Chemical bonding 1

Chemical Bonding

Chemical Bonding-By Aditya Abeysinghe 1

Why are bonds formed?Many elements and ions in nature are unstableand sometimes they are highly reactive. Howeverany element or ion tend to increase their stabilityby achieving the noble gas configurations.For example, a sodium atom which has a valenceelectron may bind with an electron deficientchlorine atom to form sodium chloride. Thus thesodium atom will achieve the noble gasconfiguration and the chlorine atom will achievethe noble gas configuration of argon and be stable.


Proof that formation of bonds makethe participant atoms stable

1. Most atoms in nature do not exist as ‘free’ atomsbut as compounds.

2. Emission of energy when bonds are formed andthe absorption of energy in the creation ofbonds.(According to the Gibb’s equation anyelement becomes more stable when it possessesless energy and is of high disorder)

3. Most rare gases are reluctant to formcompounds(However, some compounds of xenonand argon have already being discovered)


OverlappingWhen two orbitals are ‘on each other’ overlapping occurs.However the percentage of overlapping changes depending on thetype of orbital.For example the overlapping of two s orbitals is considered fullyoverlapped since the percentage overlapped remains constantregardless of the direction.(due to the spherical shape of theorbitals)In contrast, the overlapping of other orbitals such as p, d, and forbitals change because of their shape.

The overlappingpercentage remainsconstant in s orbitalsregardless of direction

Theoverlappingpercentage of porbitalschangesaccording tothe direction inwhich theyoverlapChemical Bonding-

By Aditya Abeysinghe4

What is a bond?A bond is the process by which two orbitals ofthe two atoms overlap to form a molecularorbital and produce a chemical connectionbetween the two atoms.The bond is caused by the electrostatic force ofattraction between two oppositely chargedpoles.


Types of bonds

Type of bond(Primary Bonds)

Covalent bonds

Polar

Non-polar

CoordinateIonic or

electrovalentbonds

Metallic bonds


Co-valent bonds(Co+ Valent bonding)

Covalent bonds are bonds formed by sharing electronsbetween the participating atoms. Here overlapping of two ormore orbitals produce molecular orbital(s) and form the bondas a whole.As mentioned earlier there are three types of bond by physicalnature1. Non-polar covalent bonds2. Polar covalent bonds3. Coordinate covalent bondsCovalent bonds can also be classified by the no. of bondsexisting between any two bonded atoms as1. Single bond covalent formation2. Double bond covalent formation3. Triple bond covalent formation Chemical Bonding-

By Aditya Abeysinghe7

Non polar covalent bondingIn non-polar covalent bonding,1. The electronegativity difference between the

atoms is less than 0.4 and sometimes 0.2. The dipole moment of such bonds may be

negligible.E. g: CH4 The electronegativity

of the C-H bond is0.35. However thepolarity of any three C-H bonds is neutralizedby the other bond dueto the tetrahedralshape of methane. Thus themolecule as a whole is nonpolar


Polar covalent bonding

Polar bonding results when two different non-metals unequally share electrons betweenthem.Polar covalent bonds usually exist in moleculeswhere1. There is an imbalance of the electro

negativities in a molecule as a whole.2. The electro negativity between the atoms in

a specific bond lies between 0.4 and 1.6.


E. g: When one hydrogen of methane isbeing replaced by a hydroxyl radical(-OH)the formation of methanol will result in anon-polar molecule due to the imbalanceor the residue of the electro negativities inthe molecule as a whole

Methanol(CH3- OH)

Chemical Bonding-By Aditya Abeysinghe

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Coordinate covalent bonding(Dative covalent/ Dipolar bond)

Coordinate covalent bonds result when bothelectrons in a specific bond is ‘gifted’ by a donor(by asingle element unlike the sharing of electronsprovided by both elements in the bond)A dipolar bond is formed when a Lewis base donatesa pair of electrons to a Lewis acid(for a detaileddescription of Lewis acids and Lewis bases see thenext slides).For example the Ammonium ion is formed bydonating a pair of electrons to the electron deficientHydrogen ion by the Ammonium compound.


+ [H]+

NH3 + H+ = NH4+

Similarly Boron Tetraflouride is formed when the electron deficientBoron Tri fluoride atom gains a pair of electrons from a donor fluorideion.BF3 + F- = BF4

- .Finally not only ions can act as electron donors or receptors but alsocompounds.For example the following compound is formed by the donation of apair of electrons from a ammonia compound to a boron trifluoridecompound.


