Electronegativity; however other meanings are as follows:
'Electronegativity is a way of measuring the tendency associated with an element to entice electrons to itself. ' (Philips Matthews (1992). Advanced Chemistry. United Kingdom: Cambridge School Press. 107)
)
'Electronegativity of atom is the energy of the atom in a molecule to get electrons'. (Graham Hill and John Holman, Chemistry in Framework, page114, 1978)
'Electronegativity is the relative ability of atom to attract electrons. When bounded to some other atom, the greater electronegative element draws in a set of electrons more highly than dosage the less electronegative element'. (Douglas C. Neckers and Michael P. Doyle, Organic chemistry, pg. 6)
'The ability of atom in confirmed molecule to appeal to electrons to itself is named the comparative electronegativity of any 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 house called electronegativity: the ability of the atom in a molecule to attract distributed electrons to itself'. (Steven S. Zumdahl, 3rd ed. Chemistry, pg 345, 1993).
ELECTRONEGATIVITY TRENDS WITHIN THE PERIODIC TABLE
Electronegativity of the elements display trends exactly like other properties exhibited by aspect; ionization energy, atomic radius and so on.
In a period
Electronegativity rises on moving from departed 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 consequently of the increase in nuclear charge and therefore, the added electrons can be performed more firmly.
In finish, the electronegativities increase with the upsurge in the amount of outer electrons. An example of this has been proven using the next and third durations.
Period Two
Elements
Li
Be
B
C
N
O
F
Electronegativity value
1. 0
1. 5
2. 0
2. 5
3. 0
3. 5
4. 0
Period Three
Elements
K
Mg
Al
Si
P
S
K
Electronegativity value
0. 9
1. 2
1. 5
1. 8
2. 1
2. 5
3. 0
From analysis, the graphs that will be obtained will be in a straight line line and will have a frequent gradient.
In a group
In moving down several the periodic desk; throughout, a new shell is added and the nuclear fee increases. Since the nuclear charge boosts from top to underneath, this indicates that the electronegativity of the elements that are positioned in underneath of the periodic table should be more than that of element in the most notable. This isn't the case though.
The reason is due to upsurge in the atomic radii as we move down the group. The increase in atomic radii escalates the electron screening effect, and it is much more than the increase in the nuclear fee. As such, the elements at the bottom are less electronegative than the elements located at the very top. Thus, even as move down a group, electronegativity lowers.
In brief summary, by knowing the electronegativity of two atoms in a molecule, we can forecast the sort of bond:
Electronegativity difference is greater than 1. 7 (sometimes 1. 9), it should be an ionic bond
Electronegativity difference is between 0. 5-1. 7, it ought to be a polar covalent bond (explained in the next chapter)
Electronegativity difference is under 0. 5, the relationship should be natural covalent bond
Miroslav Erdelyi. (). Electrogativity. Available: http://chemistry-4end. blogspot. com/2012/02/electrogativity. html. Previous accessed 9th november 2012.
CALCULATING ELECTRONEGAVITIES
Pauling Electronegativity
Linus Pauling was the initial scientist to spell it out the phenomena of electronegativity. The ultimate way to descbribe his method is to check out a hypothetical molecule that we will call XY. By checking the measured X-Y bond energy with the theoretical X-Y relationship energy (computed as the average of the X-X bond energy and the Y-Y relationship energy), we can describe the comparative affinities of these two atoms with respect to each other.
О Connection Energies = (X-Y)measured - (X-Y)expected
If the electonegativities of X and y will be the same, then we would expect the assessed bond energy to equal the theoretical (expected) connection energy and then the О connection energies would be zero. If the electronegativities of these atoms are not the same, we would visit a polar molecule where one atom would begin to draw electron density toward itself, causing it to be partially negative.
By doing some careful tests and calculations, Pauling developed a slightly more sophisticated equation for the relative electronegativities of two atoms in a molecule: EN(X) - EN(Y) = 0. 102 (О1/2). 1 In that equation, the factor 0. 102 is simply a alteration factor between kJ and eV to keep carefully the systems consistant with relationship energies.
By assigning a value of 4. 0 to Fluorine (the most electronegative element), Pauling could set up relative values for all the elements. This is when he first discovered the craze that the electronegativity of any atom was dependant on it's position on the regular table and that the electronegativity tended to increase as you shifted left to right and underlying part to top over the table. The range of principles for Pauling's range of electronegativity runs 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 relationship type is commonly 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 seen 10th November, 2012. )
MULLIKEN ELECTRONEGATIVITY
Unlike Pauling's method of assessment, Robert S. Mulliken however, proposed a scale which is similar to that of Pauling by associated with the electron affinity EA and ionization potential EP. His principles obtained were absolute so he needed convert them on track values.
