Chapters 7 and 8

Chemical bondingChapters 7 and 8Valence ElectronsElements with similar chemical behavior have the same number of valence electrons.For the representative elements (1A, 2A, 3A, 4A, 5A, 6A, 7A, 8A) the group number corresponds to the number of valence electron in each group (with the exception of He)When examining electron configurations, the electrons that are present in the highest principle energy level represent the valence electrons of those atoms.

Br: [Ar]4s2 3d10 4p5 Bromine has 7 valence electronsValence Electrons and Electron Dot StructuresValence electrons are the electrons that participate in chemical bondsElectron dot structures consist of the atom symbol and its valence electrons represented as dots.Br: [Ar]4s2 3d10 4p5

The Octet Rule - MetalsMetals tend to lose electrons to gain stability. When they lose electrons they attain a stable electron configuration where the next lowest-energy level has a full octet.

The Octet Rule Non-metalsNon-metals tend to gain electrons to obtain a full octet in their highest occupied p sublevel

Formation of Ionic CompoundsCations and anions combine to form ionic compounds

++NaCl

The Formation of Calcium Chloride

CaCl2Properties of Ionic CompoundsThe chemical formula for an ionic compound is called a formula unit which represents the lowest who number ration of atoms in that compound.( ex. NaCl)Most ionic compounds are crystalline solids at room temperatureIonic compounds generally have very high melting pointsIonic compounds can conduct electricity when molten or dissolved in water

Coordination Number = The number of ions of opposite charge surrounding the ion in a crystal.Metallic BondingPositive metal ions floating in a sea of electrons.

Metals are excellent conductors of electricityMetals can be easily deformed (They are malleable and ductile)Know that metals are arranged in crystalline structures, but do not worry about the arrangement of these structures: face-centered, body-centered, hexagonal.AlloysAlloys are mixtures of metalsProperties of alloys are usually superior to those of their component elements.

Substitutional AlloyInterstitial Alloy

Covalent BondingCovalent bonds occur between two or more non-metalsUnlike ionic bonds where electrons are transferred from one atom to another, electrons are shared between atoms in a covalent bond.Atoms joined together by covalent bonds are called moleculesA compound composed of molecules is called a molecular compoundMolecular and Structural FormulasA molecular formula indicates the types and numbers of each atom in a moleculeThe structural formula indicate the arrangement of the atoms in the molecule

H2OCovalent Bonds and the Octet RuleAtoms share electrons in a covalent bond so that each atom has enough electrons to satisfy the octet rule

Varieties of Covalent BondsSingle bonds (sigma bonds) enOne pair of electrons is shared between two atoms

Lone pairVarieties of Covalent BondsDouble Bonds (1sigma bond, 1 pi bond)Atoms share two pairs of electrons

Triple Bonds (1sigma bond, 2 pi bonds)Atoms share three pairs of electrons

Coordinate Covalent BondsA covalent bond in which one atom contributes both bonding electrons.

Resonance StructuresDef: A condition when more than one valid Lewis structure can be written for a molecule or ion.

Exceptions to the Octet RuleToo few electrons surrounding the central atom (ex: BH3)Boron will not have a full octet, only 6 electrons. It can only achieve a full octet when another atom shares an entire pair of electrons with it (Coordinate covalent bonding)

Too many electrons surrounding the central atom (ex: PCl5)

An odd number of electrons

How to Draw a Lewis Structure for MoleculesPredict the location of atomsIf there are more than two atoms, place the least electronegative atom in the center and surround it by the remaining atoms.Hydrogen is always terminal (outside) because it can only make one bondDetermine the total number of electrons if each atom had a full set of valence electrons (2 for H, 8 for all others)Add up the number of valence electron that you have to work withSubtract total valence electrons from total electrons and divide by two. This is the number of bonding pairs that are needed to put together the molecule.Connect the atoms with the number of bonds that you calculated aboveAdd lone pairs where needed so that each atom has a full octet (except for hydrogen which can only have two electrons)

MoleculeTotal ElectronsValence ElectronsBonding Pairs HCN Polyatomic IonsPolyatomic ions are a cluster of non-metals that carry a charge.

To draw the structure of a polyatomic ion, follow the procedure for drawing ordinary molecules but add or subtract the number of electrons gained or lost to the total number of valence electrons in your structure as indicated by the charge on the ion.MoleculeTotal ElectronsValence ElectronsBonding PairsIO3- Molecular Shape (VSEPR)Valence Shell Electron Pair Repulsion minimizes the repulsion of shared and unshared pairs of electrons around the central atom.

The shape of a molecule determines many of its physical and chemical properties.The VSEPR is based on the arrangement of bonding and lone electrons around a central atom to minimize repulsion.The repulsion of electrons creates a specific bond angle between a central atom and two terminal atoms.Lone pairs of electrons occupy more space than bonding pairs of electronsMolecular Geometry

or 2 Electronegativity and PolarityRecall: Electronegativity is the ability of an atom to attract an electron.

Chemical bonding is like Tug-o-War

Non-polar Covalent0-0.4Polar Covalent0.5-2.0Ionic>2.0 ElectronegativityBond Type Difference

Molecular PolarityMolecules are either polar or non-polarBoth polar and non-polar molecules may contain polar bonds. What determines whether a molecule is polar or non-polar is the symmetry of the molecule PolarBonds PresentSymmetryPolar/Non-PolarExamplesNoNoNon-PolarNO2NoYesNon-polarSiH4YesNoPolarNH3YesYesNon-polarCO2VSEPR shapes that can demonstrate symmetry are:

Linear Trigonal PlanarTetrahedral