Shapes of Molecules and Ions

• What primarily determines the three-dimensional shape of a molecule or ion?

  Number and arrangement of electron pairs
• Into what two categories do electron pairs fall?

  Bonding pairs and lone pairs
• What is the role of bonding pairs of electrons?

  Forming covalent bonds with other atoms
• What is the role of lone pairs of electrons?

  Not involved in bonding, remain on atom
• In an ammonia molecule (NH3), how many electron pairs surround the central nitrogen atom?

  Four
• In an ammonia molecule (NH3), how many bonding pairs are connected to hydrogen atoms?

  Three
• In an ammonia molecule (NH3), how many lone pairs are there?

  One
• What is the bond angle in methane (CH4)?

  109.5°
• What is the bond angle in ammonia (NH3)?

  107°
• What is the bond angle in water (H2O)?

  104.5°
• What is the fundamental principle of electron pair repulsion theory?

  Minimising repulsion between electron pairs
• According to electron pair repulsion theory, why do molecules adopt certain shapes?

  To minimise the repulsion between electron pairs
• How do lone pairs influence molecular shape, according to electron pair repulsion theory?

  Compressing bond angles
• Why do lone pairs cause more repulsion than bonding pairs?

  They are closer to the nucleus
• What is the order of repulsion strength between electron pairs?

  Lone pair-lone pair > Lone pair-bonding pair > Bonding pair-bonding pair
• How does the number of lone pairs affect the bond angle in molecules with the same number of electron pairs?

  More lone pairs decrease bond angle
• How are multiple bonds treated when determining molecular shape?

  As if it were a single electron pair
• What is the shape and bond angle of a carbon dioxide (CO2) molecule?

  Linear, 180°
• What is the shape and bond angle of an ethene (H2C=CH2) molecule?

  Trigonal planar, 120°
• In predicting molecular shapes, what is the first step?

  Identify the central atom
• In OF2, how many outer shell electrons does oxygen have?

  6
• In OF2, how many electrons do the fluorine atoms contribute?

  2
• In OF2, what is the total number of electron pairs?

  4
• In OF2, how many bonding pairs are present?

  2
• In OF2, how many lone pairs are present?

  2
• What is the predicted molecular shape of OF2?

  Bent or non-linear
• What is the approximate bond angle of OF2?

  104.5°
• In predicting the shape of the ammonium ion, what adjustment must be made to the total number of electrons?

  Subtract one electron
• In NH4+, how many outer shell electrons does nitrogen have?

  5
• In NH4+, how many electrons do the hydrogen atoms contribute?

  4
• In NH4+, after adjusting for charge, what is the total number of electrons?

  8
• In NH4+, what is the total number of electron pairs?

  4
• In NH4+, how many bonding pairs are present?

  4
• In NH4+, how many lone pairs are present?

  0
• What is the predicted molecular shape of NH4+?

  Tetrahedral
• What is the approximate bond angle of NH4+?

  109.5°
• To predict molecular shape, what should you first determine on the central atom?

  Total number of electron pairs
• When determining the number of outer shell electrons, what is used to find the number of electrons?

  Group number
• What should you do if the species being analysed is an ion?

  Adjust electron number for charge
• How do you determine the number of lone pairs?

  Subtract bonds from electron pairs