Topic 4: Bonding Flashcards
Describe structure of an ionic compound
- Giant ionic lattice
- Electrostatic forces of attraction between oppositely charged ions
Describe ionic bonding
- Occurs between a metal and a non-metal
- When electronegativity difference is high between atoms, electrons are transferred from the metal to non-metal
- forms ions
- electrostatic forces of attraction between oppositely charged ions
- in a GIANT IONIC LATTICE structure
Why are some molecules polar?
- Polar bonds
- Asymmetrical distribution of charge
- dipoles do not cancel
- hence net dipole moment
- draw arrow on lewis structure
Describe covalent bonding
A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei.
Occurs between NON METALS only
Describe metallic bonding
Electrostatic forces of attraction between a lattice of positive ions and the surrounding sea of delocalized electrons
Why are ionic compounds/metals brittle?
- Ionic compounds are brittle because slight movement brings ions of same charge adjacent to each other
- Repulsion between layers.
Why do metals have low electronegativites
They need to lose electrons to form an octet
VSEPR full name and what does it suggest?
Valence shell electron pair repulsion - non-bonding pairs of electrons repel each other more
Describe possible molecular geometries for tetrahedral electron domain?
Tetrahedral (109.5)
Trigonal pyramidal (107)
Bent (104.5)
Describe possible molecular geometries for trigonal planar electron domain
Trigonal planar (120) Bent (117.5)
Describe possible molecular geometries for trigonal bipyramidal
Trigonal bipyramidal (90,120)
Seesaw (90 <120)
T-Shaped (90, 180)
bent (90, <180)
How to determine whether molecules are polar or non-polar?
They are polar if:
- lewis structure has lone pairs on central atom (unless it is a molecule like XeCl2 which has 3 lone pairs on central atom so it would be non-polar)
- different atoms bonding to central atom rather than just one e.g: CCl4
How to calculate bond order?
Total number of bonding pairs/total number of positions double bond can be in
Co-ordinate covalent bond
The electrons in the shared pair may originate from the same atom.
Order single, double and triple bonds in terms of strength and length (from least to most)
Length:
triple, double, single
Strength:
single, double, triple
Describe trend in group 6 hydrides
H2O has high MP due to hydrogen bonding which is stronger than Van der Waals.
Increase in Mr means increase in electrons, more electron distortion, so more Van der Waals
Why are some molecules soluble in water
- They can form hydrogen bonds with water
- as they are polar
- negative dipole attracts positive dipole of water and vice versa
Explain what ‘like dissolves like’ means
Polar compounds soluble in polar substances and non polar soluble in non polar substances
Describe possible molecular geometries for octahedral electron domain
Octahedral (90) Square pyramidal (<90) Square planar (90)
Describe benefits of ozone
Absorbs UV light emitted from sun, protects from skin cancer
Why does water have a maximum density at 4 degrees celcius rather than when it is ice
- Hydrogen bonding between water molecules
- Each water bonds to 4 other water molecules
- Very open tetrahedral structure
- When it melts, molecules move closer to each other
Describe the characteristics of diamond as an allotrope of carbon
- Strong covalent bonds
- Each carbon atom bonds to 4 other carbon atoms
- Tetrahedral structure
- Giant covalent
- Hard
- No delocalised electrons so not a conductor of electricity
- used as a cutting tool
- Sp3 hybridisation, 109.5 bond angle
- insulator
Describe graphite
- Giant covalent
- Each carbon bonded to 3 other carbon atoms
- delocalised electrons so conducts electricity
- Weak intermolecular forces between layers
- Layers can slide over each other due to weak London Dispersion Forces
