Ionization energies or electron affinities of isolated gaseous atoms. Such quantities are a compute of the tendency of isolated atoms to lose and gain electrons. We are going to study about electronegativity of an element. The electronegativity of an element is a measure of the power of an atom in a molecule to magnetize shared electrons to it. Unlike ionization or electron affinity, it is not a directly computable physical quantity but rather, a theoretical conception for that numerous numerical scales have been developed.
In this chapter, we are going to converse the 3 most significant scales which have been developed for measuring electronegativity.
Pauling electronegativity scale:
As we familiar in homonuclear diatomic molecule similar to A2 (A-A) and B2,(B-B) the electron pair is equally shared between the atoms bonded together. In a heteronuclear diatomic molecule of the AB type though, the condition is quite different. In the procedure of formation of a bond between A or B, atom A slowly starts stripping off its electrons thereby becoming a partially cationic species ('it becomes partially positively charge'). As the positively develops on A, its tendency to attract electron amplifies. Meanwhile a alike procedure as well occur on atom B, therefore both atoms A or B start attracting the electron pair towards themselves. They continue doing thus until the tendencies of both the atoms in the bonded condition to attract the electron pair towards themselves balance.
If one of the atoms say B, has superior tendency to attract the electron pair toward itself compared to that another atom say A, the electron pair will expend more time on B than A. Fractional charges will therefore be created on A and B. This tendency of attracting the shared electron pair toward itself via an atom a molecule has been named the electronegativity of the element the model of electonegativity was 1st developed through Pauling. He described it on the basis of the samples desirable in the single bond energies of elements that were derived from the thermo chemical data. He recognized that bond energy, EA-B between 2 unlike ion similar to atom A and B is greater than √EA-A √EB-B where √E A-A or √EB-B are bond energies of A-A and B-B homonuclear bonds. He allotted the course of this excess bond energy √E A-B - √E A-A √EB-B to the electrostatic attraction between partially charged atoms are divided due to dissimilarity in electron attracting tendencies of A & B which is the difference in the electronegativity of A or B. He was able to derive a connection that can be termed as
ΔXA-B = √ EA-B (√ EA-A √ EB-B)/ 96.49
Here XA is the electronegativity of element A or XB is that of element B. getting bond energies, it is possible to analyze the dissimilarity between the electronegativities of the 2 elements. This formula only gives the difference in the electronegativities of the 2 elements or not the absolute value assigned to a particular element. The largest electronegativity difference is that between fluorine the most electronegative element and calcium, the most electropositive element which came out to be 3.3. Pauling assigned arbitrarily a entire number value 4.0 for fluorine so that values of electronegativity of all elements stay positive. Below Table shows values of electronegativity of different elements (bold faced) as calculated via Pauling using his formulae.
Mulliken - jaffe electronegativity scale:
Mulliken described electronegativity as the mean value of 1st ionization energy and 1st electron affinity. Both quantities are following positive values if loss of electron involves absorption of energy and negative values if gain of electron involves liberate of energy. Therefore electronegativity XA of atom A is given by the following connection.
XA = (I)A (EA)A/2
According to this relationship, a very electronegative element has very high ionization energy. So it will be difficult to eliminate its electrons. It as well has a very high electron affinity. Therefore, a very stable species results when electrons are added. On another hand, an element of low electronegativity will have a low ionization energy or low electron affinity. So it looses electrons readily and has tiny tendency to pick up electron.
It is very hard to compute electron affinity for all elements. Thus this process is not universally applicable. The electronegativity values on Mulliken Scale are about 2.8 times those of Pauling's values. The trends in the difference of electronegativity are though similar.
Alfred-rochow electronegativity scale:
According to Alfred Rochow, electronegativity is equated to the force of attraction between an atom and the electron separated by a distance equal to the covalent radius of the atom. The force of attraction is expressed according to coulomb's rule as: F = Z*e2 /r2
Here Z* is Slater's effective nuclear charge e the electronic charge or r- the covalent radius.
Table: Electronegativity values of element
Electronegativity is determined of the attraction which an atom has for electrons in a bond it has formed with the other atom. The ability of an atom to attract electrons depends on the charge on the atom or the hybridization of the atom. An atom that has acquired a positive charge will tend to attract electrons to it more readily than will a unbiased atom. In turn, a negatively charged atom will be less attractive to electron than a unbiased atom. Hybridization as well affects electronegativity since of lower energy and therefore, greater electron attracting power of s-orbital. Therefore hybrid orbitals having greater s-character possess higher electronegativity. An atom in sp hybridized state will be more electronegative than similar atom in SP2 hybridized state that will in turn be more electronegative than similar atom in SP3 hybridized state. Thus the carbon atom in CH4, C2H4 and C2H2 has dissimilar values of electronegativity.
So we can say that electronegativity is not a steady quantity. Everyone the electronegativity scales provide only average values of electronegativities of element in different bonding environments.
All the electronegativity scales we studied offer only relative values of electronegativities. Such values are nevertheless useful in making quantitative comparisons between elements. Electronegativities can be used to predict the value of the bonding which has a compound will have. The larger the difference between the electronegativities of the 2 elements, the more polar will be the bond between such elements. An electronegativity difference of concerning 107 corresponds to a partial ionic character of about 50%. Thus, a bond can be considered predominantly ionic, if the difference in the electronegativities of the bonded is more than 1.7. On the contrary, a difference in the range of 0.4 - 1.7 consequences in a covalent bond's partial ionic characters and polar covalent bond.
Periodicity in Electronegativity:
Electronegativity values of elements show fairly discernible periodic trend all through the periodic table. The trend is alike to that of ionization energies. Therefore, as predictable, electronegativity of elements raises sharply across a row of S and P-block element. This is because a consequence of the sharp enhance in effective nuclear charge of these elements instance from lithium to fluorine. Though, across a series of conversion elements, the enhance in electronegativity is much smaller. This is since the additional electron is being added to an inner shell that gives relatively good defending for the outer electron from the nucleus. On moving down a group of representative elements, for instance in the lithium collection, there is a general reduce in electronegativity. The diminish is relatively small except between the first two elements.
The much greater electronegativity of lithium row elements correlates well with their small size. Inevitable, the elements of period 4 from gallium onwards which is Ga, Ge, As, Se or Br contain greater electronegativities than would be expected through extrapolation from values for the first two elements in the relevant groups. This is due to the addition of transition elements since off that the effective nuclear charge of these elements is greater than that, if the change elements were not there. Likewise, the presence of the lanthanide elements is responsible for greater electronegativity of the elements of 5d series than would be expected through extrapolation from values of the elements of 3d and 4d series.
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