EL5: Shapes of Molecules Flashcards
What does VSEPR stand for?
Valence Shell Electron Pair Repulsion Theory
Define VSEPR
The shape adopted by a simple molecule/ion to keep its repulsive forces at a minimum
How are molecules held together?
Covalent bonds
Why does each covalent bond repel the other?
Covalent bonds consist of pairs of electrons
Why do bonds push each other as far apart as possible?
To reduce the repulsive forces between their pairs of electrons
Why are bonds equally spaced?
Because repulsions between each bond are equal
What bond angle does a linear molecule have?
180 degrees
How many bond pairs does a linear molecule have?
2
Define a linear molecule
An atom/ion where two groups of electrons surround its centre
Give 3 examples of linear molecules
- BeCl2
- CO2
- Ethyne
What bond angle does a trigonal planar/planar triangular molecule have?
120 degrees
How many bond pairs does a trigonal planar/planar triangular molecule have?
3
Define a trigonal planar/planar triangular molecule
An atom/ion where three groups of electrons surround its centre
Give three examples of trigonal planar/planar triangular molecules
- Ethene
- Methanol
- BF3
What bond angle does a tetrahedral molecule have?
109.5 degrees
How many bond pairs does a tetrahedral molecule have?
4
Define a tetrahedral molecule
An atom or ion where four groups of electrons surround its centre
Give three examples of tetrahedral molecules
- Methane
- Ethane
- Ammonium ion
What bond angles does a trigonal bipyramidal molecule have?
120 or 90 degrees (depending on the position in the molecule)
How many bond pairs does a trigonal bipyramidal molecule have?
5
Define a trigonal bipyramidal molecule
An atom/ion where five groups of electrons surround its centre
Give an example of a trigonal bipyramidal molecule
PF5
What bond angle does an octahedral molecule have?
90 degrees
How many bond pairs does an octahedral molecule have?
6
Define an octahedral molecule
An atom/ion where six groups of electrons surround its centre
Give one less common example of an octahedral molecule
- SF6
Where are octahedral shapes usually found?
- In the shapes of complexes of metal ions with six ligands e.g Ni(NH3)6
Why are shapes sometimes slightly distorted away from their regular shapes in a molecule/ion?
If the molecule/ion has lone pairs on the central atom, extra repulsion caused by them results in distortion of the regular shape
Order the following from the causing the most repulsion to causing the least
- Lone Pair to Bond Pair
- Bond Pair to Bond Pair
- Lone Pair to Lone Pair
Lone Pair to Lone Pair > Lone Pair to Bond Pair > Bond Pair to Bond Pair
How much does each lone pair reduce the bond angle by?
2.5 degrees
How many groups of electrons does a trigonal based pyramid molecule have?
4
What types of electron groups does a trigonal based pyramid molecule have?
- 3x single covalent bonds
- 1x lone pair of electrons
What bond angle does the trigonal based pyramid shaped molecule have?
107 degrees
What is the main example of a trigonal based pyramid molecular shape?
Ammonia
What causes ammonia to have a trigonal based pyramid molecular shape?
The lone pair in nitrogen repulses the hydrogen-nitrogen bond pairs, pushing them closer together
What shape is the trigonal based pyramid and bent/v-shaped shape based on?
Tetrahedral shape
How many groups of electrons does a bent/v-shaped molecule have?
4
What types of groups of electrons does a bent/v-shaped molecule have?
- 2x single covalent bonds
- 2x lone pair of electrons
What is the main example of a bent/v-shaped molecule?
Water
Why does water have a bent/v-shaped molecular shape?
The lone pair to lone pair repulsions of the two lone pairs in the oxygen pushes the Oxygen-Hydrogen bonds together
Define “covalent bond”
A very strong attraction between the bonding pair of electrons and the nucleui of the atoms involved in the bond
Define a “bonding pair”
An electron pair that forms a bond in a covalent structure
Define a “lone pair”
Pairs of electrons uninvolved in the bonding of a covalent structure
Define “bond”
The electrostatic attraction between atoms/ions in a structure
Define “electrostatic attraction”
A strong electrical force of attraction
What does bonding involve?
The sharing or transferring of electrons in the highest occupied electron shells
What do all substances “want” to become?
Ions with a full outer shell
Why do substances want a full outer shell?
As this is the most energetically stable arrangement
What group of elements have the most energetically stable arrangements naturally and why?
The noble gases, by the balancing of electrical forces in the atom due to the equal number of protons and electrons
Define “electronegativity”
The ability of an atom to attract the electron pair in a covalent bond to itself
Explain why a non polar bond is non polar.
A bond with similar atoms - so they have the same electronegativity so they will both pull on the electrons to the same extent and be equally shared
Explain why a polar bond is polar.
- A bond with different atoms - so they have different electronegativity
- Therefore one will pull the electron pair closer to its end
- It will be slightly more negative than the overall bond (δ−)
- The other atom will be slightly less negative - more positive (δ+)
- A dipole is formed and the bond is said to be polar
How does the electronegativity of the atoms in a polar bond relate to its polarity?
The greater the difference in electronegativity between the two atoms in the bond, the greater the bond’s polarity
What is the Pauling Scale?
A scale for measuring electronegativity
What is a non-polar covalent bond between?
The same/very similar non metals e.g Cl2
What is a polar covalent bond between?
Different non-metals e.g H2O
Order the following in order of increasing polarisation:
- Polar covalent
- Metallic
- Non - polar covalent
- Ionic
- Non - polar covalent
- Polar covalent
- Ionic
- Metallic
What is a dot and cross diagram used to show?
The bonding in covalent structures and represent the way they bond together
What do line diagrams show?
They show a pair of electrons being shared between two atoms for every line in the diagram
Define a “co-ordinative dative bond”
Both electrons of a shared pair are provided by one species (element)
Describe the bonding of a CO molecule
- Involves a triple bond
- 2 of the pairs of electrons are formed by the carbon and oxygen atoms each contributing one electron to the pair as in a normal covalent bond
- Both electrons in the third pair come from the oxygen atom
Describe the bonding of an ammonium ion (NH4+)
- The lone pair N is used to share with the hydrogen ion which needs two electrons to fill its outer shell
- The N now has a positive charge as its now sharing rather than owning two electrons
Describe the bonding in boron trifluoride - ammonia (NH3BF3)
- Boron has an incomplete shell in BF3 and can accept a share of a pair of electrons donated by ammonia
- The B becomes negative as it now shares a pair of electrons that it didn’t have before
What are the electrical properties of covalent substances and why?
They do not conduct electricity because they have no mobile ions/electrons
Are covalent substances soluble in water and why?
- No they tend to be more so in organic solvents
- The polar water molecules are more attracted to each other than the molecular substance and so do not react with it
Why do covalent bonds have a low melting point?
Because they have weak intermolecular forces which means little energy is needed to separate molecules from each other
Why does CH4 have a lower melting point than C2H6?
Because the molecule has a smaller surface area
