Bonding in Atoms and Molecules

8/13/2019 Bonding in Atoms and Molecules

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BONDING IN ATOMS AND MOLECULES

ATOMIC STRUCTURE

Every atom consists of a very small nucleus composed of protons andneutrons, which is encircled by moving electrons. Both electrons and

protons are electrically charged, the charge magnitude being 1.60 x 10-1 !

that is negative in sign for electrons and positive for protons" neutrons areelectrically neutral. #ince the normal atom is electrically neutral, thenumber of protons in the nucleus must be e$ual to the number of electrons inthe surrounding orbital. %he nucleus of an atom accounts for almost all ofthe mass of that atom, while the diameter of the atom tends to be of the order10,000 times greater than that of the nucleus.

%he &%'()! *+(BE / of an element is given by the number ofprotons or electrons/ possessed by a normal atom of that element. &n ionhas different number of electrons and protons. %he electrons, particularlythose in the outermost orbital the valence electrons/ determine manyengineering properties of materials such as chemical reactivity binding

patterns established with other atoms and thus strength characteristicselectrical properties and optical characteristics.

&tomic (ass &tomic *o. 2 *umber of *eutrons3

ELECTRONS IN ATOMS

%he number of electrons in a given shell can be obtained from the formulai.e *o. of electrons 4n5 where n is the number of shell. %hus for the1st#hell - #hell/, n 1" *o. of electrons 4 1/ 4474nd#hell 8- #hell/, n 4" *o. of electrons 4 4/4 97:rd#hell (-#hell/, n :" *o. of electrons 4 :/4 197;th#hell *- #hell/, n ;" *o. of electrons 4 ;/4 :47

%here are two ways to show how these electrons are arranged in an atom.)n the first method electrons are assumed to revolve around the atomicnucleus in discrete orbitals and the position of any particular electron ismore or less well defined in terms of its orbital.


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)n the second method electrons are permitted to have only specific values ofenergy. &n electron may change energy, but in doing so it must ma=e a$uantum >ump either to an allowed higher energy absorption of energy/ orto a lower energy emission of energy/. %hese electron energies are beingassociated with energy levels or states. Electrons are thus said to be arrangedin sub-shellswithin which no more than two electrons can occupy the sameenergy levels/ which are for our current purpose s,p,d and f orbitals.


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IONIC BOND

%his is really the attractive force existing between a positive and a negativeion when they are brought into close proximity, these ions being formedwhen the atoms involved lose or gain electrons in order to stabili?e theirouter shell. Elements are either classified as electro positive or electronegative, depending upon whether they tend to loose or gain electrons inorder to achieve this stable outer-shell configuration. )oni?ation energies or

potential measure the amount of energy re$uired to remove the successiveouter electrons from an atom3.

8ess amount of energy is re$uired to strip one electron from a metal thanfrom a non-metallic atom. %hus, metals are termed electropositive whilenon-metals are termed electronegative.

Example:

#odium !hloride, @otassium !hloride, 8ithium Aluoride and (agnesium!hloride.

Aor sodium chloride *acl/*a 11/7 1s5 4s5 4p6:s!l 1C/7 1s5 4s5 4p6:s5 :pD

CHARACTERISTICS OF IONIC COMPOUNDS

1/ %hey are electrolytes explain/4/ %hey have high melting and boiling points.:/ eactions ta=e place by electron transfer.;/ Bonds are non-directional ie. %he magnitude of bond is e$ual in all

direction around the ion.D/ !rystal lattice built from ions6/ !rystals are hard and brittle.

COVALENT BOND

%his is formed when several atoms share pairs of electrons and have theirenergies lowered as a result of this. #table covalent bonds are formed

between many non-metallic elements since the atoms of these elements


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usually possess half filled outer electron shells, which resist the directelectron transfer re$uired for the formation of an ionic bond.

