Electron affinity and periodic variation of electron affinity.

What is electron affinity ?

The amount of energy released when an electron is added to the valence shell of an isolated gaseous atom or an ion in its ground state, is called electron affinity.

The greater the energy released in the process of taking up the extra electron, the greater is the electron affinity.

The electron affinity of an atom measures the tightness with which it binds an additional electron to itself.

The electron affinities are expressed in eV per atom or Kj /mol.

How does ionization potential depends on atomic size nuclear charge and electronic configuration.

How does ionization potential depends on atomic size ?

The larger the atomic size, the smaller is the ionization potential.This is due to the fact that , with increasing ionization potential, the outer electrons lie farther away from the nucleus.

Hence, the attractive pull of the nucleus on the outer electrons decreases and it becomes easier to knock out an electron from the outer shell of the atom.

The atomic size decreases from left to write along a period.Hence,ionization potential increases. The size of atom in second period elements from Li to F are as follows,

3D structure of atom from lithium to fluorine along second period. 

What is penetration effect ?How does penetration effect influence the ionization energy ?

What is penetration effect ?

The probability of electrons coming closer to the nucleus in their motion around the nucleus is called penetration effect.

How does penetration effect  influence the ionization  energy ? 

It has been found that the ‘S’ electrons have greater probability of coming closer to the nucleus  than the P,d or f electrons of the same principal energy shell. 

In other words, ‘S’ electrons penetrate more towards the nucleus than the ‘P’ electrons and the penetrating power of the electrons in the given principal energy shell  varies as S> P> d>f.

Thus the ‘S’ electrons experience more attraction from the nucleus than the p d or f electrons of the same principal energy shell.

Therefore, the ionization potential for pulling out an ‘s’ electron is maximum  and it decreases in pulling out a p , d or f electron of the same principal energy shell in order of their motion.

What is ionization energy and why second ionization energy is greater than first ionization energy?

What is ionization energy ?

The ionization energy of an element is defined as the amount of energy required  to remove an outer most valence electron from an isolated gaseous atom of that element resulting in the formation of a positive ion that is a cation. Ionization energy is denoted by ‘I’ .

What is first ionization energy ?

The amount of energy required to  remove  the first outer most valence electron from an isolated gaseous atom resulting in the formation of one unit  positive charge , is known as first ionization energy.

Similarly , the energy required to remove second or third electron ,is known as second or third ionization energy respectively.

Unit of ionization energy .

The unit of ionization energy is KJ / mol  or Kcal / mol or  electron volt per atom .

Ionization energies are generally measured by spectroscopic techniques.

Why second ionization energy is greater than first ionization energy ?

If one electron has been taken out of an atom, it becomes increasingly difficult to remove the second and subsequent electrons from the resulting positively charged ion on account of electrostatic attraction.

This is due to the fact that after the removal of an electron , the number of electrons  decreases while the nuclear charge remains the same.

Consequently , the remaining electrons are held more tightly  by the nucleus and it becomes difficult to remove the second electron.

Therefore, the second ionization energy is greater than the first ionization energy .

Similarly, the third ionization energy is greater than the second ionization energy and so on .  

  Thus,  I₁< I₂< I₃

For example, the first ionization energy of chlorine is 1251 kJ/mol where as second ionization energy of chlorine is 2300 kJ/mol.

Variation of ionization energy in the periodic table .

In general, the ionization energy decreases as we move from top to bottom along a group and ionization energy increases when we move from left to right along a period.

The variation of ionization energy with atomic number in the second period are shown below graphically. 

Practice problem:

How ionization energy depends on atomic or ionic size ?
Why second ionization is higher than first ionization energy ? 
What is shielding or screening effect ? 

Click here : shielding or screening effect. 

Why quinuclidine is a stronger base than aniline and N,N dimethyl aniline ?

Why quinuclidine is a stronger base than N,N dimethyl aniline ?

In N,N dimethyl aniline the electron pair on N-atom can undergo resonance with the ring through pi bond. So electron on N-atom in N,N dimethylaniline is not available for protonation

But in case of quinuclidine having a bridge bond which is perpendicular to the other ring and a flat N-atom is not possible and so resonance is inhibited.

That is, due to steric inhibition of resonance the electron pair on N-atom is more available for protonation and consequently,quinuclidine is a strong base than N,N dimethyl aniline.

Quinuclidine

Why quinuclidine is a stronger base than aniline ?

In aniline the electron pair on N-atom can undergo resonance with the ring through pi bond.So electron on N-atom in aniline is not available for  protonation.

But in case of quinuclidine the bridge is perpendicular to the other ring and a flat N-atom is not possible and so resonance is inhibited.

