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Molecular Shapes Electron pairs are negative and repulse each other.

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Presentation on theme: "Molecular Shapes Electron pairs are negative and repulse each other."— Presentation transcript:

1 Molecular Shapes Electron pairs are negative and repulse each other.
valence-shell electron-pair repulsion theory (VSEPR) States that molecules will achieve a 3-dimensional shape in order to push electron-pairs as far away from each other as possible Shape 1: Linear electron pairs force bonds to be 180º from each other Ex: Any molecule with only two atoms CO2 Shape 2: Bent Linear Two pairs of unshared electrons push the bonds into a bent shape Example: H2O 1

2 Molecular Shapes Shape 3: (Trigonal) Pyramidal
A single pair of unshared electrons push bonds away into a pyramidal shape Example: Ammonia, NH3 Shape 4: Tetrahedral With a central atom that bonds 4 times(Carbon), the bonds are spaced out equally. Example: Methane, CH4 2

3 Molecular Shapes Shape 5: Trigonal Planar
A central atom bonded to three elements with no unshared pairs of electrons. Shape is triangular and flat Examples: BH3, CH2O 3

4 Molecular Polarity When Determining Polarity:
Ask: Are the bonds polar/nonpolar? Ask: Is there symmetry? Ask: What shape is it? Nonpolar molecules have symmetry. no positive/negative end charge is evenly distributed Examples: Diatomic molecules(BrINClHOF), CH4, CO2 Polar molecules do not have symmetry Linear, trigonal planar and tetrahedral shapes can be polar if there is no symmetry Bent linear and pyramidal shapes are always polar 4

5 Physical Properties of Molecules
Molecule- compounds that are formed by covalently bonded elements Molecules tend to be soft or brittle Poor conductors of heat & electricity low melting points Many are gases or liquids at room temperature Intermolecular Forces of Attraction Determines the phase of molecules(solid, liquid or gas) Relatively weak compared to ionic compounds Dipole-dipole forces- polar molecules tend to “stick” to each other because of their positive-to-negative bonding. Van der Waals Forces- non-polar molecules have a weak attraction to each other Dipole-dipole forces are stronger than Van der Walls forces both are very weak compared to ionic compounds 5

6 Naming Binary Molecules
Step 1: Determine if ionic or molecular. If it starts with a metal, see ionic rules for naming. Step 2: The most metallic(closer to francium) atom is written first Step 3: The least metallic (closer to flourine) atom is written second & ending is changed to “-ide.” Step 4: Add prefixes to indicate how many of each element is in the formula. Mono is only used for the second element. CO2 = carbon dioxide Examples: SO2 CCl4 N2H4 # atoms Prefix 1 Mono 2 di 3 tri 4 tetra 5 penta 6 hexa 7 hepta 8 octa 9 nona 10 deca sulfur dioxide carbon tetrachloride dinitrogen tetrahydride 6

7 Physical Properties of Molecules
“Like dissolves like” Polar molecules dissolve polar molecules Nonpolar molecules dissolve nonpolar molecules But nonpolar and polar do not mix! It's like oil and water. Well, it is oil(nonpolar) and water(polar)! 7

8 Physical Properties of Molecules
Ionic compounds can dissolve in polar liquids Polar molecules surround the ions positive end-to-anion Negative end-to-cation Compounds with Both ionic and Covalent bonds Polyatomic ions are made with covalent bonds Bonds between nonmetals are covalent Bond with the metal is ionic. 8

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