Because pairs of electrons are shared in covalent bonds there is somedegree of orbital overlap between the bonded atoms. %his overlap islimited by the electrostatic repulsion of the positive charged nuclei, theresultant shapes of the bond orbital determines whether the bond will bedirectional or non-directional in space with respect to the nuclei. %hegreater the degree of orbital overlap the stronger the bond, sinceextensive overlap means greatly lowered energy levels of the bondelectrons. E.g. !l, A,


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and this electrons cloud. & metallic bond thus conceived, can exist onlybetween a large aggregate of metallic atoms and must therefore be non-directional. E.g.7 )ron, #ilver, old, *ic=el, and lead etc.

NB7 )f there are FnG atoms in a metal in a given lattice, and theelectrovalence of the metal is 21, then FnG electrons will circulate freelythroughout the lattice.

CHRACTERISTICS OF METALLIC COMPOUNDS

1/ %hey are conductors4/ %hey have variable melting and boiling points:/ eactions ta=e place by electron transfer;/ Bonds are non-directional

D/ !rystal lattice built from positive particles and demorali?edelectrons

6/ !rystals vary in hardness.

SECONDARY BONDS

%hese are those comparatively wea= intermolecular bonds formed as aresult of dipole attractions, the dipole forming as a result of the

unbalanced distribution of the electron in asymmetrical molecule.%emporary dipoles attract each other and the bond formed between themis very wea= due to Han der Iaal forces.

#econdary bonding exists between virtually all atoms or molecules but itspresence may be obscured if any of the three primary bonding type ispresent. %hey are also evident for inert gases, which have stable electronstructures and in addition, between molecules in molecular structures thatare covalently bonded.

Examples are


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of the electronic clouds of the atoms opposing these forces of repulsionare forces of attraction in which the positively charged nucleus of oneatom is attracted by the negatively charged electrons of the other atoms.%his is sometimes referred to as the COULOMBIC FORCE. &ne$uilibrium results when the attractive and repulsive forces are e$ual. i.e.when the net force A* is e$ual to ?ero from the e$uation below7

A* A!2 A, where A!!oulombic force" A epulsive A!2 A0But A! - ?1$/ ?4, $/ -------1/ a14 4 a1 4 e$uilibrium separation between atoms

Jo, o permittivity of free space

= a x 10H mKc7 ?1and ?4are the valencies of the two ion types. $L electronic charge 0.16 x 10-19!

&lso,A - bn -----4/ b, n are empirical constants

an 2 1

n 9, for ionic solids.

!omparing 1/ and 4/, A!x aM5 and Ax aM

%his means that the attractive forces predominate at greater distances ofatomic separation whereas the repulsive forces predominate at closer atomicspacing.

Bond energy E and bond force A, are related by

E LAdaEn L Anda where En is the net energy and An is the net force.En LA2 A!/ da

&s atoms are brought together energy is released i.e. Ec is negativeExothermic, N


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CRYSTAL STRUCTURE

&lmost all metals, some ceramics and some polymers crystalli?e whenthey solidify i.e. the atoms are arranged in a : O N repeating patternwhich extends over a long range of many atomic distances. %here are anumber of ways in which the atoms in solids are arranged in : O N. %hisarrangement constitutes the SPACE LATTICE (B&H&)# 8&%%)!E/.

& space lattice is therefore defined as a : O N array of discrete pointswhose arrangement and orientation remain the same irrespective ofwhere the lattice is viewed. #pace lattices are characteri?ed by unit cellswhich is defined as the fundamental building bloc= with which the latticeis generated along its crystallographic a1, a4, a:/ direction. #omeexamples of space lattices are7

i. #imple cubicii. Body centered cubic

iii. Aace centered cubiciv. acent unit cells met at a corner point. %herefore onlyone eight part of any corner atom is effectively within one unit cell. *umber of atoms in each unit cell 1K9 x 9


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1&ll ionic solids are belonging to this class.

