4. Chemical bonding & structure Flashcards
Defi of ionic bond
EFOA between cations and anions
Defi of covalent bond
EFOA between positively-charged nuclei and negatively-charged shared electrons
Defi of metallic bond
EFOA between metal ions and sea of delocalised electrons
What are 2 factors that can increase the strength of ionic attraction?
Decreasing the size of ions
Increasing charge of ions
Explain why ionic lattices have high melting points & boiling points
Large amount of thermal energy required to overcome strong EFOA
Explan why ionic lattices are hard
Not easy to scratch surface because ions are bound strongly to the lattice & not easily displaced
Explain why ionic lattices are brittle
Distortion causes ions of like charges to come close together & repel each other
Explain electric conduction of ionic lattices
Solid: cannot conduct
Molten/aqueous: can conduct
Explain why ionic lattices are soluble in water and polar solvents
Polar molecules pull an ionic lattice apart & surrounds each ion
What are the 2 types of electron pairs?
Bonding pairs & lone pairs
What atoms can hold fewer than 8 electrons after bonding?
Compounds of group 2 & 3 elements
What atoms can hold more than 8 electrons in its valence shell?
Period 3 onward
What is a pure ionic compound?
One that has a complete transfer of electrons from metallic to non-metallic atoms, forming cations and anions
What is a pure covalent compound
One that has an equal sharing of electrons between covalently bonded atoms
What is the trend of electronegativity across a period? Why?
Increases across the period, but noble gases are not electronegative
Atoms get smaller, nuclear charge increases –> more electrostatic attraction between the bonding and nuclei
Define electronegativity
The relative attraction that an atom has for the shared pair of electrons in a covalent bond
Trend of electronegativity down a group? Why?
Decreases down the group.
Atoms get larger, increase in distance results in decrease in electrostatic attraction between bonding electrons and nuclei of atoms –> Decrease in electronegativity
What is polarisation?
The distortion of the valence electron cloud
What is a polar bond?
A covalent bond where electrons are shared unequally because of the electronegativity difference
What are the 3 factors affecting covalent bond strength?
Bond length
Bond multiplicity
Bond polarity
Explain how bond length affects bond strength
Longer bond length –> Shared electrons further from nuclei –> Lower bond energy
What is the exception for bond length? Explain
F-F bond in fluorine molecule
It has a short BL but because of the repulsion between the lone pairs of electrons, it has a relatively weak bond
What is bond multiplicity?
Multiple covalent bonds between 2 atoms
What is the ranking of electron pair strength? Reason?
LP-LP > LP-BP > BP-BP
Due to bonded pair electrons being localised between 2 nuclei, but lone pair is not localised and extend to a greater region of space
Shapes & angles for 2 electron domains
Linear, 180 deg
Shapes & angles for 3 electron domains
Trigonal planar, 120
Bent, <120, arnd 115
Shapes & angles for 4 electron domains
Tetrahedral, 109.5
Trigonal pyramidal, 107
Bent, 104.5
Shapes & angles for 5 electron domains
Trigonal bipyramidal, 120,90
Seesaw, 185, 118, 88
T-Shape, 90
Linear, 180
Shapes & angles for 6 electron domains
Octahedral, 90 Square pyramidal, 88 Square planar, 90 T-Shape, 90 Linear, 180
What are the physical properties of giant covalent structures? [3]
- High mp/bp because of strong covalent bonds
- Low electrical conductivity due to lack of mobile charges
- Insoluble in polar solvents and water
What are the physical properties of simple covalent molecules? [3]
- Low mp/bp because of weak Van der Waals intermolecular forces of attraction
What is the structure of diamond like?
Every carbon atom tetrahedrally bonded to four other carbon atoms
What is the structure of graphite like?
Each carbon atom bonded to 3 other carbon atom, leaving a sea of delocalised electrons –> Can conduct electricity
Arranged in layers, with weak LDF between layers, so they can slip easily
What is the structure of fullerene like?
C60, consists of exactly 60 carbon atoms
What is the structure of graphene like?
A thin layer of pure carbon, carbon atoms bonded together in a hexagonal honeycomb lattice
One layer of graphite
What causes a permanent dipole?
A difference in electronegativity between bonded atoms
What causes an instantaneous induced dipole?
