ELECTRON AFFINITY AND ELECTRONEGATIVITY

ELECTRON AFFINITY
This is also known as electron gain enthalpy.
Electron affinity refers to the change of energy obtained when an electron is added to one mole of a gaseous atom or ion. This can be in different forms which include the following:-

1. First electron affinity
2. Second electron affinity

(A) FIRST ELECTRON AFFINITY
Is the energy released when one mole of isolated gaseous atoms each acquire an electron to form one mole of unnegative ions.
For example:-
  +      →             …………………..  ∆H
-xKJ/mole = First electron affinity

(B) SECOND ELECTRON AFFINITY
Is the energy which is absorbed when one mole of uni-negative (mono negative) gaseous ions each acquire an electron to form one mole of bi negative (dinegative) ions.
For example:-
  T- +     e-  →           …………………………..  ∆H
+xKJ/mole = Second electron affinity

The first electron affinity is negative while the second electron affinity is positive. For the first electron affinity to be negative, it is because the first electron is attracted a gaseous atom. Presence of the attraction means the process is thermally favoured and hence heat is evolved during the process, where for the second electron affinity to be positive it is due to repulsion from the first added electron. Presence of repulsion means the process is not thermally favoured and extra energy must be absorbed to overcome the repulsion before adding the second electron.

PROPERTIES OF ELECTRON AFFINITY.

•  It is a fixed and measurable value. It is being measured in units of KJ/mol for both first and second electron affinity.
• It deals with isolated atoms only. It does deal with atoms which are isolated only and not bonded atoms which are very reactive to attract electrons, for example Cl and O
• An atom has the absolute value of electron affinity. The electron affinity has values of positive and negative for the first and second electron affinities respectively. (i.e. +KJ/mol and  -KJ/mol)
• Does not change regularly in a period or group. When the first electron affinities are arranged together for different isolated atoms, does not change in the same intervals; there is no common difference.

FACTORS AFFECTING THE MAGNITUDE OF ELECTRON

i). Size of an atom.
As the size of an atom become small, nucleus of the atom will have have great tendency of attracting extra electrons towards itself and hence higher energy released during the process of adding extra electron and hence greater electron affinity obtained and vice versa.

ii). Effective nuclear charge.
As the magnitude of effective nuclear charge increase, the tendency of atom to attract the extra electron is more thermally favoured and hence higher electron affinity of an atom.

iii). Electronic configuration of an atom.
Atoms with stable electronic configuration has low electron affinity while those of less stable electronic configuration have high electron affinity.

For example:-
Group II elements:
These have zero electron affinity due to small atomic radii and high effective nuclear charge, they have outermost electronic configuration of ns².
Group VIII elements (noble gases).
They have practically zero electron affinity due to their stable double and octate electron structure of completely paired orbitals.

Consider the following for more understanding

ELECTRON GAIN ENTHALPY OF “Be, Mg, N, P, AND NOBLE GASES.

Be and Mg
Be   have electronic configuration of (1S2   2S2) And Mg have (1S2   2S2 2P6  3S2 )  Have fully filled S-orbital ,due to the extra stability of fully filled orbital in Be (1s2) and Mg (3S2) This show that have no tendency to gain electron .hence Be and Mg have nearly zero electron gain enthalpy.

N and P
N (1S2   2S2   2P3) And P (1S2 2S2 2P6 3S2   3P3) Have half filled 2P and 3P orbital respectively. These configuration being more stable, N and P have little tendency to gain electrons. This accounts for low electron gain enthalpy values for N and P.

Noble gases
All noble gases have fully filled valence shells (nS2   nP6) .due to their highly stable nS2  nP6    configuration ,noble gases have absolutely no tendency to take up any additional electron ,hence ,noble gases have zero electron gain enthalpy.

ELECTRONEGATIVITY
Electronegativity is the measure of the tendency of an atom to attract the shared pair of electrons in covalent bond towards itself. Its opposite term is electropositivity.
An element with an atom of large tendency of attracting a bonding pair of electrons is said to be strong electronegative element or weak electronegative element, while element with an atom of Small tendency of attracting a binding pair of electrons is said to be weak electronegative element or strong electropositive element.

FACTORS AFFECTING THE SIZE OF ELECTRONEGATIVITY
The attraction that a bonding pair of electrons feels for a particular nucleus of one of bonded atoms and hence higher electronegativity of the atom.

i). The size of charge in the nucleus.
The greater number of protons present in the nucleus, the higher the nuclear charge and hence higher electronegativity of the atom.

ii). The amount of screening by inner electrons. 

The greater the screening effect exerted by inner electrons of the atom to the bonding pair of electrons, the less attraction between bonding electrons and hence low electronegativity.

iii). The distance from the nucleus.

