Presentation on theme: "Molecular Shapes Electron pairs are negative and repulse each other. valence-shell electron-pair repulsion theory (VSEPR) States that molecules will achieve."— Presentation transcript:
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 CO 2 Shape 2: Bent Linear Two pairs of unshared electrons push the bonds into a bent shape Example: H 2 O
Molecular Shapes Shape 3: Pyramidal – A single pair of unshared electrons push bonds away into a pyramidal shape Example: Ammonia, NH 3 Shape 4: Tetrahedral – With a central atom that bonds 4 times(Carbon), the bonds are spaced out equally. Example: Methane, CH 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), CH 4, CO 2 Polar molecules do not have symmetry – Linear and tetrahedral shapes can be polar if there is no symmetry – Bent linear and pyramidal shapes are always polar
Physical Properties of Molecules Covalently-bonded substances are called molecules Molecules tend to be soft or brittle Poor conductors of heat & electricity low melting points Intermolecular Forces of Attraction Determine phase of molecular substance(solid, liquid, gas) Intermolecular Forces are weak compared to ionic bonds Dipole-dipole forces(positive-negative ends of polar molecules) Van der Waals Forces(weak attraction of nonpolar molecules)
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)!
Physical Properties of Molecules Salts(ionic substancescan 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.
Naming Binary Molecules Step 1: Determine if ionic or molecular. If it starts with a metal, see ionic rules for naming. Step 2: The least non-metallic atom is written first. This is the one closest to francium. Step 3: The more non-metallic 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. Examples: – SO 2 – CCl 4 –N2H4–N2H4 sulfur dioxide carbon tetrachloride dinitrogen tetrahydride