Entry Test Preparation 2015, Chemistry BOOK 1 Chapter # 5 Chemical Bonding , Theory and Key Concepts 

Entry Test Preparation 2015

Chemistry

Chapter # 5

chemical bonding

Chemical Bond:

A chemical bond is the force which holds together two or more atoms or ions to form a compound.

Electropositive Elements:

Elements whose atoms give up one or more electrons easily.

They have low ionization potential.

Examples: Na, Ca, Mg

Electronegative Elements:

Elements which gain electrons. They have higher value of electronegativity.

Examples: Cl, O, Br

Ionic Bonding

An ionic bond is formed when a metal atom transfer one or more electrons to other atoms.

Na  Na⁺ + e-

F + e^‑  F-

The oppositely charged ions get attraction through electrostatic force of attraction to form an Ionic Bond.

If the electronegativity difference between the atoms is equal to or greater than. 1.7, the bond forms between the atoms have more than 50% ionic character.

• In the solid state, each cation surrounds itself with anions and each anion with cations. In this way a very large number of ions are arranged in an ordinary network called ionic crystals.
• They are good conductors of electricity in fused sate or aqueous solution.
• They are soluble in polar solvents and insoluble in non polar solvents.
• Have high melting point and boiling point.
• Have strong force of attraction between cation and anion (Columbic force).

…………………………………..

Covalent Bonding

If duplet (2) or octet (8) is completed by sharing of electrons between two electronegative elements then the bond formed is called covalent bond.

Non-Polar Covalent Bond:

A covalent bond which is formed between two elements of same electronegativity (e.g. H₂, O₂, O₂ etc.) is called a non polar covalent bond because the shared pair of electrons remains exactly midway between the two atoms.

Polar Covalent Bond:

If a covalent bond is formed between two elements of different electronegativity, then the shared pair of electrons shifts slightly towards the more electronegative elements and such a bond is called polar covalent bond.

Coordinate Bonding or Dative Bond:

A bond formed by transfer and followed by sharing of a pair of electrons between a Lewis Base and Lewis Acid is called coordinate bond.

·       It is represented as ( )

·       Atom/ion/molecule donating electron pair is called donor or Lewis base.

·       Atom/ion/molecule accepting electron pair is called acceptor or Lewis acid.

·       Coordinate bonds are formed between the atoms only if they are already bonded covalently.

Example: NH₃ has three NH covalent bonds and a lone pair.

This lone pair is donated to an H⁺ ion to form NH₄⁺.

Hybridization:

Intermixing of orbitals of same energy or of slightly different energy to produce entirely new orbitals of equivalent energy, identical shaped and symmetrical in plane.

·       Only the orbitals of an isolated single atom can undergo hybridization.

·       The hybrid orbitals generated equal in number to that of pure atomic orbitals which are intermixed.

·       A hybrid orbital, like an atomic orbital, can have two electrons of opposite spins.

HYDROGEN BONDING

Hydrogen Bonding is said to be formed when slightly acidic hydrogen attached to a strongly electronegativity atom such as F, O and N is half with weak electrostatic forces by the non bonded pair of electrons of another atom. That is, it is a dipole-dipole interaction.

·       Of all the electronegative donor atoms, only, F, O, and N enter into stable H-bond formation.

·       Greater the electronegativity difference between the bonded atoms, stronger is the Hydrogen bonding.

For example:

The electronegativity difference between F and H is greater than that of between Cl and H, therefore, HF is more polar than HCl.

Intramolecular Hydrogen bonding:

This type of Hydrogen bonding occurs between polar H and other electronegative atom present in the same molecule.

Intramolecular Hydrogen bonding:

This type of Hydrogen bonding takes place between hydrogen and electronegative element present in the different molecules of the same substance (H₂O and H₂O) or different substances (H₂O and NH₃).

RESONANCE Some molecules can be constructed in more than just one way. For example, nitrogen dioxide (NO2) is a stable molecule. It can have either a single bond between the nitrogen and the first oxygen and a double bond between the nitrogen and the second oxygen, or vice versa. Either one is a valid structure, or the true structure is a blend of the two. Thus, both oxygen atoms can be said to have a 1.5 bond with the nitrogen. This molecule is said to have a resonance structure.

Bond Energy:

It is defined as the energy required to break one mole of bonds to form neutral atoms. It is also called bond enthalpy as it is a measure of enthalpy change at 298 K. the enthalpy change in splitting a molecule into its component atoms in called enthalpy of atomization.

The strength of the H-X types of bonds is in the order.

H – F > H – Cl > H – Br > H – 1

Their bond energies also vary in the same order.

The bonds with higher bond energy values have shorter bond lengths.

The bond energies of C to C bonds being in the order C ≡ C > C = C > C – C , their bond lengths are in the reverse order i. e., C – C > C = C.

Bond length:

The distance between the nuclei of two atoms forming a covalent bond.

It is experimentally determined by physical techniques like X-rays diffraction, electron diffraction.

The observed bond energy is greater than calculated value and that means a more stable bond. This stability is due to percentage ionic character in that compound.

Dipole Moment

A diatomic molecules with a polar bond has a dipole. Whether or not a molecule has , a dipole depends not only upon bond polarity but also upon molecular geometry and the presence of lone pair of electrons.

Dipole moment is a measure of the separation of charge in a molecule. It is the product of charge times the distance that separate it from a charge of equal magnitude but opposite sign as:

Dipole moment = charge  distance

 = d  e

in S.I units dipole moments are measured in Coulomb – meter (C.m). However, commonly used unit is Debye D.