The shorter the distance of bonding pair of electrons from the nucleus of the atom, the more attraction between electrons and hence high electronegativity of the atom.

Trend across the period:-
As you move across the period from left to right, the electronegativity increase due to the increase in the size of nuclear charge as the result of increase in number of protons in the same atoms.

Trend down the group:-
As you move down a group electrinegativity decrease due to:-
An increase in distance from the nucleus of the atom to the bonding pairs of electrons due to an increase in number of shells of the atom on descending the group.
An increase in screening effect due to an increase in number of inner electrons on descending the group.

PROPERTIES OF ELECTRONEGATIVITY

• Computed from the atom.

Electronegativity can not be measured in the same way as the value of electron affinity, instead it is computed from the properties of the atom.

• Change in regularity.

The electronegativity change regularly from one another. Their intervals are in fixed ratio from one atom to another.

• Relative value.

The value of electronegativity is relative depending ypon its bonding state. For example sp hybridized carbon is more electronegative than sp² hybridized carbon.

POLARITY
 Polarity of the bond increase as electronegativity difference between the bonding atom become large and the compound or molecules tends to acquire large ionic character .different compound have different character ranging from ionic properties amphoteric    properties to covalent properties similarly different compound have different pH reaction ranging from basic ,neutral and acidic these properties which exhibited by compound are result of electronegativity difference between the bonding atom .

TO JUSTIFY this statement we will consider the following compound
Hydrides of period 2
Hydrides of period 3
Chloride of period 3
Hydroxide of period 3
Hydride of period 2

HYDRIDES OF PERIOD 2

Important hydride of period 2 are NH3, H2O and HF: IN this hydride hydrogen have less electro negativity than N, O and F . This means that  in their chemical bonding the bond electron pair are withdraw more away from  H  Leaving it positive polarized
Extent to which hydrogen is positive polarized increase progressively from NH3, H2O and HF. this is because increase electro negativity of period II progressively from N, O, F .Polarization   of hydrogen determine acidity of hydrides
Acidity of hydride increase as hydrogen become more positive polarized .this means that acidic of hydride increase progressively from NH3   to HF .TO  justify this statement that acidic strength increase from NH3  to HF  we shall consider  the action of each hydride with water

ACTION OF NH3 WITH WATER
Ammonia dissociates in water and form ammonia hydroxide which makes the solution basic
NH3 + H2O -----------------> NH4OH
Ammonia solution is basic in nature this is due to formation of ammonium hydroxide being basic ammonium hydroxide react with alkali to form salt and water .as ammonium solution are base in turn red litmus paper blue

WATER (H2O)
Water is amphoteric has both acidic and basic properties the action of water as acid of base depends on medium in which water is present whether is strong acid or strong medium
Water as base
Water behaves like base and accept hydrogen proton if it treated with substance which is strongly acid than the water itself

WATER AS ACID
Behave like acid and supplies hydrogen proton especially if  it treated with substance which is stronger base than the water it self

ACTION OF HF WITH WATER
HYDROGEN fluoride is stronger than acid. It dissolve in water and form strong acid solution
HF   +   H2O ---------------->H3O     +     F-

HYDRIDE OF PERIOD 3
HYDRIDE of period 3 include NaH, MgH2 ALH3, SIH4, PH3 H2S and HCL

ACTION OF WATER WITH HYDRIDES
In the first four hydrides NaH , MgH2 ALH3,   SIH4  hydrogen is stronger electro negativity than period elements Na  ,Mg   and AL and SI . Hence in their chemical bonding the bond electron pairs are withdrawn more towards hydrogen and make it negatively polarized in aqueous solution such hydrides ionize and release hydrogen (H-) Ion which combine with hydrogen proton released by H2O and form  hydroxide which is given off combine with metallic ion and form metal hydroxide
Example

NaH    +   H2O----> NaOH   +   H2

MgH2   +      2H2O--------> Mg(OH)2      +    2H2

ALH3   +   3H2O--->

SIH4      +  4H2O---->

In case of  phosphine (PH3) hydrogen and phosphorus have equal electro negativity  values the bond electron pair are equally shared between hydrogen and phosphorus and no dipole movement exist on hydrogen and phosphorus atom in phosphine. Phosphine is there for pure covalent compound it is non polar. Has non polar phosphine does not dissolve in polar solvent like water phosphine has no chemical reaction with water
PH3   +     H20-------> NO REACTION
IF phosphine is forced into the water at high pressure the resulting phosphine –water will be neutral and has no action to any litmus paper
In last two hydride  H2S an. d HCL is less electronegative than   S and CL this means that the bond electron pairs are drawn away from hydrogen leaving it positive polarized in aqueous solution the hydrides ionized and released hydrogen proton which make aqueous of these hydride to be acid in nature
H2S    +    2H2O------> 2H3O+       S2-
HCL   +   H2O----------> H3O+       CL-