That is, due to steric inhibition of resonance the electron pair on N-atom is more available for protonation and consequently,quinuclidine is a strong base than aniline.

Why 1-phenylpiperidine is less basic than 3,4 dihydro-2H-1,4 ethanoquinoline?

The lone pair on N-atom in 1-phenylpiperidine undergoes resonance with the pi electrons of benzene ring , so it is unavailable for donation to Lewis acid.

But in 3,4 dihydro-2H-1,4 ethanoquinoline compound, due to presence a  long  bridge bond resonance is not possible .

Because the bridge bond is perpendicular to the other ring.Hence,lone pair of electron on N-atom  is available for protonation.

Consequently the compound is more stronger base than 1-phenylpiperidine.

Summary : 

   
      Why quinuclidine is a stronger base than aniline and N,N dimethyl aniline ?
      Why 1-phenylpiperidine is less basic than 3,4 dihydro-2H-1,4 ethanoquinoline ?

Inert pair effect cause and consequences

Importance of hydrogen bonding in biological system.

What is hydrogen bonding ?

When a proton donor group interacts with an electron donor having a lone pair of electrons or a pi bond, a weak bond is formed represented by dotted line, known as hydrogen bonding. 

There are two types of hydrogen bonds.(I)Inter-molecular hydrogen bond and (II) Intra-molecular hydrogen bond which are shown below.

 Importance of hydrogen bonding in biological system.

Both intra-molecular and inter-molecular hydrogen bonding has a important role in biological system.

The long helical structure of protein molecules and double helical structure of DNA molecules are stabilised by the existence of H-bonding .

For example, a protein molecule contains a large number of  >C= O and  >N一H groups and hydrogen bonding occurs between these two groups are as follows.

A single protein molecule contains a large number of these hydrogen bonds.Although each of them is very weak yet the overall effect of all the hydrogen bonds is highly significant and brings stability to the molecule of protein.

The hydrogen bonding force in protein and cellulose makes them able to absorb dyes.



Summary


What is hydrogen bonding? 
importance of hydrogen bonding in biological system.

What is VSEPR theory?

Centre of symmetry and alternating axis of symmetry

What is elements of symmetry ?

Organic molecules are covalent compounds having definite bond length and bond angles among the constituents atoms.

Organic molecules can be confidently taken as idealised statics entities that can be represented by rigid mechanical models.

Mechanical models having definite geometric shapes, like symmetric and non symmetric depending on certain basic elements of symmetry.

In stereo chemistry,the elements of symmetry can be considered as a specific operations that generate the repetition of the geometric shapes of the molecule which observed before carrying out symmetry operation.

What is centre of symmetry ?

A centre of symmetry is an imaginary point in the molecules such that if a line is drawn from any group of the molecule to this point and then extended to an equal distance beyond the point,it meets the mirror images of the original group.

Centre of symmetry is also called centre of inversion, denoted by Ci .For example, 2,4-dimethyl cyclobutane-1,3-di-carboxylic acid exhibit centre of symmetry and so it is optically inactive.

2,4-dimethyl cyclobutane-1,3-di-carboxylic acid 

Why  cis form of 2,5-dimethyldihydropyrimidine-4,5-dione is optically active,but trans form optically inactive ?

The cis form of 2,5-dimethyldihydropyrimidine-4,5-dione has no centre or plane of symmetry, so it is optically active whereas trans form of 2,5-dimethyldihydropyrimidine-4,5-dione has a centre of symmetry, hence it is optically inactive.

What is alternating axis of symmetry ? 

A molecules possess an alternating axis symmetry ( Sn) of n fold if rotation of the molecule about the axis by 2π / n followed by reflection through a plane perpendicular to this axis produces an indistinguishable with the original.

It is also called rotation or reflection of symmetry. For example,1,3,7,8-tetramethyl-5-azaspiro-[4,4] nonan-5-ium have alternating axis of symmetry.

This compound possess alternating axis of symmetry and also has a plane of symmetry, so it is optically inactive.




Summary

• Elements of symmetry
• Centre of symmetry.
• Alternating axis of symmetry.

Click here: Racemic modification and resolution

shielding effect Slater's rules and its limitations.

What is shielding effect ?

In multi-electron atoms, there are a number of electrons .Some of them,occupying lower energy orbitals, tend to reduced the effect of nuclear charge on the electrons present in higher energy orbitals. 

This is due to the fact that these orbitals intervene between the nucleus and the electrons in the higher energy orbitals.This effect is known as shielding effect .

What is effective nuclear charge ?

