P- Bonding Flashcards
What is a coordinate/ dative covalent bond?
A shared pair of e- w/ both e- SUPPLIED by one atom
Electronegativity
The power of an atom to attract the pair of e- in a covalent bond
What is the way of drawing atoms showing e- w/o shells called?
Lewis structures
What is electronegativity measured in?
Pauling scale (0-4)
Describe the trend in electronegativity in the periodic table.
- ↓ down the group
∵ shielding ↑ - ↑ across the period
∵ nuclear charge ↑ (↑ + charge to pull e- )
What is the atom that accepts the e- pair (slightly positive end of the molecule) called?
Electron deficient (/ delta positive)
What is the bond between 2 atoms with an electronegativity difference?
Polar (covalent) bond
How do we know whether 2 elements form covalent or ionic bonds?
electronegativity diff. < 1.7 – covalent
> 1.7 – ionic
Octet rule
atoms are most stable when they have 8 outer e-
(esp. if they are in bonds)
Why are molecular structures poor conductors of electricity?
- they have an overall neutral charge
- x charged particles to carry charge
How are coordinate covalent bonds represented in a diagram?
Arrow pointing towards atom accepting e- pair
Condition needed for the formation of a dative covalent bond
lone pair + incomplete octet –> dative covalent bond
What is the name of the compound called when some very reactive molecules (eg. AlCl3) reacts with itself?
Dimer- ↑ reactive
What is a dipole?
A covalent bond whose ends have opp. partial charges.
Why is F more electronegative than Cl?
- smaller atom
- shared e- closer to nucleus when covalent bond forms
Starter: How does sodium conduct electricity?
- delocalised e-
- move & carry charge THROUGH the structure
- when potential difference is applied
What are intermolecular forces?
Weak forces of attraction between 2 molecules
Give one unusual physical property molecules with H bonds have compared to molecules that don’t.
Molceules w/ H bonds- ↑ bp
2 conditions needed for H bonding
NEEDS BOTH!!!
- N, O or F directly bonded to a H atom
- Lone pair on N, O or F atom
Examples where H bonding is seen
- Ammonia (NH3)
- N is v. electronegative + has 1 lone pair
- Water
- O has 2 lone pairs
- Alcohol- O-H
- Carboxylic acid- O-H
- Amine- N-H
- Protein- N-H
Why does ice have a low density?
- maximises amount of H bonds between molecules
- open hexagonal structure w/ large spaces within crystal
- melting–> liquid occupies ↓ space –> ↑ dense
Why is a DNA molecule helical?
- C=O bonds in long chains
- H atoms + electronegative O atom –> H bond
- molecule spirals
INTRAmolecular H bonds- attraction between H & electronegative atom on the SAME MOLECULE
What kind of substances are affected by Van der Waals forces?
Atoms & molecules (x ions) apart from diamond & silicon dioxide (SiO2)
How is a temporary dipole formed?
- e- cloud in constant motion –> temp dipole when more at one pole
- attraction between 2 molecules induces dipole on the other molecule–> VdW
Factors increasing strength of VdW forces
- ↑ e- cloud
- ↑ Ar of atoms/ Mr of compounds
- s.a.
How does shape of a molecule affect its boiling point?
- straight chains have ↑ bp (than spherical)
∵ ↑ s.a. for VdW forces
↑ interactions + stronger
Factors determining bp
- Mr
- Are there dipole-dipole/ H bonds as well as VdW?
- Shape
**H bonds= strongest IM–> high bp (eg. water- unusually high bp)
What is ‘absolute zero’?
0 K- everything is solid
Explain why temp of water doesn’t change during boiling/ melting.
Melting- energy is used to break SOME of the H bonds
Boiling- ALL H bonds
– x IM forces in gas ∵ all H bonds broken when boiling
Explain what energy transferred is used for during melting of a substance.
To overcome SOME forces holding atoms/ ions/ molecules in proximity
Explain what energy transferred is used for during boiling of a substance.
