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149393_7covalent2.pdf CHAPTER 2
INTERATOMICFORCES
InteratomicBinding
Allof th emecha nismsw hichcausebondingbetweentheato msderivefromelectr ostaticinterac tionbetweennucleiande
lectrons. Thediffe ringstrengthsanddiffer ingtypesof bondaredet erminedby theparti cularelectronicstructuresof th eatomsinvolve d. Theexist enceof a stablebondingar rangement impliestha tthe spatialconfiguratio nof positiveioncoresandouterelec tronshas les s totalenergy thananyotherconfigur ation(includi nginfin itesepa ration of therespectiv eatoms). Theene rgydeficitof theconf igurationcompared withisol atedatoms is knownas cohesiveene rgy, andrangesin valuefrom0.1 eV/atom forsoli dswhichcan musteron lytheweakvander Waalsto7e v/atom ormo rein somecovalent andioniccompoundsandsomem etals .Energiesof Interacti ons
BetweenAtoms
The energyof thecrystali s lowerth anthatof thefreeatomsbyan am ountequalto theenergyrequiredtopullthecr ystalapar t intoa set of freeatoms . Thisis calledthebinding (cohesive) energyof thecrystal.
NaClis mo restablethana coll ectionof freeNaan dCl. Gecr ystalis morestablethana collectionof freeGe. The potential energy of either atom will be given b y: or simply:U= decreasein potentialenergy+in creasein potentiale nergy (duetoattra ctio n)(duetorepulsion) n m r b r a r U + - = ) ( U(r): thenet potentialenergyof interactionas functio nof r r : thedista ncebetweenatoms, ions, orm olecules a,b: proportionalityconstantof attractionandr epulsion, respectivelym , n: constantcharacteristics of eachtypeof bondandtype of structure This typical curve has a minimum at equilibriumdistance R 0 the potential increases gradually, approaching 0 as R theforce i s attractive the potential increases very rapidly, approaching at small radius. theforce is repulsive Typesof Bondi ngMechanisms
1 1 - -IONIC BONDING
IONIC BONDING
Ionic bonding is the electrostatic force of attrac t ion between positiv ely and negatively charged ions(betweennon-metalsand metals). These ions have been produced as a result of a tran sfer of electrons between two atoms with a large differe nce in electro negativities. Allion iccompoundsarec rystallinesolidsat roomtemperature. NaClis a typicalexampleof ionicbonding.
The metalli c elements have only up to the valence electrons in their outer shel l will los e their elec tronsand beco me positive ions, whereas electronegative elements tend to acquire addit ional elect rons to complete their octedand be come negative ions, or anions.
NaCl Notice that when sodium loses its one valence elect ron it gets smaller in size, while chlorine grows larger wh en i t ga ins an additional valanceelectron.After the reaction takes place, the charged Na+ and Cl-io ns are held t ogether by electr ostatic forces, thus forming anionicbond. NaCl When the Na
+ and Cl - ions approach each other closely enough so that the orbits of the electron i n the ions begin the overlap each other, then the electro n begins to repel each other by virtue of the repulsi ve electrostatic coulomb force. Of course the closer together the ions are, the grater the repulsive force . Pauli exclusion principle has an important ro le in repulsive force. To prevent a violation of the excl
usion principle, thepotential energy of thesyst em increas es very rapidly.Most ionic com pounds are britt le; a crystal willshatter if we try to distort it. This happens because distortion cause ions of like charges to come close together then sharply repel.
Brittleness
Most ionic com pounds are hard ; the surfaces of their crystals are no t easily scratches. Thi s is because the ions are bound strongly to the lattice and aren't easily displaced.
Hardness
Solid ionic compoun ds do not conduct electricitywhen a potential is applied because there are nomobile charged particles.
No free e lectrons causes the ions to be firmly
bound and cannot carrycharge by moving.Electrical
conductivityThe melting and boiling points of ionic compounds are high be cause a large am ount of thermal energy is required to separate the ions which are bound by strong electrical forces.
Melting point
and boiling pointExplanation
Property
2 2 - -COVALENT BONDING
COVALENT BONDING
Covalent bonding takes place between atoms with sma ll differences in electronegativity which are close t o each other in periodic table( between non-metal s and non- metals ). The covalent bonding is formed by sharing of outer shell electrons (i.e., s and p electrons) between atoms r ather than by electron transfer. This bondingcan be attai ned if the tw o atoms eac hshare one of the other'selectrons. So the noble gas electron configurationcan be attai ned. Each electron in a shared pair is attracted to both nuclei involved in thebond. Theapproach, elect ron overlap, and attraction can be visualized as shown in t hefol lowi ng figure representing thenuclei and electrons in a hy drogen molecule. e eCovalent network substances are brittle.If sufficient force is applied to a cr ystal, covalent bond are broken as the lattice is distorted. Shattering occurs rather than deformation of a shape.