HYDROXIDE OF PERIOD 3
Hydroxide of period III include NaOH , Mg (OH)2   ,Al(OH)3,  H2SiO3
From Si on ward this compound are not hydride as such but are oxy acid of non metal sodium and magnesium hydroxide have strong tendency of releasing of hydroxyl group in the form of OH ion .there for are the true hydroxide, Al (OH)3. The acidic character of oxy acid is due to the losing of hydrogen ions from hydroxyl group. Sodium and magnesium are less electro negativity than oxygen , this metal tend to released electrons to the OH group thus setting it free as OH ion , but Si ,P  ,S  and CL are electronegative enough  to withdraw electrons buy inductive effect from OH bond thus polarizing it and facilitate the release of hydrogen as proton , therefore hydroxide of Si ,P  ,S  and CL are acidic in nature

Interpretation
It appears from above that change in electro negativity of element in period 3 is accompanying with change in metallic properties of element. Early found far to left have low electro negativity are basic ,on other hands element far to the right have higher electro negativity and have non metallic properties by forming acid oxide which dissolve in water and give stronger acidic solution

ACTION OF HNO3 WITH OXIDES OF PERIOD 3
The oxide of period 3 include   Na2O, MgO, AL2O3 SiO2 P2O5, SO2, CL2O

Na2O, MgO are basic oxide hence react with concentrated nitric acid
Na2O +   2HNO3-------> 2NaNO3        H2O
MgO  +   2HNO3  ---->   Mg (NO3)2      H2O

AL2O3 have no chemical reaction with concentration of nitric acid due to the formation of protective OXIDE LAYER over its surface, the remain oxide are acidic oxide therefore do not react with concentration of nitric,

SiO2 +   HNO3---->    NO REACTION

ACTION OF HNO3 WITH HYDRIDE OF PERIOD 3 ELEMENTS
The hydride of period 3 include NaH, MgH2 ALH3,SiH4,PH3,H2S,and HCL.

Hydrides of element far to the right have Large non metallic character and dissolve in water and give acidic solution
HCl  + H2O ---> H3O+   CL-
H2S +  H2O -------> H3O+       S2-

Being acidic such hydrides have no chemical reaction with nitric acid
HCL +  HNO3 ---->NO CHEMICAL REACTION

Hydride of the element far to the left have large metallic character thus they dissolve in water and give strong basic solution, as base can react with nitric acid
 NaH  + HNO3----> NaNO3    +     H2

Interpretation
Reaction of oxide and hydroxide of period 3 element with nitric acid given above justify that metallic character increases across the period from right to left. Oxide and hydroxide of element far to the right have non metallic character and hence have no chemical reaction with nitric acid. Oxide and hydroxide of the element far to the left have large metallic character and hence react with nitric acid the increase of metallic character in that order is caused by the increase in atomic size and decreasing electro negativity progressively from right to left of the period

CHLORIDE OF PERIOD 3 ELEMENTS
The chloride of period 3 include NaCL, MgCL2, ALCL3, SiCL4, PCL3 and SCL2

ACTION WITH WATER
In this chloride CL is most electronegative than period three element hence in all chloride the bond electron pair are drawn toward  the chlorine atom however as moving from NaCl ,MgCl2.............SCL2. 

The electro negativity difference became small between chlorine and period 3 element result to decrease ionic character progressively from NaCl to SCl2 ,  sodium chloride and magnesium chloride are stronger ionic salt.
Being strong ionic salt sodium chloride and magnesium chloride have no chemical reaction with water. This so because the result base and acid are strong electrolyte which in aqueous solution do not remain in molecule form , instead they ionize almost completely by they do not hybridized in water
NaCl + H2O---->  NO reaction

In aqueous solution sodium chloride and magnesium chloride just ionized and give free ions
NaCl   +  H2O -------> Na+(aq)      Cl-(aq)
The remaining chloride is covalent and hence hydrolysis increase progressively to SCl2, the extent of hydrolysis follows this trend

NaCl <MgCl<AlCll<SiCl,<PCl<SCl.

Overall reaction
Al3+   3H2O -------> Al (OH)33H+

REFERENCES
Modern physical organic chemistry, Eric V.Anslyn and Dennis A.Dougherty,university Science Books,2006

Robert S. MULLIKEN ,Journal of chemical physics,1934,2,782.
Stiven S.Zumdahl,chemical,principles,university ,of Illinois,1998