- lubricant
- 120 degrees
- sp2
Describe graphite
- Giant covalent
- Each carbon bonded to 3 other carbon atoms
- delocalised electrons so conducts electricity
- Weak intermolecular forces between layers
- Layers can slide over each other
- lubricant
- sp2
Graphene
- Single layer of gaphite
- Simple covalent
- Each carbon bonded to three others
- delocalised electrons, so conducts electricity
- sp2
Fullerene
- 60 carbon atoms
- arranged in hexagons and pentagons
- forms a spherical shape
- Poor electrical conductor
- each carbon bonded to 3 others
- sp2
EXAM TIPS
- Some molecules have non-polar covalent bonds due to low E.M difference but are still polar molecules overall because of their geometry (e.g: lone pairs on central atom) such as ozone, as well as PH3 which has non-polar covalent bonds but is a polar molecule overall due to lone pairs however if there is no lone pair and only non-polar covalent bonds then molecule can’t be polar
The whole point of a polar molecule is an uneven charge distribution
- remember to draw lone pairs on lewis structures, don’t rush it
- when doing ozone wavelength calculations, remember to first MULTIPLY BY 1000 and then DIVIDE BY 6.02 X 1023. Then at the end once calculator gives you the value, MULTIPLY BY 10^9 TO CONVERT INTO NM
- If it asks to find hybridisation in a given lewis structure for numbered atoms, REMEMBER LONE PAIRS MAY EXIST THAT HAVN’T BEEN DRAWN ON sneakily. Lone pairs do influence hybridisation to remember to take them into account
- You’ll do amazing. You’ve got this. Keep revising.
Hydridization
sp, linear, 180
sp2, trigonal planar, 120
sp3, tetrahedral, 109.5
Ozone calculation:
Wavelength = HC/E h = plank's constant c = speed of light E = energy - calculated using data booklet.
The energy is the bond enthalpy which is on page 11 in data bk. Draw lewis structure of molecule they want to see if it is a single/double/triple bond between the 2 atoms and find enthalpy accordingly.
Multiply this value by 1000, then divide by 6.02 x 1023 for 1 bond. This is the E value which you can substitute into the equation to find the wavelength
ONCE WAVELENGTH FOUND, multiply by 10^9 to convert it into nanometres. This will be the final answer.
What is hybridisation?
Mixing of atomic orbitals to form molecular orbitals of degenerate energy
Describe Sp3 hybridisation
1 electron from 2s orbital goes into empty 2p orbital. The 4 orbitals mix together to form 4 degenerate sp3 orbitals. These form sigma bonds only because no unhybridised p-orbitals are left
Describe formation of a sigma bond
Axial, head on head overlap of orbitals trapping electron density between nuclei of atoms. Forms between an s and an s, p and a p, or s and a p orbital
Describe formation of a pi bond
Sideways overlap of unhybridised p orbitals, trapping electron density above and below internuclear axis
Describe sp2 hybridisation
1 electron from 2s orbital enters empty 2p orbital.
2 p orbitals and the 2s orbitals mix together, forming 3 sp2 orbitals
1 unhybridised p orbital left
Discuss the bonding in the resonance structures of ozone.
one pair on p orbital of O atom overlaps/delocalizes with pi electrons from double bond
both O–O bonds have equal bond length
both O–O bonds have same/1.5 bond order
both O–O are intermediate between O–O AND O=O
both O–O bonds have equal bond energy
Pi electrons are delocalized
Describe, by means of equations, how nitrogen(II) oxide, NO, catalyses the depletion of ozone.
NO (g) + O3 (g) -> NO2 (g) + O2(g)
NO2(g) -> NO(g) + O. (g)
NO2(g) + O. (g) -> NO(g) + O2
NO2 + O3 -> NO + 2O2
O3 + O. -> 2O2
State the source of ozone depleting pollutants
Nitrogen dioxide CFCs propellants in aerosol cans air conditioners fire extinguishers solvents plastic foams
Explain how the bonding in O2 and O3
affects the wavelengths of UV light they absorb.
