Introduction To Bonding BONDING Notes

12/15/2016BONDINGNotesMr. BuchananIntroduction to Bonding Ionic Bonding occurs between ametal and a nonmetal. Metallic bonding occurs betweentwo metals . Covalent bonding occurs between anonmetal and a nonmetal. A positive ion is called a cation. A negative ion is called an anion.Introduction to Bonding Atoms are generally found in nature incombination held together by chemicalbonds. A chemical bond is a mutual electricalattraction between the nuclei andvalence electrons of different atomsthat binds the atoms together. There are three main types ofbonding: ionic, metallic and covalent.Introduction to BondingWhat determines the type of bondthat forms? The valence electrons of the two atomsinvolved are redistributed to the most stablearrangement. The interaction and rearrangement of thevalence electrons determines which type ofbond that forms. Before bonding the atoms are at theirhighest possible potential energy1

12/15/2016Introduction to BondingIntroduction to Bonding There are 2 philosophies of atom to atominteraction– One understanding of the formation of achemical bond deals with balancing theopposing forces of repulsion and attraction– Repulsion occurs between the negative eclouds of each atom– Attraction occurs between the positivenuclei and the negative electron clouds When two atoms approach eachother closely enough for their electronclouds to begin to overlapIntroduction to BondingIntroduction to Bonding As the optimum distance is achievedthat balances these forces, there is arelease of potential energy Another understanding of the formation of a chemical bond between twoatoms centers on achieving the moststable arrangement of the atoms’valence electrons– The atoms vibrate within the windowof maximum attraction/minimumrepulsion The more energyreleased the strongerthe connecting bondbetween the atoms– The electrons of one atom begin torepel the electrons of the other atom– And repulsion occurs between thenuclei of the two atoms– By rearranging the electrons so thateach atom achieves a noble gas-likearrangement of its electrons creates apair of stable atoms (only occurswhen bonded)2

12/15/2016Introduction to BondingIntroduction to Bonding Sometimes to establish this arrangement one or more valence electronsare transferred between two atoms A good predictor for which type ofbonding will develop between a set ofatoms is the difference in theirelectro-negativities.– Basis for ionicbonding Sometimes valenceelectrons are sharedbetween two atoms– Basis for covalentbonding– Remember, electro-negativity is ameasure of the attraction an atom hasfor e-s after developing a bond The more extreme the differencebetween the two atoms, the lessequal the exchange of electronsIntroduction to Bonding Let’s consider the compound CesiumFluoride, CsF.– The electro-negativity value (EV) forCs is .70; the EV for F is 4.00. The difference between the two is3.30, which falls within the scale ofionic character. When the electro-negativity differencebetween two atoms is greater than 1.5the bond is mostly ionic.3

12/15/2016Introduction to Bonding The take home lesson on electronegativity and bonding is this:– The closer together the atoms are onthe P.T., the more evenly their einteract, and are therefore more likelyto form a covalent bond– The farther apart they are on the P.T.,the less evenly their e- interact, andare therefore more likely to form anionic bond. metal w/nonmetal ionicnonmetal w/nonmetal covalentIonic BondsCovalent Bonding Forming ionic bonds can berepresented as a transfer ofelectrons.to give Na :Cl:.Simply as Na Clor NaCl4

12/15/2016Covalent BondingCovalent Bonding In a covalent bond: Covalent Bonding is between two ormore non-metals. Covalent bonds are formed whenelectrons are shared between twoatoms. If they share 2 electrons, they form asingle bond; 4 electrons is a doublebond; If two atoms share 6 electrons, theyform a triple bond.– The electro-negativity differencebetween the atoms involved is notextreme So the interaction between theinvolved electrons is more like asharing relationship– It may not be an equal sharingrelationship, but at least the electronsare being “shared”.Covalent BondsCovalent Bonding Polar bonds usually involve nitrogen,oxygen or fluorine (NOF) Non-Polar bonds usually involvecarbon-hydrogen bonds In polar bonds, the electrons areshared unequally In non-polar bonds, the electrons areshared equally. Covalent compounds can exist in anystate (solid, liquid or gas). They havelow melting and boiling points.Lets look at the molecule Cl2ClClCl ClSharedElectrons5

