chemical bonding and molecular structure

molecules

group of atoms

since atoms cant exist independently except noble gases

chemical bond

the attractive force that holds various constituents together like molecules, atoms, and ions in different chemical species is called…..

bonding is natures way of attaining stability

Kossel-Lewis approach

1916

first to provide an explanation of valence which was based on inertness of noble gases.

Lewis and his theory

he pictured atom in terms of positively charged ‘kernel”{nucleus and electrons} and the outermost shell that could accommodate 8 electrons that would occupy each corner of a cube

Lewis postulated

atoms achieve the stable octet when they are linked by chemical bonds

kossel brough attention to with respect to chemical bonding

1] the highly negative halogens and highly positive alkali metals are separated by noble gases

2] by formation of negative ion from a halogen and positive ion from alkali metal is associated with gain and loss of an electron

3] the cation and anion thus attain noble gas configuration

4] the anion and cation are stabilized through electrostatic attraction

electrovalent bond and electrovalence

the bond formed as a result of the electrostatic attraction between cations and anions was termed as …..

the electrovalence is equal to the number of unit charge of the ion

octet rule : 1916

electronic theory of chemical bonding

atoms can combine either by transfer of valence electrons from one atom to another

OR

by sharing of valence electrons in order to attain stable octet

covalent bond by Langmuir 1919

he abandoned the idea of a kernel and stationary cubical arrangement of the octet

each bond is formed as result of sharing electron pair between atoms

each combining atom at least contributes one electron to the octet

the combining atoms attain noble gas configuration

formal charge :

in case of polyatomic ions the net charge is possessed by the ion as a whole and not by a particular atom

the lowest energy structure is the one with smallest formal charge on atoms

factor based on a pure covalent view of bonding in which electron pairs are shared equally by neighbouring atoms

limitations of octet rule :

1} incomplete octet [LiCl', BeH2}

2}odd electron molecules

3}expanded octet

4}though the rule is based on the inertness of noble gases it fails to explain why Xenon makes compounds

5} does not account to the shape of the molecule

5} does not explain the relative stability of the molecules

formation of ionic compound depends upon

the ease of formation of the positive and negative ions from their respective parent neutral atoms

the arrangement of the positive and negative ions in the lattice of the crystalline compound

ionic bonds will be formed easily between elements with comparatively low ionization enthalpies and elements with highly negative electron gain enthalpy

electron affinity

the negative of the energy change accompanying electron gain

ionic compounds in their crystalline state

consist of orderly 3-D arrangements of cations and anions held together by coulombic force energies

lattice enthalpy

defined as the energy required to completely separate one mole of solid ionic compound into gaseous constituent ions

bond length

defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule

measured by spectroscopic methods , X-ray diffraction and electron diffraction

covalent radius

measured approx as the radius of an atom’s core which is in contact with the core of another atom

Van Der Waal radius

The Van Der Waals radius is a measure of the size of an atom that is not bonded to other atoms. It is defined as the distance at which the electron clouds of two non-bonded atoms begin to overlap, indicating the effective size of the atom in terms of steric hindrance and molecular interaction. This radius is typically larger than the covalent radius, as it accounts for the presence of van der Waals forces, which include attractions and repulsions between atoms, crucial for understanding molecular behavior, packing in solids, and the interactions between biomolecules.

bond angle

defined as the angle between the orbitals containing bonding electron pairs around the central atom in molecules/complex ion

bond enthalpy / dissociation enthalpy

it is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms

KJ mol^-1

bond enthalpy is directly proportional with bond strength

in polyatomic molecules

second time it undergoes a change because of a change in chemical environment

hence average bond enthalpy is used

bond order

number of bonds formed between two atoms in a particular molecule

isoelectronic species and ions have same bond orders

bond order is directly proportional to bond enthalpy

inversely proportional to bond length

resonance

a single Lewis structure is inefficient to represent a molecule, hence a number of structures with similar energies position of nuclei and bonding and non bonding pairs of e-.

this is called resonance hybrid

represented by double headed arrow

features of resonance

it stabilizes the molecule as the energy of the hybrid is less than all the constitute canonical structures

misconceptions of resonance

canonical forms are of no real existence

the molecule does not exist as a single canonical form for a fraction of time.

there is no such equilibrium between the canonical forms as we have between tautomeric forms {keto and enol}

the molecule has a single structure which is the resonance hybrid

polarity of bonds

heteronuclear molecules :

shared pair of electron pair between gets displaced more towards more electronegative element

this also known as dipole moment

dipole moment= charge(Q) * distance of separation ( r)

it is the vector sum of all the dipole moments of various

Fajan’s rule

ionic bonds have partial covalent character

1] the smaller the size of cation and larger the size of anion the greater the covalent character

2] greater the charge of the cation = greater the covalent character

3] for cations with the same size, transition metals are more polarizing than the on with noble gas configuration{alkali/alkaline }

cations polarizes the anion pulling electronic charge towards itself increasing the electronic charge between the two.