Mathematically,
X=
Mulliken level= (3. 48X)-0. 602
ALLRED-ROCHOW ELECTRONEGATIVITY
Allen and Rochow, established their electronegativity scale by putting into consideration the charge with an atom. To calculate the electronegativity, the demand is assessed per unit area of the atomic surface of this atom using. Here, Slater's rules is very important, it is employed determine nuclear demand experienced by valence electrons. However, surface area of the atom in a molecule is necessary so that we can analyze 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 relationship between Allred-Rochow electronegativities (x-axis, in -2) and Pauling electronegativities (y-axis). The correlation between Allred-Rochow electronegativities (x-axis, in -2) and Pauling electronegativities (y-axis).
SANDERSON'S ELECTRONEGATIVITY
Sanderson's electronegativity size is dependant on idea about how atoms have a tendency to attract other atoms or substances to themselves taking into consideration the charge experienced by the valency electron from its nucleus
This charge is exactly what determines how the atom could draw in other atoms in a chemical substance reaction. The size by using an atom is inversely proportional to the nuclear fee. Therefore, if the nuclear fee is high, the atom becomes compacted and the atom becomes small. This was what built the lands of Sanderson's electronegativity scale. He proposed his theory that was that the tendency of an atom to catch the attention of or repel other atoms should be consequently of how compacted or compressed an atom is. He show this mathematically,
Electronegativity=
Sanderson's major contribution to chemistry was the theory of electronegativity equalization, which says that whenever atoms of in another way electronegative combine chemically, they adjust their electronegativities so that similar values of the merged electronegativity is preserved.
COVALENT BONDING
In the absence of metals to contribute electrons, the atoms of non-metals usually attain a stable settings by donating equal quantity of electrons for showing. This type of bonding is recognized as covalent bonding.
FAJANS' RULES
Kasimir Fajans suggested some rules that predict the kind of bond that is likely to take place between ions. He discussed that whenever two oppositely charges ions get together to create a connection, the positively costed ion allures the negatively costed one. But due to the positive demand in the nucleus, the bonding positively charged ion is being repelled and fascinated at the same time. This continuous repulsion and appeal results to distortion of the negatively recharged ion as effect covalent bonding occurs whereby the positively charged ions discuss the negatively recharged ions in order to realize stability
Fajan's rule
http://oscience. info/chemistry/fajan%E2%80%99s-guideline/
An increase in the amount of polarization of anion is favoured by:
Large demand and little size of the cation.
Large fee and large size of the anion.
The power of distortion of any anion is related to its softness
INTERMOLECULAR FORCES OF ATTRACTION
DIPOLE-DIPOLE 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 procedure of separating charge from a molecule. A dipole-dipole attraction however is thought as an intermolecular pressure of fascination developed between complete opposite partially billed ends of substances. This can be describe best as; when the electrons shared between two atoms in a covalent relationship which have high difference in electronegativity. The electrons be will be drawn more for the electronegative factor. This will make a more electronegative partial negative charge and hence creates a dipole. The chargers are mainly found between your ends two polar molecules. Dipole- dipole occurs when there can be an unequal writing of debris (electrons) between your atoms of a molecule. Dipoles are most dominant in polar substances such as drinking water and hydrogen fluoride. The bond between the incomplete positive charged wall surfaces of the polar molecule and the partial negative charged walls of 1 polar molecule is a very weak bond made which is termed as asymmetric circulation of fee.
HYDROGEN BOND
This type of dipole-dipole attraction including hydrogen in one relationship and highly electronegative bond in the other water contains hydrogen connection tends to produce an usually higher boiling point than expected. Example is within water where the boiling point is assume to be below 60oC it goes up up to 100oC. this is therefore of hydrogen bond.
VANDER WAALS FORCES
+H H -
= H2
H
H+H H -
+H H -
Vander Waals advised that in the absence of difference in electronegativity between two atoms in a relationship, a dipole does not develop. In such instances, effects of external makes can cause the molecule to be asymmetrical. The appeal will are present between molecules is instantaneous and fragile. It was known as after the creator; Vander Waals makes of interest.
