3Bonding Let’s consider the compound Cesium Fluoride, CsF. The electro-negativity value (EV) for Cs is 0.70; the EV for F is 4.00.The difference between the two is 3.30, which falls within the scale of ionic character.When the electro-negativity difference between two atoms is greater than 1.7 the bond is mostly ionic.
6Ionic Bonding In an Ionic bond: The electro-negativity difference is extreme,So the atom with the stronger pull doesn’t really share the electronInstead the electron is essentially transferred from the atom with the least attraction to the atom with the most attraction
7Ionic BondingWhen a metal bonds with a nonmetal an: Ionic bond is formedAn ionic bond contains a positive and negative ion.A positive ion is called a cation.A negative ion is called an anion.An Ionic bonding always involves the transfer of an electron from the metal to the nonmetal.The cation and anion are held together by electrostatic attraction.
8Characteristics of Ionic Compounds Ionic compounds do not consist of individual molecules. Instead there is a huge network of positive and negative ions that are packed together in a solid brittle crystal lattice.Because their bonds are strong, ionic compounds tend to have very high melting and boiling points-Ionic compounds are electrolytes, which means they can conduct electricityWhen forming ionic compounds the positive and negative charges must balanceIonic crystals cannot conduct electricity because the ions must be able to move.
10nonmetal w/nonmetal = covalent BondingThe take home lesson on electro-negativity and bonding is this:The closer together the atoms are on the P.T., the more evenly their e- interact, and are therefore more likely to form a covalent bondThe farther apart they are on the P.T., the less evenly their e- interact, and are therefore more likely to form an ionic bond.metal w/nonmetal = ionicnonmetal w/nonmetal = covalent
13Covalent Bonding In a covalent bond: The electro-negativity difference between the atoms involved is not extremeSo the interaction between the involved electrons is more like a sharing relationshipIt may not be an equal sharing relationship, but at least the electrons are being “shared”.
14Covalent Bonding Covalent Bonding is between two or more non-metals. Covalent bonds are formed when electrons are shared between two atoms.If they share 2 electrons, the form a single bond; 4 electrons is a double bond;If two atoms share 6 electrons, they form a triple bond.
15Covalent BondingPolar bonds usually involve nitrogen, oxygen or fluorine (NOF)Non-Polar bonds usually involve carbon-hydrogen bondsIn polar bonds, the electrons are shared unequallyIn non-polar bonds, the electrons are shared equally.Covalent compounds can exist in any state (solid, liquid or gas). They have low melting and boiling points.
16Lets look at the molecule Cl2 Covalent BondsLets look at the molecule Cl2ClClCl+SharedElectrons
17Notice 8 e- in each valence shell!!! each atom must have 8 valence e'sClClNotice 8 e- in each valence shell!!!Shared electrons are counted with both atomsCl2
18How about the molecule HCl? (Polar Covalent) shared, but not evenly Covalent BondsHow about the molecule HCl?HClClH+2.13.0(Polar Covalent) shared, but not evenly
19So what’s the bottom line? To be stable the two atoms involved in the covalent bond share their electrons in order to achieve the arrangement of a Noble Gas.
20Drawing Lewis Structures The Octet RuleConcept of Formal Charge
21Lewis StructuresLewis structures are representations of molecules showing all valence electrons, bonding and nonbonding.
22The Octet RuleThe most important requirement for the formation of a stable compound is that the atoms achieve a noble gas electron configuration.
23Writing Lewis Structures Find the sum of valence electrons of all atoms in the polyatomic ion or molecule.If it is an anion, add one electron for each negative charge.If it is a cation, subtract one electron for each positive charge.PCl3(7) = 26
24Writing Lewis Structures The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds.Keep track of the electrons:= 20
25Writing Lewis Structures Fill the octets of the outer atoms.Keep track of the electrons:= 20; = 2
26Writing Lewis Structures Fill the octet of the central atom.Keep track of the electrons:= 20; = 2; = 0
27Writing Lewis Structures If you run out of electrons before the central atom has an octet……form multiple bonds until it does. Usually occurs with carbon, nitrogen, oxygen
28More than One Lewis Structure Resonance StructuresMore than One Lewis Structure
29ResonanceThis is the Lewis structure we would draw for ozone, O3.-
30ResonanceBut this is at odds with the true, observed structure of ozone, in which……both O-O bonds are the same length.…both outer oxygens have a charge of -1/2.
31ResonanceOne Lewis structure cannot accurately depict a molecule like ozone.We use multiple structures, resonance structures, to describe the molecule.
32Resonance Just as green is a synthesis of blue and yellow… …ozone is a synthesis of these two resonance structures.
33ResonanceIn truth, the electrons that form the second C-O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygens and the carbon.They are not localized; they are delocalized.
35Exceptions to the Octet Rule There are three types of ions or molecules that do not follow the octet rule:Ions or molecules with an odd number of electronsIons or molecules with less than an octetIons or molecules with more than eight valence electrons (an expanded octet)
36Odd Number of Electrons Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.
37Fewer Than Eight Electrons Consider BF3:Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.This would not be an accurate picture of the distribution of electrons in BF3.
38Fewer Than Eight Electrons Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.
39Fewer Than Eight Electrons The lesson is: if filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, don’t fill the octet of the central atom.
40More Than Eight Electrons The only way PCl5 can exist is if phosphorus has 10 electrons around it.It is allowed to expand the octet of atoms on the 3rd row or below.Presumably d orbitals in these atoms participate in bonding.
41More Than Eight Electrons Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.
42More Than Eight Electrons This eliminates the charge on the phosphorus and the charge on one of the oxygens.The lesson is: when the central atom in on the 3rd row or below and expanding its octet eliminates some formal charges, do so.
43Hydrate FormationIn the construction of a crystal lattice, depending on the ions involved there can be small “pores” develop between ions in the ionic crystal.Some ionic compnds have enough space between the ions that water molecules can get trapped in between the ionsIonic compounds that absorb water into their pores form a special type of ionic compound called a hydrate.
45Hydrate FormationHydrates typically have different properties than their dry versions - A.K.A. anhydridesAnhydrous CuSO4 is nearly colorlessCuSO4•5 H2O is a bright blue colorWhen Copper (II) Sulfate is fully hydrated there are 5 water molecules present for every Copper ion.The hydrated name would be Copper (II) Sulfate Pentahydrate
46Hydrate FormationHave you ever bought a new purse or camera and found a small packet of crystals labeled – do not eat?These crystals are there to absorb water that might lead to mildew or moldThe formula of a hydrate is XAYB • Z H2O (Z is a coefficient indicating how many waters are present per formula unit)
47Molecular Polarity All above are NOT polar Molecules will be polar if a) bonds are polar ANDb) the molecule is NOT “symmetric”All above are NOT polar