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Chemistry 13: Molecular Structure PowerPoint PPT Presentation


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Chemistry 13: Molecular Structure. Christopher Chui. Electron Distribution. For a polyatomic ion, a Lewis electron dot diagram is preferred Outer electron pairs attracted by two nuclei are called shared pairs

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Chemistry 13: Molecular Structure

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Chemistry 13 molecular structure l.jpg

Chemistry 13: Molecular Structure

Christopher Chui

Molecular Structure - C. Chui


Electron distribution l.jpg

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


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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


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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


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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


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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


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