O2 has a double bond
O3 1.5 bond order
O2 shorter and stronger bond than O3
O2 absorbs shorter wavelength than O3
Bonding in the nitrate ion involves electron delocalization. Explain the meaning of electron delocalization and how it affects the ion
pi electrons shared by more than two atoms/nuclei / a pi/
π bond/overlapping p-orbitals that extends over more than two atoms/nuclei;
all (N–O) bonds equal length/strength/bond-order / charge on all oxygen/O atoms equal / increases stability/lowers PE (of the ion);
State one environmental problem caused by these atmospheric pollutants
Depletion of ozone layer
Explain, on a molecular level, why ozone dissociates with radiation of a longer wavelength than oxygen
O3 needs less energy/weaker bonding
O3 has bond order 1.5/ intermediate between double and single
Nitrogen(II) oxide, NO, is a primary pollutant that depletes the ozone layer. State two equations that show how this oxide catalyses the depletion of ozone in the stratosphere
NO∙(g) + O3(g) → NO2∙(g) + O2(g)
NO2∙(g)+O∙(g)→NO∙(g)+O2(g);
Discuss one advantage and two disadvantages of using hydrocarbons as alternatives to CFCs
Do not damage ozone layer
Decompose less readily than CFCs
Cheaper than CFCs
HOWEVER, both contribute to global warming
Why are CFCs expected to remain the atmosphere for many years
- Low reactivity
- Some countries still producing CFCs
- Produce radicals
- slow mixing between troposphere and upper atmosphere
Why does PCl5 have a higher MP than PCl3
- stronger Van der Waals forces
- due to more electrons and a greater mass
What are the 3 types of intrAmolecular bonding?
Ionic
Covalent
Metallic
4 types of intermolecular?
Hydrogen bonding
Dipole-dipole
Induced dipole
London dispersion forces
Which elements form hydrogen bonding and why
FON
Most electronegative atoms that’s why
Explain an instantaneous dipole
An instantaneous dipole is produced due to the random motion of electrons around the nucleus of atoms.
Electrons become asymmetrical around the nucleus of an atom.
When the electrons in nearby atoms come near each other, they induce a dipole in the nearby atom due to repulsion between the electrons, leaving a positive dipole on the nearby atom, attracted to the negative dipole on the atom that induced the dipole.
Explain the factors that affect the strength of London dispersion forces
Molecules with a larger SURFACE AREA will have a greater likelihood of coming into contact with each other and therefore there will be greater repulsion between the electrons, leading to stronger LDF between molecules.
The greater the SIZE of the molecule, the stronger the LDF because there will be more electrons in a larger molecule. The more electrons, the further they’ll be from the nucleus, less attraction on these so form dipoles more easily
Any ion that ends in “ide” means…
It gains electrons
Carbon ion formula?
C4+
Carbide ion formula
C4-
Draw the line which separates metals from non-metals
Boron to Te
Describe volatility of ionic compounds
Ionic compounds have a low volatility because ions are held together in the giant ionic lattice with strong electrostatic forces, no free to move
If asked to determine which has a stronger ionic bond, what would you do?
Determine charge density by seeing which atoms are larger
Larger atoms will have a smaller charge density
Is water dissolving ionic compound exo or endothermic
Water attracted to ion and forming bonds with it is EXO
Ion being pulled away from lattice - ENDO because it is bond breaking
Polyatomic ions:
NH4 + OH - NO 3- HCO 3- CO3 2-, SO4 2- PO4 3-
Polyatomic ions:
NH4 + OH - NO 3- HCO 3- CO3 2-, SO4 2- PO4 3- HSO4-
Why does bond polarity occur?
Bond polarity results from the difference in electronegativities of the bonded atoms.
This causes an unequal distribution of charge.
What do lewis structures show?
Lewis (electron dot) structures show all the valence electrons in a covalently bonded species.
What is the octect rule?
The “octet rule” refers to the tendency of atoms to gain a valence shell with a total of 8 electrons.