NH . B

F

+


Lewis acidA Lewis acid is an electron pair acceptor.A Lewis acid may be formed by1. By compounds in which central atom is

electron deficient2. When multiple bonds exist between different

atoms3. Atoms having sextet configuration.4. By compounds containing a central atom with

an unfilled d- orbital.E. g: Various metal cations, such as Mg2+ and metalcompounds such as AlCl3 are Lewis acids becausethey have unfilled valence orbitals.


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Lewis baseLewis base is a compound with a pair of nonbonding electrons.Thus H2O with its two pairs of non bondingelectrons on oxygen acts as a Lewis base bydonating an electron pair to an H+ thus formingthe hydronium ion, H3O+ .

Lewis bases can also be formed in similar methods asthat in Lewis acid formation.


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Ionic bondingIonic bonding usually occurs when a bond is formed by twoor more elements with electro negativity difference of 1.7or more.Ionic bonds are also called electrovalent bonds since thetwo charges in any ionic bond have electrostatic propertiesand a valency unlike that in any covalent compound.In 1916, W. Cosol mentioned that ‘ atoms tend to achievethe nearest noble gas electronic configuration whenparticipating in a chemical reaction’.According to his theory, NaCl may be formed as follows:

Na- [Ne] 3s1 Na+-[Ne]

Cl- [Ne] 3s2 3p5 Cl–- [Ar]NaCl


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Electro-valencyElectro-valency is the process by which one ormore electrons are drifted between atoms andresult in the valency of an element.The magnitude of the valency of an atomdepends upon the no. of electrons received ordonated in the process of electro-valency.For example Mg2+ means that the magnesiumatom has donated two electrons either to arecipient atom/compound or to an aqueoussolution. Also N3- means that nitrogen has eithergained three electrons from a donor ion/compound or from an aqueous solution.


Characteristics of metals

1. Luster and reflection of incident light2. High electrical and thermal conductivityproperties3. High density4. Malleability and elasticity5. High melting and boiling points and also highevaporation heats6. Photoelectric effect7. Formation of cations/positive ions in chemicalreactions


Drude and Lorentz theoryThe model proposed by Drude was developedby Lorentz and stated a new theory stemmingaway from the structures of covalent and ionicbonding.The model states that a metal is composed of arigid positive ion lattice and the electrons inthat lattice do not form any attractions with thepositive ions and exist as a ‘cloud’ covering thelattice.


Explaining the metallic bondingusing the hypotheses of Drude

1. Electrons which are at a lower level may promote to ahigher level when given energy to the atom. Onretuning to a lower energy level the energy is emittedmainly as heat and light. Thus luster is visible to bereflected over a specific metallic surface whenexposed to a light or radiation source.

2. The availability of free electrons makes it easier forelectrical and thermal conductions throughout themetal. In contrast, due to the sharing of electrons in acovalent bond tight attractions between bondingatoms make its electron mobility extremely low. Thusmetals transfer the heat and electricity throughmotion and vibration of electrons within the metal.


3. High density of the metal may be due tothe close of electrons and the positive chargeswithin the metal.4. Malleability and elasticity

As shown in the next page if we pullopposite sides along A-B of a metal and of anionic compound, the metallic bond formsattractive forces while the ionic compoundforms repulsive forces.


Metallic CompoundIonic Compound

A B

Chemical Bonding- By Aditya Abeysinghe22

Covalent compounds too break up when applied withstress due to weak intermolecular forcesE.g.: S, I, NaphthaleneHowever, giant covalent molecules like diamond do notbreak easily. If broken, they do not form small sheets orrods as in the case of metals

Due to the increase in the no. of mobility electronsin a given area, the density of alkaline earth metals(Mg, Be) is greater than that of alkaline metals(Li, Na).


5. High melting, boiling points and evaporationheats can be described in a similar way to that ofthe high density.Similarly the photoelectric effect can be explainedby the luster given off by a metal due to theincidence of light.6. Formation of positive ions in a chemicalreactionDue to the presence of high mobility electronsthe metal may tend to release one or moreelectrons to a recipient element to make positiveions equal in charge to the no. of electronsreleased to make the ion.


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For more information about theintermolecular attractions andother information on chemicalbonding visit the presentation

‘Chemical Bonding-2By Aditya Abeysinghe’


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Chemical bonding 1