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Lewis Dot Structures andMolecular GeometriesDr. Walker

What will you know? What willyou do? (3c) ·Lewis dot diagrams are used to represent valence electrons in anelement. Structural formulas show the arrangements of atoms and bonds in amolecule and are represented by Lewis dot structures.Draw Lewis dot diagrams to represent valence electrons in elements anddraw Lewis dot structures to show covalent bonding.Use valence shell electron pair repulsion (VSEPR) model to draw and namemolecular shapes (bent, linear, trigonal planar, tetrahedral, and trigonalpyramidal).Polar bonds form between elements with very different electronegativities.Non-polar bonds form between elements with similar electronegativities.Polar molecules result when electrons are distributed unequally.Recognize polar molecules and non-polar molecules.(6a) · Draw Lewis dot structures, identify geometries, and describe polaritiesof the following molecules: CH4, C2H6, C2H4, C2H2, CH3CH2OH, CH2O, C6H6,CH3COOH.

Lewis Dot Structures Created by Gilbert Lewis in 1916 Shows structural formulas for compounds– Arrangement of atoms and bonds within acompoundStructural formula forMethane, CH4

Lewis Dot Structures Uses valence electrons One dot one valence electron One dash a covalent bond twoelectrons

Lewis Dot Structureshttp://www.roymech.co.uk/images14/lewis elements.gif

Practice How many dots will the following elementscontain?– Fluorine– Boron– Carbon

Practice How many dots will the following elementscontain?– Fluorine 7– Boron 3– Carbon 4 Equal to number of valence electrons– For main groups, equal to last number of groupnumber

Lewis Structures Lewis structures show how valence electronsare arranged among atoms in a molecule. Lewis structures reflect the idea that stabilityof a compound relates to the octet rule Shared electrons pairs are covalent bonds andcan be represented by two dots (:) or by asingle line ( - )

HONC, HONC. The HONC Rule– Hydrogen (and Halogens) form one covalent bond– Oxygen (and sulfur) form two covalent bonds One double bond, or two single bonds– Nitrogen (and phosphorus) form three covalentbonds One triple bond, or three single bonds, or one double bondand one single bond– Carbon (and silicon) form four covalent bonds. Two double bonds, or four single bonds, or one triple andone single, or one double and two singles

Lewis Dot Structures Compounds Make the atom wanting the most bonds the central atom(if more than 2 total atoms) Draw proper number of dots ( valence electrons aroundeach atom). Join atoms on the outside with the central atom usingelectron pairs, obeying the HONC rule Make sure every atom has a full valence shell (2 e- for H, 8for everything else)– Boron the only exception we’ll cover, he gets 6 valenceelectrons

Additional Note on Octet Rule Atoms in the third row and below candisobey the octet rule at various times.We will not cover those structures in thiscourse. (DE anyone?)– This is for a simplification of material with adegree of honesty.

Examples – On Board H2ONH3BH3CCl4CO2HCNDiatomics

Examples

Carbon Based Molecules With multiple carbon compounds, connectcarbons together Arrange other elements around carbon, filloctets

Carbon Based Molecules Practice (on board)––––C2H6C2H4C2H5OHFormaldehyde (CH2O)

Carbon Based MoleculesEthyneEthaneEtheneFormaldehydeEthanol

Carbon Based MoleculesBenzeneAcetic Acid

Molecular Geometry Based on Valence Shell Electron PairRepulsion (VSEPR) theory Electron pairs around a central atomarrange themselves so they can be as farapart as possible from each other.

Molecular Geometry You will be responsible for five molecular shapes Compounds take a three-dimensional shapebased on:– Number of atoms attached– Number of unbonded electrons present These are general rules for binary compounds– There are always exceptions!!! (includingorganics)

Linear Carbon is central atomSurrounded by two oxygen atomsNo unbonded electrons on carbonLook for AX2 geometry– Central atom is group 14

Bent Oxygen is central atom– Central atom is typically group 16. Surrounded by two atoms (H or halogen) Two unbonded electron pairs on oxygen,push hydrogens out of the plane

Bent vs. LinearWhat’s The Difference?Unbonded electronson oxygenBent Both have a similarformula (AX2) Look at the central atom– If the element is group14, it is linear– If the element is group16, it is bent– Look for presence orabsence of unbondedelectronsNo unbonded electronson carbonLinear

Write allTrigonal Pyramidal Nitrogen surrounded by three hydrogen atoms(or halogens) One pair of unbonded electrons, push hydrogensout of olecshapes4.jpg

rig pyr top.jpg

Trigonal Planar Boron is central atom surrounded by threefluorine atoms (or H or other halogen) Boron can defy octet rule, happy with sixelectrons No unbonded electrons on boron, fluorine atomsstay within a single plane

Planar vs. Pyramidal Both have similar formula (AX3) Look at the central atom– If it has unbonded electrons, itwill be trigonal pyramidal– If it doesn’t have unbondedNo unbondedelectrons (only boron!), it will Electrons on boronbe trigonal planar

Tetrahedral AX4 formula Carbon (or silicon) surrounded by four hydrogens(or halogens) Only shape we’re concerned with foursurrounding atomshttp://www.elmhurst.edu/ chm/vchembook/204tetrahedral.html

DescriptionExampleLinear(AX2)2 outside atoms0 lone pairsCO2Bent (AX2)2 outside atoms2 lone pairsH2Otrigonal planar3 outside atoms0 lone pairsBF3Tetrahedral (AX4)4 outside atoms0 lone pairsCH4trigonal pyramidal(AX3)3 outside atoms1 lone pairNH3Molecular GeometryDiagram(AX3, A boron)

Write allPolarity– Bond Polarity Difference in electronegativitybetween two atoms in a chemicalbond Unequal sharing of electrons between elements

Write allBond Polarity Ionic– Elements on opposite sides of periodic table (metal nonmetal)– Examples NaCl, LiF, ZnCl Polar Covalent (unequal sharing)– Two elements on right side (both nonmetals) of periodictable– C-O, S-O, P-Br Nonpolar covalent (equal sharing)– Two of the same element on the right side of the periodictable– H-H, Cl-Cl, O O

Write allMolecular Polarities Polar molecules occur when electrons are NOTdistributed equally Look for symmetry within molecule– Only one line of symmetry – Polar molecule Polar shapes– Trigonal pyramidal– Bent These rules will apply regardless of the number ofatoms on the molecule with these shapes

Write allMolecular Polarities Nonpolar molecules occur when electrons aredistributed equally Look for symmetry within molecule– More than one line of symmetry – Nonpolar molecule Nonpolar shapes– Linear– Trigonal Planar– Tetrahedral These are just guidelines for binary compounds(two elements). Compounds with multipleelements and organics do not apply to theserules.

Skills to Master Drawing Lewis dot structures from a givenmolecular formula Assigning a shape based on a molecularformula (or Lewis dot structure) Determine whether a bond is polar ornonpolar Determine whether a molecule is polar ornonpolar based on formula (or Lewis dotstructure)

Terms To Know Lewis Dot StructureStructural formulaLinearBentTrigonal pyramidalTrigonal planarTetrahedralPolarityElectronegativity (review)PolarPolar covalentNonpolarNonpolar covalent

(3c) ·Lewis dot diagrams are used to represent valence electrons in an element. Structural formulas show the arrangements of atoms and bonds in a molecule and are represented by Lewis dot structures. Draw Lewis dot diagrams to represent valence electrons in elements and draw Lewis dot structures to show covalent bonding.