VSEPR theory

valence shell electron pair repulsion theory

theory is based on the repulsive interactions of the electron pairs in valence shell

Sidgwick and Powell in 1940

developed by = Nyholm and Gillespie {1957}

postulates of VSEPR

1] shape of a molecule depends upon the number of valence shell electron pairs around the central atom

2] pairs of electrons in valence shell repel one another since the electron clouds are negatively charged

3] these electron pairs tend to occupy such positions in space that minimize repulsion and maximize the repulsion

4] the valence shell is taken as a sphere with electron pairs localizing on the spherical surface at maximum distance from one another

5] multiple bond is treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as single super pair

6] theory is applicable to resonance structure

repulsive interaction between types of electron pairs

lone pair-lone pair > lone pair- bond pair > bond pair -bond pair

difference between the lone pair and bond pair

lone pair are localized on the central atom. occupies more space

bonded pair is shared between two atoms. occupies less space

division of molecules in 2 categories

1] molecules in which central atom has no lone pair

2] molecules in which central atom has one or more than one lone pair

limitations of VSEPR

DOES NOT EXPLAIN THE SHAPES OF POLYATOMIC MOLECULES

Valence bond theory

Heitler and London 1927

discussed in terms of quality not quantity

attractive forces:

1] nucleus of one atom and its own electrons

2] nucleus of one atom and other atoms

repulsive forces:

1]electrons of two atoms

2] nuclei of two atoms

magnitude of new attractiveness is more than new repulsive forces

two atoms approach each other and potential decreases

stage: net force of attraction balances the net repulsion and is said to be bonded .

bond enthalpy

energy gets released after when the bond is formed and hence the bond is more stable than that of isolated atoms.

the energy released is called ………..

orbital overlap concept

in the formation of a molecule there is a minimum energy state where the atoms are so close that their atomic orbitals merge.

this partial merging of atomic orbitals is called overlapping which results in pairing of electrons.

extent of overlapping = the strength of the covalent bond {directly proportional }

they can be positive, negative, neutral/zero depending on the phase and direction of the orientation of amplitude of orbital wave function

how is covalent bond formed

covalent bond is formed as result of pairing two electrons with opposite electron spins

positive overlap

orbitals forming bonds should have same sign/phase and orientation is space

sigma bond

axial/head on overlap

this type of covalent bond is formed by the end to end {head on} overlap of bonding orbitals along the internuclear axis

s-s overlap

there is overlap of two half filled s-orbitals along the internuclear axis

s-p overlap

occurs when half filled s-orbitals of one atom and half filled p-orbitals of another atom overlap

p-p overlapping

between half filled p orbitals

pi bond

sidewise overlapping

atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axes.

consists of two saucer type charged clouds above and below the plane of the participating atoms

strength of sigma and pi bonds

strength depends upon the extent of overlapping.

sigma : overlapping takes place till large extent. it stronger than pi bond

formation of multiple bonds

pi bond is formed in addition to sigma bond

hybridization

introduced by Pauling

according to Pauling the atomic orbitals combine to form new set of equivalent bonds known as hybrid bonds.

definition: process of intermixing of the orbitals of slightly different energies so as to redistribute their energies. resulting in formation of new set of orbitals of equivalent energy and shape

features of hybridization

1] the number of hybrid orbitals is equal to the number of atomic orbitals that get hybridized.

2] the hybridized orbitals are always equivalent in energy and shape

3] hybrid orbitals are more effective in forming stable bonds then pure atomic orbitals

4]these orbitals are directed in space is a specific preferred direction to minimize repulsion between electron pairs

5] the type of hybridization indicates the geometry og the molecule formed

important conditions for hybridization to happen

1] the orbitals present in the valence shell of the atom must be hybridized

2] the orbitals undergoing hybridization should have almost equal energy

3] promotion of electrons is not necessary condition prior the hybridization

4] it is not necessary that only half filled orbitals participate in hybridization

hybridisation of elements involving d orbitals

the energy of 3d is comparable to 3s and 3p and also to those of 4s and 4p

molecular orbital theory {features}

f Hund and RS Mullikan 1932

1] the electrons i a molecule are present in various molecular orbitals

2] atomic orbitals of comparable energy and similar energy combine to form molecular orbitals

3] in molecular orbital an atom is influenced by more than one nuclei depending on the number of the atoms present in the molecule { atomic orbital- monocentric: molecular orbital - polycentric

4] the number of molecular orbitals formed is equal to the number of atomic orbitals formed

5] when two atomic orbitals combine one bonding and one anti bonding molecular orbital is formed

6] B.O has lower energy and is more stable than ABO

7] the electron distribution probability is give by molecular orbitals

8] filled according to aafbau’s rule, pauli,s exclusion and hunds rule

Bonding molecular orbitals

bonds formed by addition of atomic orbitals.

the two orbitals reinforce each other due to constructive interference

electron density is located between the nuclei of the bonded atoms because of which the repulsion is least

electron tend to hold nuclei together and stabilize the molecule and hence has low energy

energy is lower than the parent atomic orbital

Antibonding molecular orbitals

the electron waves cancel each other due to destructive interference

most of the electron density is located away from the nuclei hence maximum repulsion

electron destabilizes the molecule because of mutual repulsion is more than the attraction

the energy is more than the parent atomic orbital

conditions for combination of atomic orbitals

1] the combining atomic orbitals must have same or similar energy level

2] the combining atomic orbitals must have the same symmetry about the molecular axis [ by convention z axis is taken as molecular axis]

3] the combining atomic orbitals must overlap to maximum extent[ greater the overlap greater the electron density

stability of molecules

it is stable if no of bonding atomic orbitals > no of antibonding atomic orbitals

bond order

a positive bond order means stability

negative bond order means instability

bond length

the bond length decreases as the bond order increases

magnetic nature

if all molecular orbitals are doubly occupied, the substance is diamagnetic ( repelled by magnetic field}

if one or more molecular orbitals and singly occupied then it is paramagnetic { attracted by magnetic field}

hydrogen bonding

def—- attractive force which binds hydrogen atom of one molecule with the electronegative atom of another molecule

it is weaker than covalent bond

intermolecular hydrogen bond

it is formed between two different molecules of the same or different compounds

intramolecular hydrogen bond

it is formed when the hydrogen atom id between the two highly electronegative atoms present in the same molecule 2