6.1 Shapes of Molecules and Ions Flashcards
How does the electron-pair repulsion theory work?
- Electrons all have negative charge, so electron pairs repel one another
- This causes them to be arranged as far as possible from each other to minimise repulsion
- The bonded atoms are therefore held in a definite shape (so electron pairs surrounding the central atom determine the shape of the molecule/ ion)
How do you represent molecules in 3 dimensions?
- Imagine the 3D molecule in front of you, and a piece of paper (that’s surface is facing you) going through it vertically
- Change the orientation of the molecule so the piece of paper “cuts” through as many of the atoms as possible
- Draw the atoms in the plane of the paper (the ones the paper cuts through) using regular solid lines in their 3D positions
- Draw atoms coming out (in front) of the plane of paper with with a plain wedge
- Draw atoms going into (behind) the plane of paper with a dashed wedge
- You may have to rotate the molecule so that all of the atoms can be drawn, even if this means more will be drawn with wedges (e.g. sulfur hexafluoride)
What are the 2 types of electron pairs? What does each mean?
- Bonded pairs of electrons are involved in a covalent bond
- Lone pairs of electrons are not in a covalent bond
How do lone pairs differ to bonded pairs in terms of repulsion? Why?
- Lone pairs sit closer to the central atom, and take up more space
- They therefore repel more strongly than a bonded pair
List the the relative repulsions between lone pairs and bonding pairs in order of increasing repulsion.
- Bonded pair/ bonded pair
- Bonded pair/ lone pair
- Lone pair/ lone pair
Molecules with 4 electron pairs are based off of what shape? How do lone pairs affect this?
- Tetrahedral
- Lone pairs repel more strongly than bonded pairs
- The bonded pairs are repelled by the lone pairs which push them closer together, which reduces their bond angle; the angle between a bonded pairs of electrons
- Each additional lone pair reduces the bond angle by 2.5°
List the number of electron, lone and bonding pairs in methane, and therefore the name of its shape and the bond angle.
- 4 electron pairs
- 4 bonding pairs, no lone pairs
- Tetrahedral
- 109.5°
List the number of electron, lone and bonding pairs in ammonia, and therefore the name of its shape and the bond angle.
- 4 electron pairs
- 3 bonding pairs, 1 lone pair
- Pyramidal
- 107°
List the number of electron, lone and bonding pairs in water, and therefore the name of its shape and the bond angle.
- 4 electron pairs
- 2 bonding pairs, 2 lone pairs
- Non-linear
- 104.5°
When drawing molecular shapes, how are multiple bonds treated? Give an example.
- Multiple (e.g. double/ triple) bonds are treated as a single bonding region
- Carbon dioxide has 2 double bonds, so this is counted as 2 bond regions
List the number of bonding regions in carbon dioxide, and therefore the name of its shape and the bond angle.
- 2 bonding regions, 0 lone pairs
- Linear
- 180°
List the number of lone and bonding pairs in BF3, and therefore the name of its shape and the bond angle.
- 3 bonding pairs, 0 lone pairs
- Trigonal planar
- 120°
What does it mean if a molecule is planar?
- All of its atoms are in the same plane
List the number of lone and bonding pairs in SF6, and therefore the name of its shape and the bond angle.
- 6 bonded pairs, no lone pairs
- Octahedral
- 90°
How is a NH4+ ion bonded together? How is this shown in a displayed formula?
- A molecule of ammonia and a positive hydrogen ion bond together as the hydrogen ion shares what was nitrogen’s lone pair with the atom of nitrogen, through a dative covalent bond (which behaves as a regular bonding pair)
- As the hydrogen ion was missing an electron, the overall charge of the ion is 1+
