Shapes of molecules and ions

• The electron pairs surrounding a central atom determine the shape of the molecule or ion.
• Electron pairs repel one another so are arranged as far apart as possible.
• The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape.

  Different numbers of electron pairs result in different shapes
• Methane CH_4:

  • four bonded pairs of electrons (4 areas of electron density) surround the central carbon atom
  • the four electron pairs repel one another as far apart as possible in 3 dimensional space
  • This results in a tetrahedral shape with four equal bond angles of 109.5 degrees
• A lone pair of electrons is slightly closer to the central atom, and occupies more space, than a bonded pair of electrons.

  This results in a lone pair repelling more strongly than a bonded pair.

  Relative repulsions:

  bonded pair/bonded pair< bonded pair/lone pair< lone pair/lone pair
• Lone pairs repel bonded pairs slightly closer together, decreasing the bond angle.

  The bond angle is reduced by about 2.5 degrees for each lone pair
• In molecules containing multiple bonds, each multiple bond is treated as a bonding region. E.g. in CO2 4 bonded pairs around a central carbon atom are arranged as two double bonds that count as two bonded regions

  The two bonded regions repel each other as far as possible giving CO2 a linear shape with all three atoms in a straight line
• Boron trifluoride has three bonded pairs around a central boron atom. Electron pair repulsion forms a trigonal planar shape with a bond angle of 120 degrees
• Sulfur hexafluoride has six bonded pairs of electrons around the central sulfur atom. Electron pair repulsion forms an octahedral shape with equal bond angles of 90 degrees
• Carbonate ions (CO3 2-) and nitrate ions (NO3 -) both have three areas of electron density surrounding the centre atom so they have a trigonal planar shape with a bond angle of 120 degrees.
• Sulfate ions have four areas of electron density around the central sulfur atom. This gives them a tetrahedral shape and a bond angle of 109.5 degrees
• Electron pairs repel one another because each electron has a negative charge. The electron pair repulsion theory is a model used in chem for explaining and predicting the shapes of molecules and polyatomic ions
• The electron pairs surrounding a central atom determine the shape of the molecule or ion.
• The electron pairs surrounding a central atom repel on another so that they are arranged as far apart as possible
• The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape. Different numbers of electron pairs result in different shapes
• A molecule of methane CH4 is symmetrical with four C-H covalent bonds

  • four bonded pairs of electrons surround the central carbon atom
  • the four electron pairs repel one another as far appart as possible in 3D space
  • The result is a tetrahedral shape with four equal H-C-H bond angles of 109.5
• A lone pair of electrons is slightly closer to the central atom, and occupies more space, than a bonded pair. This results in a lone pair repelling more strongly than a bonding pair
• With increasing repulsion:

  • bonded pair / bonded pair
  • bonded pair / lone pair
  • lone pair / lone pair
• Methane, ammonia, and water all have four electron pairs surrounding the central atom, but in water and ammonia the electron pairs are a mix of lone pairs and bonded pairs:

  • four electron pairs repel as far apart as possible int tetrahedral arrangement
  • lone pairs repel more strongly than bonded pairs
  • therefore, lone pairs repel bonded pairs slightly closer together, decreasing the bond angle
  • the bond angle is reduced by about 2.5 degrees for each lone pair
• Bond angle is reduced by about 2.5 degrees for each lone pair
• In molecules containing multiple bonds (e.g. double bond), each multiple bond is treated as a bonding region
• In CO2:

  • the 4 bonded pairs around the central carbon atom are arramged as two double bonds
  • the two bonded regions repel one another as far apart as possible
  • this gives the carbon dioxide molecule a linear shape with all three atoms aligned in a straight line
• The greater the number of electron pairs, the smaller the bond angle
• Boron trifluoride has only three bonded pairs around the central boron atom. Electron pair repulsion gives a trigonal planar shape with equal bond angles of 120 degrees
• Sulfur hexafluoride has six bonded pairs of electrons around the central sulfur atom. Electron pair repulsion gives an octahedral shape with equal bond angles of 90 degrees.
• The reason why SF6, with 6 bonded pairs forms an octahedral is because the six fluorines are positions at the corners of an octahedron. Eight sides are joined by six corners
• Carbonate and nitrate ions have three regions of electron density surrounding the centre atom so they are trigonal planar
• Sulfate ions have four centres of electron density around the central sulfur atoms and have the same shape as a methane molecule