Chapter 6: Shapes of Molecules & Electronegativity 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?
- Lone pairs repel more strongly than a bonded 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°
Describe what a 3D representation of methane would look like.
- Central carbon atom
- Regular bonds attaching 2 hydrogen atoms to it, one above and one to the left (triangle shape)
- Regular wedge towards the bottom right attaches the third hydrogen atom
- Between the top hydrogen and the one with a regular wedge is the fourth hydrogen with a dashed wedge
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 bonding 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 would SF6 be drawn in 3D?
- The central sulfur atom would have 6 fluorine atoms attached to it at equal angles
- The top and bottom would be attached with regular bonds
- The other 2 on the top would have a plain wedge
- The 2 on the bottom would have a dashed wedge
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+
List the number of lone and bonding pairs in NH4, and therefore the name of its shape and the bond angle.
- 4 bonding pairs, no lone pairs
- Tetrahedral
- 109.5°
What is covalent bonding? Required.
- The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
What is electronegativity? Required.
- The ability of a bonded atom to attract the bonding electrons in a covalent bond
What is a Pauling electronegativity value?
- A value assigned as a measure of the relative attraction of a bonded atom for the pair of electrons in a covalent bond
Where are electronegativity values the highest in the periodic table?
- Up a group
- Across (to the right of) a period
- (With the exception of group 0)
Why do electronegativity values increase up a group?
- The atomic radius is smaller
- The bonded pairs of electrons are attracted more strongly to the nucleus of the atom
Why do electronegativity values increase across a period?
- The nuclear charge increases
- This attracts the bonding pairs of electrons more strongly
Which elements are the most electronegative?
- Fluorine
- Oxygen
- Nitrogen
What is a non-polar bond, and what is it also known as?
- A bond where the electrons are shared equally between the bonded atoms
- A pure covalent bond
What causes a bond to be non-polar? Give examples.
- The atoms have the same/ similar electronegativities, like carbon and hydrogen
- The atoms are the same, such as in diatomic molecules
What is a polar bond? Give an example.
- A bond where the electron pair is shared unequally
- Hydrogen chloride
What happens to atoms as a result of the existence of a polar bond?
- A permanent dipole is formed; there is a separation of opposite partial charges
- The pair of electrons sit closer to the atom with the higher electronegativity value, which causes the bond to be polarised as it has a small partial negative charge (δ-) on one side, and a small partial positive charge (δ+) on the other
Why is the delta sign used to show dipoles?
- It shows that the charges are small
How can electronegativity values be used to guess the type of bonding between atoms?
- If there is no difference in the electronegativity values of 2 bonded atoms, they will form a pure covalent bond
- A small difference creates a polar covalent bond
- If there is a large difference between the electronegativity values, one of the atoms will have a much greater attraction for the bonded pair of electrons, and would even take control of the electrons and form an ionic bond
What makes molecules polar?
- If it is not symmetrical, the molecule may have an overall dipole as the dipoles wouldn’t cancel out
How can you tell if a molecule is polar?
- If the dipoles act in opposing directions (think of them pointing from positive to negative), they cancel out and the molecule is non-polar
(- If you were to pull at the dipoles around the central atom in its 3D structure, would it move? If the central atom would move, the molecule is polar.)
Polar molecules only dissolve in what type of solvents? What about non-polar molecules?
- Polar solvents
- Non-polar molecules only dissolve in non-polar solvents
How do ionic compounds (e.g. NaCl) dissolve in water?
- Water molecules attract and surround the Na+ and Cl- ions
- The ionic lattice breaks down as it dissolves
- Na+ ions are attracted to the oxygen of the water molecules (δ-)
- Cl- ions are attracted to the hydrogen of the water molecules (δ+)