BODY CENTRED CUBIC (BCC) STRUCTURE

%he unit cell for this structure consists of a sphere at the center of a cube

with eight other spheres at the corners of the cube and touching the centralsphere. !orner atom 9 x 1

9 1

Body centered atom 1%otal *umber of atoms per unit cell 4E.g. Hanadium, (olybdenum, )ron P/, chromium

FACE CENTRED CUBIC (FCC) STRUCTURE

%he unit cell of this structure is made of a sphere in the center of each of thesix faces of the cube.

a

!orner atoms 1Aace centered atom Q x 6 :


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%otal number of atomsKunit cell ;E.g. &luminium, *ic=el, !opper, #ilver, old, 8ead, @latinum, )ron R/

CLOSEPAC!ED HE"AGONAL STRUCTURE (CPH)

!orner atom 1 x 14 6

4Base centered atom Q x 4 1!entered atom :%otal number atomsKunit cell 6E.g. Beryllium, (agnesium, inc, !admium

PAC!ING FRACTIONS#ATOMIC PAC!ING FACTOR

%his is the fraction of the unit cell that is occupied by atoms or the fractionof solid sphere assuming atoms to be solid spheres/ volume in a unit cell.i.e. &@A Holume of atoms in a unit cell

%otal unit volume

*umber of atoms per unit cell x volume of sphere %otal volume of unit cell


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F$% BCC

&

B

a

a 4

A

,

,

,

,

B

&

B

a

a 4

,

,

,

,

)n B!! atoms bonds along body diagonal which goes right throughKinto thebody of the atom. is the radius of the atom.

;/4

a4

2 4a4

from @ythagoras theorem/ 164 :a4 and raising to a power of :K4 16:K4 x 4x:K4 ::K4 x a4x:K4.

a: 16/:K4 : ------1/ :

&lso, number of atoms per unit cell is 4.Holume of sphere ;K::

Holume of unit cell a:&@A 4 x ;K::

16/:K4 :

:

Aurther simplification gives


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&@A 0.69%his means that atoms occupy 69S volume of the unit cell.

INFLUENCE OF STRUCTURE AND BONDING ON

PROPERTIES OF CRYSTALLINE FORMS

#%E*acent layers. A!! has four ;/ setsof close-pac=ed planes per unit cell. %he further apart the layers are, thewea=er is the bond between these layers.


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POLYMERISATION

%he term polymeri?ation means Tmany mersU, where mer is the buildingbloc= of the long-chain or networ= molecule. & covalent bonding holds merstogether.E.g.

@olymers are therefore substances whose molecules are composed of greatnumber of repeating groups called ('*'(E#.

@olymeri?ation is simply the process by which monomer units are >oined

over and over, to generate each of the constituent giant molecules to givelarge, complex molecular forms with very high molecular weight.

(ost generally the raw materials for synthetic polymers are derived fromcoal and petroleum products, which are composed of molecules having lowmolecules weights. %he reactions by which polymeri?ation occurs aregrouped into two general classifications O addition and condensation.

ADDITION POLYMERISATION

&s chain reaction, polymeri?ation is a process by which bi-functionalmonomer units are attached one at a time in chain li=e fashion to form alinear macro molecule" the composition of the resultant product molecule isan exact multiple for that of the original reactant monomer. %hree distinctstages O initiation, propagation, and termination - are involved in addition to

polymeri?ation. Nuring the initiation step an active center capable ofpropagation is formed by a reaction between an initiator or catalyst/ speciesand the monomer unit.

@ropagation involves the linear growth of the molecule as monomer unitsbecome attached to one another in succession to produce the chain molecule.

NB. !hain growth is relatively rapid. %he period VV to grow a moleculeconsisting of say 1000 over units is on the over of 10VVVVV.


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@ropagation may end or terminated by either the reactivity between the twoends of active propagating chains or an active chain and may react with aninitiator.

%his reaction is used in the synthesis of polyethylene, polypropylene,polyvinyl chloride, and polystyrene etc. Eg. Aormation of polyethylene.%he process begins with an initiator ie hydroxyl free radials & free radical isa reactive action or group of atoms containing an unpaired electron.