An electron in one of the atomic orbitals of a multi-electron atom felt electrostatic repulsion by other electrons in the same or other orbitals. 

As a result of this, the nuclear charge felt by this electron will be less than the actual nuclear charge. 

This electron is said to be screened from the influence of the nuclear charge and the reduced charge felt by the electron is known as the effective nuclear charge.   

                           Zeff =Z actual – s

where 's' is the shielding constant whose magnitude determines the extent to which the electron shielded by other electrons in the atom from the nuclear charge.

What is Slater rules ?

Slater has given a set of empirical rules for calculating the shielding constant for an electron  is known as Slater rules. These  rules are,

Case-I.   For  S  or  P  electrons.

1.The electronic configuration should be in the following order and grouping.

(1s)   ( 2s  2p)   ( 3s  3p)   ( 3d )   ( 4s 4p )     (4d )    (4f)   (5s  5p) .

2.Electrons in any group to the right of the  ( ns  np ) group contribute nothing to the shielding constant.

3.( a) All other electrons in the ( ns np ) group shields the valence electron to an extent of 0.35 each.

(b) Contribution is 0.30 if the group is 1s.

4.All electrons in the ( n- 1) shell shield to an extent of 0.85 each.

5.All the electrons in ( n – 2) shell or lower shells shield completely ,that is contribute 1.00 each.

Case-II. For d or f –electrons.

Rules  (i), (ii) 3(a) remains same 3(b),4 and 5 are replaced by rule 6

6. All electrons in groups lies to the left of nd  or nf  group contribute 1.00

periodicity and different atomic radius and ionic radius.

What is periodicity ?

In periodic table, it has been found that elements withsimilar properties fall in the same group.

The repetition of similar properties of elements after certain regular intervals when they are arranged in the order of increasing atomic numbers is called periodicity.

What is the cause of periodicity ?

According to modern periodic table, all the physical and chemical properties of elements  depends on their outer electronic configuration or outer valence electrons.

For this,the elements  belonging  to each group have similar properties but they differ from the elements belonging to the other groups.

For example , all the alkali metals of group-I having outer electronic configuration ns1, are strongly metallic and electropositive . 

The elements of group zero  with electronic configuration ns2 np6, are inert and exhibit high ionization potential.

Hence, we can define that the cause of periodicity in properties of elements  appears due to the repetition of similar outer electronic configuration of their atoms at certain regular intervals.

What is covalent radius ?

One half of the distance between the nuclei of twocovalently bonded atoms of the same element in a molecule, is taken as the covalent radius of the atom of that element.

While defining the covalent radius, it is assumed that atoms are hard, incompressible spheres which are touching each other in normal covalent formation.

The covalent bond  distance is measured by  X-ray  diffraction or spectroscopic studies.

What is Vander waals’ radius ?

The van der waals’ radius is defined as one half of the distance between the nuclei  of two adjacent identical atoms belonging to two neighbouring  molecules of an element in the solid state.

It is essentially the distance between two non bonded atoms of two adjacent molecules.

The van der waals’ radii are obtained from X-ray studies of various elements in the solid state.

What is ionic  radius ?

The ionic radius may be defined as the distance from the nucleus of an ion upto which it has influence on its electron charge cloud.

The inter nuclear distance in any ionic compound is determined from X-ray measurements.

This distance is taken as the sum of the radii of the two ions involved. Knowing the radius of one , that of the other can be calculated.

What  is metallic radius ?

Metallic radius is defined as one half of the distance between the two adjacent metal atoms in a metallic cluster.

Summary: 


What is periodicity ?
What is the cause of the periodicity ?
What is covalent radius ?
What is ionic radius ?
What is Vander waal's radius ?
What is metallic radius ?
  

Nucleophilic substitution reaction 

ring-chain isomerism and metamerism.

Ring-chain isomerism and metamerism.

What is ring-chain isomerism ? 

Two structural isomers having the same molecular formula but possessing open chain and cyclic structures are called ring-chain isomers and this phenomenon is called ring-chain isomerism. 

For example, 1-butene and cyclobutane  show this type of isomerism. 

What is chain isomerism ? 

When two structural isomers with the same molecular formula exhibit different physical and chemical properties due to the different chain formation , then this type of isomerism is called chain isomerism. 

For example, n-pentane , isopentane and neopentane.

What is metamerism ? 

When a bivalent ,trivalent  or tetravalent atom is attached with two different or same alkyl groups resulting in the  formation of two or more structural isomers, then they are called metamers and this phenomenon is called metemerism.

Practice problem.
What is isomerism ?
What is structural isomerism ? 
What is functional group isomerism ? 
What is position isomerism ? 

Click here: Isomerism and classification.