Fluctuation of electron clouds –> Temporary positive and negative charges
What causes an induced dipole?
When a molecule with a dipole comes close to a non-polar molecule
What is another name for London Disperson Forces (LDF)?
Instantaneous dipole-Induced dipole interactions
ID-ID interactions
In what molecules is LDF present?
In all molecules because all molecules will experience a fluctuation of the electron cloud
What are the 2 factors that affect the strength of LDF?
- Size of electron cloud/no. of electrons
2. Surface area of contact (for isomers)
Explain how surface area of contact of isomers affect the strength of LDF?
Straight chain molecules will have greater surface area in contact with other molecules
Therefore forming stronger LDF between molecules
What does the polarity of a molecule depend on? [2 things]
The polarity of bonds
Shape of molecule
What are dipole-dipole forces?
The electrostatic attraction between the oppositely charged ends of 2 polar molecules with permanent dipoles
What affects the strength of DP-DP forces?
Polarity of molecule
The greater the electronegativity difference the stronger the DP-DP force
What are the 2 criteria for hydrogen bonding?
- Hydrogen atom must be directly bonded to N, O or F atom
2. N, O and F atom has at least one lone pair of electrons
What 2 factors do you consider when you determine the strength of a H-bond?
- Number of lone pairs
- Number of H atoms
For every 1 H-atom directly attached to N, O or F and one lone pair of electrons on N, O, or F, there is 1 hydrogen bond.
What are 2 applications of hydrogen bonding into real life?
- Dimerisation of ethanoic acid in benzene or in CCl4 (very strong intermolecular hydrogen bonding between 2 acid molecules)
- Structure of ice (open structure, 4 hydrogen bonds per H2O molecule)
Rank the 3 types of intermolecular forces, from weakest to the strongest
LDF < Dipole-Dipole forces < Hydrogen bonding
What substances are soluble in polar solvents?
Substances that can form hydrogen bonding with polar water molecules
What substances are soluble in non-polar solvents?
Substances that have comparable LDF to the solvent
What is the pattern of mp/bp of hydrides in group 14?
Lower than the rest of the hydrides
Because they are non-polar, they only have LDF
What is the general trend for mp/bp for hydrides?
Group 14 < 15 < 16 < 17
What is the equation for lattice energy?
LE is directly proportionate to (the charges on the ions divided by the sum of the 2 radii)
Explain what an ion-dipole attraction is
When the dipoles in H2O are attracted to the anions and cations in an ionic compound, and forms bonds with the surface ions in an ionic crystal (releasing energy in the process)
Explain how ionic-dipole attraction causes the detachment of ions from the crystal lattice & subsequently solvation (dissolving)
When the energy released in bond forming during ion-dipole attraction is greater than the energy required to break the intermolecular ionic bonds, then the ionic compound will be soluble
What are the 3 factors that affect the strength of a metallic bond?
- The number of valence electrons per atom
- The nuclear charge
- The metallic radii (decreasing will lead to an increase in strength)
What is charge density?
Charge density is proportionate to the (nuclear charge over radius)
Why are metals good thermal conductors?
Because when heat is supplied, KE of the electrons increase and KE is transmitted through the system of delocalised electrons to other parts of the metal
Why are metals malleable?
The regular arrangement of metal atoms allow the particles to slide past each other when an external force is applied, but the delocalised electrons still hold the whole structure together
What does ‘polarising power’ mean?
The ability of the cation to distort the anion’s electron cloud
What does ‘polarisability’ mean?
The ease with which the anion’s electron cloud can be distorted
What are the 2 factors affecting the degree of polarisation?
- Charge density of cation (charge/radius)
2. Size of anion (charge also but it’s less commonly used)
How to maximise the covalent character of an ionic compound?
Increase the charge density (increase charge, decrease radius)
Increase size and charge of anion
Does the covalent character in an ionic compound increase or decrease the strength of an ionic bond?
Increases it!!!
What is the definition of lattice energy?
The amount of energy evolved when one mole of the ionic solid is formed from its constituent gaseous ions under standard conditions
How you determine where the extra electrons in a negative polyatomic ion goes to?
To the more electronegative atom
How do you determine which atom loses electrons in a positive polyatomic ion?
To the less electronegative atom