I&''a'$&: %his reaction converts the double bond of one monomer into asingle bond of one monomer into a single bond and once completed, the oneunsatisfied bonding electron is free to react with the nearest ethylenemonomer. < or constituent, whilethe minor constituent confers speci\c chemical properties on thecopolymer. Ie now illustrate these principles by discussing someexamples.

#%XE*EOB+%&N)E*E !'@'8X(E#@olybutadiene is an elastomeric material with good elastic properties andoutstanding toughness and resilience.


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sheet and [ooring. ubber shoe soles are made almost exclusively from#B. !opolymers containing about 4DS styrene are also usefuladhesives, especially in the form of a$ueous dispersion or in solution. )fthe ratio of styrene to butadiene is in the range 607;0 and higher, thecopolymer is nontac=y and is used in hot-melt adhesives and latex paints.Aor example, emulsion copolymers composed of C;S styrene and 4DS

butadiene by weight/ \nd wide applications in paints. )n thoseapplications, the hardness of polystyrene is partially retained, but its

brittleness is modi\ed by the presence of butadiene.&!X8'*)%)8E-B+%&N)E*E-#%XE*E &B#/&B# is the generic name of a family of engineering thermoplastics

produced by a combination of three monomers7 acrylonitrile, butadiene,and styrene. %he overall property balance of the terpolymer is a result ofthe contribution of the uni$ue characteristics of each monomer. @olymer

chemical resistance and heat and aging stability depend on acrylonitrile,while its toughness, impact resistance, and property retention at lowtemperature are developed through butadiene. !opolymer rigidity, glossysurface appearance, and ease of processability are contributions fromstyrene. %he terpolymer properties are controlled by manipulation of theratio and distribution of the three components.&B# resins consist essentially of two phases7 a rubbery phase dispersedin a continuous glassy matrix of styreneOacrylonitrile copolymer #&*/through a boundary layer of #&* graft. %he dispersed rubbery phase is

rubber polymeri?ed from butadiene. #tyrene and acrylonitrile are graft-polymeri?ed to the rubber thus forming the boundary layer between thedispersed rubber phase and the continuous glassy matrix. )ncreasedmolecular weight of #&* improves product strength and ease of

processability, while the concentration, si?e, and distribution of therubber particles in[uence product toughness and impact strength. By acareful variation of the parameters controlling the phases, a family of&B# with a broad range of properties has been developed.E%ection-molded products such as insulated wires and cables. )ts physical

properties are dictated by three structural variables7 density, molecularweight, and molecular weight distribution. &s density increases, barrier

properties, hardness, abrasion, heat, and chemical resistance, strength,and surface gloss increase. 'n the other hand, decreasing density resultsin enhanced toughness, stress-crac= resistance, clarity, [exibility, and


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elongation and in reduced creep and mold shrin=age. (elt index ()/ is ameasure of molecular weight" it decreases with increasing molecularweight. )ncreasing () improves clarity, surface gloss, and moldshrin=age. Necreasing () leads to improved creep and heat resistance,toughness, melt strength, and stress-crac=ing. *arrower molecular weightdistribution gives better impact strength, but reduced resin processability.& broader molecular weight distribution is more shear sensitive and,conse$uently, leads to shear viscosity at high shear rates and is thuseasier to process. By copolymeri?ing ethylene with polar P-ole\ns, it is

possible to produce a variety of materials ranging from rubbery to lowmelting products suitable for hot-melt adhesives to those thatdemonstrate unusual toughness and [exibility. %his class of copolymerscan be represented by the general formula

where is a polar group li=e Ethylene O ethyl acrylate, Ethylene O vinylacetate etc. %he introduction of comonomers with a polar pendant group,, produces a highly branched random copolymer but with increasedinterchain interaction. %hus, relative to the homopolymer, thesecopolymers have enhanced [exibility, toughness, stress-crac=ingresistance, oil and grease resistance, clarity, and weatherability. )ngeneral, the range of properties of the copolymer can be varied byvarying the proportion and molecular weight of the comonomer.


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Bonding in Atoms and Molecules