D = 3.3410^-30 C.m

  

Dipole moments of some substances

Compound	Dipole Moment (D)
H2	0.00
HCl	1.03
HBr	0.78
Hl	0.38
H2O	1.85
H2S	0.95
NH3	1.49
SO2	1.61
CO2	0.00
CO	0.12
NO	0.16
H2O2	2.20
CH4	0.00
CH3F	1.81
CH3Cl	1.45
CH3Br	1.85
CH3l	1.35
C2H5OH	1.69

Dipole moment provides two types of information about the molecule structure.

Percentage Ionic Character:

Form the experimentally determined dipole moments, the percentage ionic character in a bond can be calculated. The percentage ionic character of H – F, H – Cl, H – Br and H – l bonds are 43, 17, 12 and 5 respectively.

Bond Angles or the Geometry of Molecules:

The dipole moment of water molecule Is 1.85 D and the bond angle is 104.5o. CO has a dipole moment while CO₂ does not have any dipole moment. The reason is that CO₂ has a linear structure where the dipoles being equal and opposite, cancel out each other’s effect. Benzene has zero dipole moment, as it is a symmetrical planar hexagonal molecule.

VALENCE SHELL ELECTRON PAIR REPULSION THEORY

The Valence shell Electron Pair Repulsion (VSEPR) model:

·       It is based on the number of regions of high electron density around a central atom.

·       It can be used to predict structures of molecules or ions that contain only non-metals by minimizing the electrostatic repulsion between the regions of high electron density.

·       It can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons.

·       It does fail in some cases.

Shapes of Molecules Containing Two Electron Pairs:

The two bond pairs of electrons in BeCl₂ arrange themselves as far apart as possible in order to minimize the repulsion between them. The only arrangement which can be given to satisfy this condition is linear.

Shapes of Molecules Containing Three Electron Pairs:

There are three bond pairs around boron on BCl₃. it should, therefore, be a planer triangular molecule.

VALENCE BOND THEORY

The valance bond theory assumes that a covalent bond is formed by pairing of electrons by the overlap of orbitals of two atoms. Two orbital each containing one electron (half filled) would overlap to form a single covalent bond. The electrons of overlapping orbitals share a common region of high electron density along the line between the two nuclei called bond axis.

Sigma Bond ( Bond):

When two partially filled atomic orbitals overlap in such a way that the probability of finding the electron in maximum around the line joining the two nuclei, it is called a  bond. All single covalent bonds are sigma bond.

Pi () Bond:

The bond which is formed due to the sidewise or parallel overlap of P orbitals of the two already bonded atoms in known as Pi () bond.

Sp³-Hybridization:

The mixing of one s and three p orbitals to form four equivalent Sp³ hybrid orbitals is called Sp³ orbital has 25% s-character and 75% p-character.

·       These Sp³ orbitals are directed from the centre of a regular tetrahedron to its four corners.

·       The angles between tetrahedrally arranged orbitals are 109.5^o.

For example, in the molecule of CH₄ there is sp³ – hybridization. One s and three p orbitals of carbon give rise to four equivalent sp³ hybridized orbitals. The four sp³ hybrid orbitals of the carbon atom overlap with 1 s orbitals of four hydrogen atoms to form a methane molecule which contains four  bonds (each due to sp³ – s overlap), and each  H – C – H bond angle is 109.5^o.

Sp²-Hybridization:

The mixing of one’s and two p orbitals to form three equivalent sp² hybrid orbitals is termed as sp²-hybridization.

·       Each sp² orbital consists of s and p in the ratio of 1:2.

·       These three sp² hybrid orbitals are coplanar (lie in the same plane) at 120^o angle.

A typical example of this type of hybridization is of ethylene molecule (C₂H₄). In ethylene, two sp² hybrid orbitals of each carbon atom with a linear overlap with Is orbitals of the two hydrogen atom form two  bonds, while the remaining sp2 orbital on each carbon atom overlap axially to form a  bond. The remaining two unhybridized p orbitals on two carbon atoms are parallel and perpendicular to the axis joining the two nuclei. These undergo a parallel overlap and result in the formation of a  orbital. Thus

A molecule of ethene contains five  bonds and one  bond.

Sp Hybridization:

When one s and one p orbital combine to give two hybrid orbitals, the process is called sp hybridization. The sp hybrid orbital lobes are at an angle of 180o from each other. It means that the axis of the two sp orbitals form a single straight line (they are coaxial).

In acetylene, the two sp hybridized orbitals of two carbon atom linearly overlap and form a  bond between them. The 2py and 2pz unhybridized orbitals are perpendicular to each other and to line through the centres of two sp hybrid orbitals. They overlap to form the  bonds between two carbon atoms.

MOLECULAR ORBITAL THEORY

According to this theory the overlap of two atomic orbitals must produce two molecular orbitals. In this process energy must be conserved. The two molecular orbitals result from the overlap of two atomic orbitals in such a way that the electron waves either reinforce each other or cancel each other.

In the first case the bonding molecular orbital is formed. A bonding molecular orbital is always of lower energy than either of the atomic orbitals that have combined.

In the second case, and antibonding molecular orbital is formed in which the electron density is located away from the space between the nuclei. The energy of the antibonding orbital is higher than the energy of the atomic orbitals from which it is formed.

Bond Order:

The number of bonds formed between two atoms after the atomic orbitals overlap, is called the bond order and is taken as half of the difference between the number of bonding electrons and ant bonding electrons.

Anticoding Molecular Orbital:

The molecular orbital which is of higher energy than the isolated atomic orbitals from which it is formed is called ant bonding molecular orbitals.

Paramagnetic Substance:

A substance which is attracted by a magnetic field is called paramagnetic. This is due to unpaired electron present in the substance.

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