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Chemistry 13: Molecular Structure

Chemistry 13: Molecular Structure

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Chemistry 13: Molecular Structure

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  1. Chemistry 13: Molecular Structure Christopher Chui Molecular Structure - C. Chui

  2. Electron Distribution • For a polyatomic ion, a Lewis electron dot diagram is preferred • Outer electron pairs attracted by two nuclei are called shared pairs • Outer electron pairs that are not involved in bonding are called unshared pairs, or lone pairs • Electron pairs spread as far apart as possible to minimize repulsions • Electron pair repulsions in a molecule may not be equal • Unshared-unshared repulsion>unshared-shared repulsion> shared-shared repulsion Molecular Structure - C. Chui

  3. Orbitals • s and p orbitals can overlap when electrons are shared • Carbon: 1s 2sp34 hybrid orbitals in a tetrahedron • Methane: 4 H 1s orbitals + 4 C sp3 orbitals  CH4 • A sigma (s) bond can be formed by overlapping: 2 s orbitals; s and p orbitals; 2 p orbitals; 2 hybrid orbitals; or 1 hybrid orbital and s orbital • If 2 p orbitals overlap sideways, they form a pi bond • A double bond consists of 1 s bond and 1 p bond • A triple bond consists of 1 s bond and 2 p bond • Double and triple bonds are less flexible but stronger than single bonds • Compounds that contain double or triple bonds between C atoms are unsaturated compounds Molecular Structure - C. Chui

  4. C Bonding • All single bonds  name ending –ane • Double bond  name ending –ene • Triple bond  name ending –yne • A double bond consists of 4 electrons occupying the space between bonded atoms • A triple bond consists of 6 electrons occupying the space between bonded atoms • H—C—H 116o; H—C=O 122o; C=C=O 180o; H-C=C 120o; H-C-=N 180o • Some compounds can contain more than 8 electrons in their outer level • The delocalization of p electrons among the carbon atoms in benzene results in greater stability of the compound Molecular Structure - C. Chui

  5. Isomers-1 • Whenever multiple p orbital overlap can occur, the molecule is said to contain a conjugated system • Conjugated systems can occur in chains or rings of atoms • Isomers have the same molecular formula but different structures • A double bond prevents free rotation of the atoms on each end of the bond • Cis and trans are 2 forms geometric isomers • Cis: Like atoms or groups are on the same side of the double bond. Trans: Like atoms or groups are on opposite side of the double bond • Positional isomers are formed in hydrocarbon molecules where new particles can occupy 2 or more different positions • Functional isomers are formed when a new element can be bonded in 2 or more ways Molecular Structure - C. Chui

  6. Isomers-2 • The mass spectrometer can be used to distinguish between isomers that have very similar properties • Atoms of elements other than carbon may have hybridized orbitals • Chemists use different theories to explain molecular structures. Two of these theories are: 1. Electron-pair repulsion theory; 2. Hybrid orbital theory • Use the simplest explanation, electron-pair repulsion, to explain molecular structure before using more complex concepts Molecular Structure - C. Chui