12/15/2016Covalent BondsHow about the molecule HCl?Cl ClNotice 8 e- ineach valenceshell!!!Shared electronsare counted withboth atomsBohr and Lewis DiagramsP: 11N:12Bohr modelof SodiumAtomNaLewisStructure ofSodium AtomHCl(Polar Covalent)shared, but not evenlyH Cl2.13.0Lewis Structures A Lewis structure is a diagram showingthe arrangement of only the valenceelectrons of the atoms in a molecule. We use dots to represent valenceelectrons involved in bonding. Valence electrons are electronspresent in the outermost energy level ofan atom.6

12/15/2016Valence electrons for ElementsBondsRecall that the valence electrons for the elements can bedetermined based on the elements position on theperiodic table.Examples of Lewis Dot SymbolValence electrons and number of bondsBondsNumber of bonds element can form depends on thenumber of valence electrons.Fam i l yH al ogen sF, B r, C l , IC al cogen sO, SN i tr ogenN, PC arb onC , Si# C ov al en t Bon d s*1 bondoftenO 2 bondoftenN 3 bondoftenC 4 bondalwaysX4 bonds 3 bonds 2 bondsO C O :N N : O O1 bond:F – F:: :: : In general - Atoms can share more than one electron pair. They may double or triple up pairs of electronsto satisfy the octet rule. A Single bond is the sharing of one pair ofelectrons between two atoms. A Double bond is the sharing of two pairs ofelectrons between two atoms. A Triple bond is the sharing of three pairs ofelectrons between two atoms.The above chart is a guide on the number of bonds formed by these atoms.7

12/15/2016Rules for Writing Lewis StructuresBonds1. Determine whether the compound is covalent orionic. If covalent, treat the entire molecule. Ifionic, treat each ion separately.2. Determine the total number of valence electronsavailable to the molecule or ion by summing thevalence electrons of all the atoms in the unit.3. Organize the atoms so there is a central atom(usually the least electronegative) surrounded byligand (outer) atoms. Hydrogen is never thecentral atom. So forming covalent bonds can beshown asSo what’s the bottom line?Ionic BondingTo be stable the two atomsinvolved in the covalent bondshare their electrons in orderto achieve the arrangementof a noble gas.8

12/15/2016Ionic BondingIonic Bonding In an Ionic bond: When a metal bonds with a nonmetal– The electro-negativity difference isextreme, So the atom with the stronger pulldoesn’t really share the electron– Instead the electron is essentiallytransferred from the atom with theleast attraction to the atom with themost attractionCharacteristics of Ionic Compounds Ionic compounds do not consist of individualmolecules. Instead there is a huge network ofpositive and negative ions that are packedtogether in a solid brittle crystal lattice. Because their bonds are strong, ioniccompounds tend to have very high melting andboiling points -Ionic compounds are electrolytes, whichmeans they can conduct electricity When forming ionic compounds the positiveand negative charges must balance Ionic crystals cannot conduct electricitybecause the ions must be able to move.an: Ionic bond is formed An ionic bond contains a positive andnegative ion. -A positive ion is called a cation. -A negative ion is called an anion. -An Ionic bonding always involves thetransfer of an electron from the metal to thenonmetal. -The cation and anion are held together byelectrostatic attraction.An electron is transferred fromthe sodium atom to the chlorine atomThe picture can't be display ed.---- ------- -- ---- - ----9

12/15/2016Both atoms are happy, they bothachieve the electron arrangementof a noble gas.Notice 8 e- ineach valenceshell!!!---- -- --- -------Very Strong Electrostaticattraction established -- - ----IONIC BONDSSo what’s the bottom line?To be stable the two atomsinvolved in the ionic bond willeither lose or gain their valenceelectrons in order to achieve astable octet arrangement ofelectrons.10

12/15/2016Metallic BondingMetallic Bonding The metallic bond consists ofpositively charged metallic cationsthat donate electrons to the sea. The sea of electrons are shared byall atoms and can move throughoutthe structure.Metallic BondingMetallic Bonding In a metallic bond:– The resulting bond is a cross betweencovalent and ionic bonding Valence electrons are transferredfrom one metal atom to thesurrounding metal atoms But none of the involved metal atomswant the electrons from the originalatom, nor their own so they passthem on11

12/15/2016Introduction to Metallic Bonding What results is a sharing/transfer ofvalence electrons that none of theatoms in the collection own thevalence electrons– It resembles a collection of positiveions floating around in a sea ofelectronsSea of Electrons12

Metallic Bonding Metallic Bonding The metallic bond consists of positively charged metallic cations that donate electrons to the sea. The sea of electrons are shared by all atoms and can move throughout the structure. Metallic Bonding Metallic Bonding In a metallic bond: – The resulting bond is a cross between covalent and ionic .