Topic 2 - STRUCTURE AND BONDING Flashcards
What are the 3 types of chemical bond?
- Ionic
- Covalent
- Metallic
What are the 4 types of structure?
- Simple molecular
- Giant covalent lattice
- Giant ionic lattice
- Giant metallic lattice
What is the definition of a metal?
Something that forms positive ions
What is the definition of ionic bonding?
The electrostatic attraction between oppositely charged ions in all directions (metal/non metal)
What affects the strength of the electrostatic forces in ionic compounds?
The electrostatic forces will be stronger when:
- The ion is smaller
- The magnitude of charge is greater
Why does NaF have a high melting point?
- Giant ionic lattice structure
- Many strong electrostatic forces in all directions]
- Which require lots of energy to break
Would you expect CaF2 or NaF to have a higher melting point? Why?
- Ca 2+ has a higher magnitude of charge but a bigger ionic radius
- Na + has a smaller charge and a smaller ionic radius
- Hence CaF2 has a higher melting point
Why can’t solid ionic substances conduct electricity?
The ions are in fixed positions, hence cannot move
In what form can ionic substances conduct electricity?
Molten/aqueous ionic substances
- Ions are free to move
What is the definition of metallic bonding?
The electrostatic attraction between positively charged metal ions and delocalised electrons
Why do metallic substances have high melting points?
A large amount of energy is required to separate the particles
Why are metallic substances insoluble in water?
There are no interactions with the water molecules
What is the definition of covalent bonding?
The electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
What are lone pairs?
Electron pairs that aren’t shared
- ie. electrons that are not involved in a bond
What is the octet rule, and why is it sometimes broken?
- 8 outer electrons is the most stable configuration
- However, sometimes the atom has less than 4 electrons to share
- Eg BF3 (Boron only has 3 electrons to share)
How many electrons do sulphur, chlorine and phosphorous all have the ability to accommodate in their valence shell?
18
What is the definition of double covalent bonds?
The electrostatic attraction between 2 shared pairs of electrons and the nuclei of the bonded atoms
What is the definition of triple covalent bonds?
The electrostatic attraction between 3 pairs of electrons and the nuclei of the the bonded atoms
What is the definition of dative bonding?
A dative bond is a covalent bond where only one of the bonded atoms donates both electrons being shared
Give an example of dative bonding
Boron trifluoride
Why is X-ray diffraction useful?
It allows bond lengths and spacing between ions or structures of crystals to be investigated
How is covalent bond strength determined by average bond enthalpy?
- Single bonds are longer than double bonds, which are longer than triple bonds
- Large atoms form longer bonds as bonding pair of electrons is more shielded from the nucleus
- Shorter the bond, the stronger the bond enthalpy
What substances have monoatomic structures?
- Group 8 elements
- Made of atoms
What substances have simple molecular structures?
- Most non-metal elements (except group 8)
- Most non-metal compounds
(- Molecules)
What substances have giant covalent structures?
- Diamond
- Graphite
- Silicon dioxide
(- Atoms)
What substances have ionic structures?
Substances containing both metals and non metals
(- Ions)
What substances have metallic structures?
Metals
(- Ions and delocalised electrons)
What does a solid line represent in 3D drawings?
A bond in the plane of the paper
What does a solid wedge represent in 3D drawings?
Something coming out of the plane of the paper
What does a dotted wedge represent in 3D drawings?
Something going into the plane of the paper
Give the:
- Name of the shape
- Bond angle
Of a molecule with 2 electron pairs
- Linear/planar
- 180degrees
Give the:
- Name of the shape
- Bond angle
Of a molecule with 3 electron pairs
- Trigonal planar
- 120degrees
Give the:
- Name of the shape
- Bond angle
Of a molecule with 4 electron pairs
- Tetrahedral
- 109.5degrees
Give the:
- Name of the shape
- Bond angle
Of a molecule with 5 electron pairs
- Trigonal bipyramidal
- 120 + 90 degrees
Give the:
- Name of the shape
- Bond angle
Of a molecule with 6 electron pairs
- Octahedral
- 90degrees
Why are molecules shaped the way they are?
- The electron pairs try to repel each other as far as possible
- Hence the molecule is very stable
How does every extra lone pair around an atom affect bond angles?
For every lone pair, bond angles reduce by 2.5degrees
What is the shape produced when there are:
- 2 bonding pairs
- 0 lone pairs?
Linear
What is the shape produced when there are:
- 3 bonding pairs
- 0 lone pairs?
Trigonal planar
What is the shape produced when there are:
- 2 bonding pairs
- 1 lone pairs?