Brittleness
They are hard because the atoms are strongly bound in the lattice, and are not easily displaced.Hardness
Poor conductors because electrons are held either on the a toms or withi n covalent bonds. They cannot move through the lattice.
Electrical
conductivityVery high melti ng points because each atom is bound by strong cova lent bond s. Many covalent bonds must be broken if the solid is to be melted and a large amount of thermal energy is required for this.
Melting point
and boiling pointExplanation
Property
Comparison
Comparison
of of Ionic Ionic and andCovalent
Covalent
Bonding
Bonding
3 3 - -METALLIC BONDING
METALLIC BONDING
Metallic bonding is the t ype of bonding found in metal elements. This is the electrostatic force of attraction between positively charged ions and delocalized outer electrons.
The me tallic bond is weaker than the ionic and the covalent bonds.METALLIC BONDING
METALLIC BONDING
Valance electrons are relatively bound to the nucle us and therefore they move fr eely through th e metal and th ey are spread out among th e atoms in t he form of a low-d en sity electron cloud. A me tallic bond result from th e
sharing of a variable number of electrons by a variable num ber of atoms. A metal may be described as a cloud of free electrons.
Therefore, metals have high
electrical and thermal conductivity. +++ +++ +++ All valence electrons in a metal combine to form a "sea"of electrons that move freely between the atom cores. The more electrons, the stronger the attraction. This m eans the melting and boiling p oints are hi gher, and the meta l is stronger and harder. The positively charged cores are held together by t hese negatively charged electrons. The free electrons act as the bond (or as a "glue") between the positively charged ions. This type of bonding is nondire ctional and is rathe r insensitive to structure. As a result we have a high ductility of metals -the "bonds" do not "break"when atoms are rearranged -m etals can experience a significant degree of plastic deformat ion. 4 4 - -VAN DER WAALS BONDING
VAN DER WAALS BONDING
Itis a weakbond, witha typicalstre ngthof 0.2 eV/ato m. Itoc cursbetweenneutrala tomsandmolecules.
The explanation of these weak forces of attraction is that there are natural fluctuation in the electron densi ty of allmolecules and these cause s mall temporary dipoles within the molecul es. It is th ese temporary dipoles
that attract one molecule to another. They are called va n derWaals' forces.
The bigger a molecule is, the easier it is to polar ise(toform a dipol e), and so the van derWaal'sfo rces get stronger, so bigger molecules e xist a s liquids or s
olids rather than gases. The shape of a molec ule i nfluences its abili ty to form temporary dipoles. Long thin molecules can pack clo ser to each other t han molecules t hat are mo re spherical. The bigger the 'surface area' of a molecule, the greate r the van der Waal's forces will be and the higher the meltin g and boiling points of the compound will be. Van der Waal's forces are of the order of 1% of the strength of a covalent bond.Homonuclearmolecules,
such as iodine, develop temporary dipoles due to natural fluctuations of electron density within the moleculeHeteronuclearmolecules,
suchas H- Clhavepermanent dipolesthatattractthe opposite polein other molecules. The dipolescan be formedas a resultof unbalanceddis tribution of electr onsin asymettricalmolecul es. Thi sis caused byth e instantaneouslocationof a fewmoreelect ronson onesid eof the nucleusthanon theot her. symmetricasymmetric Thereforeatomsormol eculescontainingdi polesareattr ac ted toea chotherbyelec trostaticforces.
Theseforces areduetotheelec trostaticattrac tionbetweenthenucleus of oneatom andt heelectronsof theother.
Van der waalsinteractionoccursgene rallybetweenatomswhichhave noblegasconfiguration. vander waalsbonding 5 5 - -HYDROGEN BONDING
HYDROGEN BONDING
A hydrogenatom, havingoneel ectron, can be covalentl ybondedto onlyoneatom. However, the hydrogenatom can involveitselfin an additiona lelectrostaticbo ndw
itha secondatom of highlyelectronegativecharactersuchas fluorineoroxygen. Thi ssecon dbondpermitsa hydrogen bond betweentwoatom sorstrucures. Thestr engthof hydrogenbondingvar iesfrom0.1 to0.5 ev/atom. Hydrogen bondsconnectwatermoleculesin ordinaryice. Hydrogen bonding is also very important in proteins and nucleic acids and therefore in life processes.