Which atoms might form stable compounds with incomplete octets of electrons
Some atoms, like Be and B, might form stable compounds with incomplete octets of electrons.
Why does resonance occur
Resonance structures occur when there is more than one possible position for a double bond in a molecule.
Difference between London dispersion and Van der Waals
The term “London (dispersion) forces” refers to instantaneous induced dipole- induced dipole forces that exist between any atoms or groups of atoms and should be used for non-polar entities.
The term “van der Waals” is an inclusive term, which includes dipole–dipole, dipole-induced dipole and London (dispersion) forces.
Explain the physical properties of covalent compounds (volatility, electrical conductivity and solubility) in terms of their structure and intermolecular forces.
Simple covalent molecules have a high volatility due to weak intermolecular forces, e.g: London dispersion and Van der Waals that do not need too much energy to be overcome
Insoluble
Do not conduct electricity because VALENCE electrons are tightly bound in covalent bonds - no free charged particles to move and carry a current
Which molecules form dative covalent bonds and which do not
NH3
Oh4 - hydronium - both electrons come from oxygen
CO
Which 2 elements have greatest difference in electronegativity
Francium to Fluorine
Describe EN values for polar, non-polar covalent bonds
0 - 0.3 = non-polar covalent
- 4 - 1.7 = polar covalent
- 8 + = ionic
How to determine if NaF is ionic or covalent
Melt it and see if it conducts electricity.
If it does then it is ionic
If it doesn’t then it is covalent
Tetrahedral examples
CH4
NH4+
3 bonding, 0 lone?
Trigonal planar
120 degrees
4 bonding, 0 lone?
Tetrahedral
109.5
2 bonding, 0 lone?
Linear
180 degrees
2 bonding, 1 lone?
Bent
117.5
3 bonding, 1 lone?
Trigonal pyramidal
107
2 bonding, 2 lone
Bent
104.5
5 bonding, 0 lone
Trigonal bipyramidal
90, 120
4 bonding, 1 lone
Seesaw
90, <120
3 bonding, 2 lone
T-shaped
<90, <180
2 bonding, 3 lone
Bent
180
6 bonding, 0 lone
Octahedral
5 bonding, 1 lone
Square pyramidal
4 bonding, 2 lone
Square planar
If central atom has 3 lone pairs what shape is it?
Linear
180 degrees
Bond order formula
Bond order = (Number of electrons in bonding molecules) - (Number of electrons in antibonding molecules)]/2
Describe structure of silicon + silicon dioxide
Macromolecule
Forms 4 bonds
High MP
Which covalent compounds are electrical conductors
Graphite - delocalised electrons between layers as carbon bonds to only 3 other carbons
NH4NO3 - in solid and gas state, NOT solid
Buckminister fullurene - poor conductor
Why are metals malleable
Can be bent/re-shaped
Positive ions are rearranged because positive ions repel each other
Formal charge equation
of valence electrons - (1/2 x bonding electrons + # of lone pair electrons - THIS IS FOR ONE ATOM)
Which molecule is the most favoured
Closest to 0
Calculation of wavelength needed to break ozone Oxygen-Oxygen bond
363 KJ mol- x 1000 = 363000 to get into Joules per mol
Divide 363000 by 6.02 x 10^23 = to find for ONE O-O
This is the ENERGY (E) value in the equation E = hv
find velocity
substitute velocity into c = wavelength x velocity
Find wavelength
Discuss the depletion of the ozone layer
CFC rises up to the ozone layer
High energy UV breaks off Chlorine radical
Chlorine radical breaks off ozone radical
This causes 3 oxygen molecules to be produced
Oxygen does not protect us from UV rays which can cause skin cancer
Equation of the CCl2F2 and ozone depletion
C Cl2 F2 -> C Cl F2. + Cl.
O3 + Cl. -> O2 + ClO.
ClO. + O. -> O2 + Cl.
Overall equation for ozone depletion
O. + O3 -> 2O2