Bent (V-shaped)
What is the shape produced when there are:
- 4 bonding pairs
- 0 lone pairs?
Tetrahedral
What is the shape produced when there are:
- 3 bonding pairs
- 1 lone pairs?
Trigonal pyramidal
What is the shape produced when there are:
- 2 bonding pairs
- 2 lone pairs?
Bent (V-shaped)
What is the shape produced when there are:
- 5 bonding pairs
- 0 lone pairs?
Trigonal bipyramidal
What is the shape produced when there are:
- 4 bonding pairs
- 1 lone pairs?
Trigonal pyramidal or see-saw
What is the shape produced when there are:
- 3 bonding pairs
- 2 lone pairs?
Trigonal planar or T-shape
What is the shape produced when there are:
- 6 bonding pairs
- 0 lone pairs?
Octahedral
What is the shape produced when there are:
- 5 bonding pairs
- 1 lone pairs?
Square pyramid
What is the shape produced when there are:
- 4 bonding pairs
- 2 lone pairs?
Square planar
Steps for working out the shape of molecules
- Number of valence electrons on central atom
- Number of atoms bonded to central atom
- Number of lone pairs
- Bonding pairs + lone pairs
Explain why the bond angle in water is less than the bond angle in ammonia (PPQ)
- Oxygen has 2 lone pairs of electrons but nitrogen only has one
- So the repulsion from the oxygen lone pairs is greater, reducing bond angle
What is the definition of electronegativity?
The relative ability of an atom to attract a pair of electrons in a covalent bond
Which element has the highest electronegativity?
Fluorine
Why does electronegativity increase along a period?
- More protons
- Nuclear charge is greater
Why does electronegativity decrease down a group?
- Bonded electrons are more shielded
How does magnitude affect ability to polarise ions?
- Bigger the magnitude of charge the more easily it can polarise ions
- The bigger the ion the more easily it can polarise ions
What are non-polar bonds and when do they occur?
When the bonded electron pair is shared equally
- Within an element
- Between 2 elements with similar electronegativities
What are polar bonds?
When there is an uneven distribution of the bonded electrons
How can you work out if a molecule is symmetrical?
- All the atoms around the central atom are the same
- There are no lone pairs on the central atom
How can you tell if a molecule in unsymmetrical?
- The central atoms has two or more different atoms
OR - The central atom has one or more lone pairs
What are the 3 types of intermolecular force?
- Hydrogen bonding
- Permanent dipole-dipole forces
- London forces
What are London forces?
- Type of intermolecular force
- Temporary, induced dipole-dipole force
- Occur between non-polar molecules
Why do London forces occur?
- The electrons are constantly moving around
- There will be an uneven electron distribution at any time
London forces are very weak; However, they strengthen when?
- The number of electrons in the molecule or an atom increases
- The surface contact of the molecules increases
Where do permanent dipole-dipole interactions occur?
Between polar molecules
What is the definition of hydrogen bonding?
The interaction between the lone pair of electrons on a nitrogen, oxygen or fluorine atom and a hydrogen atom which is bonded to an N,O or F atom
What are the properties of molecular substances?
- Low mp/bp: Little energy required to overcome IM forces
- Neutral: Cannot carry charge
- Polar molecules dissolve in polar solvents: PD-PD interactions form
- Non-polar molecules dissolve in non-polar solvents: London forces can form between the solvent and solute
Describe the properties of graphite
- Allotrope of carbon
- Each C atom is covalently bonded to 3 other C atoms
- Hexagonal arrangement
- Remaining electron is delocalised between layers
- Insoluble in water: No interactions
Describe the properties of diamond
- Allotrope of carbon
- Each C atom is covalently bonded to 4 other C atoms
- Tetrahedral arrangement
- Insoluble in water: No interactions
Describe the arrangement and motion of particles in a solid
- Regular and tightly packed
- No movement of particles from place to place, particles only vibrate
Describe the arrangement and motion of particles in a liquid
- Random and close
- Particles can move past/over each other
Describe the arrangement and motion of particles in a gas
- Random and far apart
- Particles move rapidly from place to place
Why does water have a higher melting point than expected?
It has hydrogen bonds which are very strong forces between molecules
Why is solid water less dense than the liquid?
In a solid, it forms an open lattice which is less dense than liquid water
What is the common cause of the anomalous properties of water?
Hydrogen bonding
How do we test for polar molecules?
Diverting streams method:
- Use a burette to create a stream of the liquid you are testing
- Charge a plastic rod
- If the liquid is polar the stream will be deflected