To overcome ALL forces between atoms/ ions/ molecules in proximity
∴ x IM forces in gases
What is a crystal?
A substance in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating three-dimensional patterns.
- most crystals are solids
What are the forces to overcome when a molecular crystal is heated?
- the strongest type of IM force present
- any other IM forces (Van der Waals, permanent dipole-dipole)
What are the forces to overcome when an ionic crystal is heated?
Ionic bonds
x IM forces!!
What are the forces to overcome when a metallic crystal is heated?
Metallic bonds
x IM forces!!
What are the forces to overcome when a macromolecular crystal is heated?
Covalent bonds
x IM forces!!
Explain why hexane is a liquid at room temp while butane is a gas.
- hexane- larger molecule
- ∴ ↑ e-
- ↑ VdW between molecules
Explain why covalent molecules are gases, liquids, or solids with low melting points.
- covalent bonds only between the 2 atoms they bond
- small attraction between individual molecules
Phosphorus (P4) has a mp of 317K while Sulfur (S8) has a mp of 386K.
Explain this difference in terms of bonding.
- ↑ VdW
- ∵ ↑ e-
Co-ordination no.
no. of bonds around a central atom
What kind of bond has the highest repulsion?
lone pair-lone pair
– to ↓ repulsion, outer e- spread out as far apart in space as poss.
What is VSEPR?
Valence shell electron repulsion theory
Name + bond angle w/ 2 bonding pairs
Linear- 180
Name + bond angle w/ 3 bonding pairs
Trigonal planar- 120
Name + bond angle w/ 4 bonding pairs
Tetrahedral- 109.5
– achieved thru 3D!! (max 90 if 2D)
Name + bond angle w/ 5 bonding pairs
Trigonal bipyramidal- 90 &120
Name + bond angle w/ 6 bonding pairs
Octahedral- 90
How would the bond angle change when there are lone pairs on a tetrahedral (4 co-ordinate) shaped molecule?
Take off 2.5 for every lone pair
How would the bond angle change when there are lone pairs on a linear/ trigonal planar shaped molecule?
x affected by lone pairs
Explain the shape of an ammonia (NH3) molecule
- trigonal pyramid- 107
- bonding pairs attracted to H as well as N
- vs lone pair only attracted to N
- ∴ pulled closer
- H-N-H angle ↓
What is different in bonding between a macromolecular and molecular crystal?
Molecular:
- strong covalent bonds between atoms
- weaker intermolecular forces between molecules
vs
Macromolecular:
- all atoms are covalently bonded
Do ionic compounds conduct electricity?
yes when molten
- ions x free to move in solids
- free to move & carry charge when separated
Are ionic compounds soluble?
yes
- ∂+ & ∂- ends of H2O attracted to ions
Why are ionic compounds brittle?
- layers slide–> same charges next to each other
- repulsion cracks crystal
Melting points of monatomic substances
v. low
- v. weak forces between ATOMS
Do monatomic substances conduct?
no
- atoms are NEUTRAL
- x mobile charged particles
Why are metallic compunds strong?
- layers can slide
- while maintaining metallic bonding
Why does graphite conduct electricity?
- delocalised e- free to move
- BETWEEN LAYERS
Why is graphite brittle?
- weak IM forces between layers (graphene)
- can slide
(pi-pi stacking interactions)
Describe the structure of ice
- 3D open structure
- held tgt by covalently bonded H2O molecules (2 H bonds)
- each O bonded to 4 H atoms
- tetrahedral shape
mp & bp of ice
low
- weak forces between H2O molecules
Why does ice have a lower density than water?
- molecules in a lattice ∵ H bonds
- vs water molecules closer ∵ x in lattice
Why is a dative covalent bond represented by an arrow?
to show both e- come from the same atom
Why is Br in p block?
OUTER e- in p orbital
Explain why the bond angles in CH4 are all 109.5.
- so that 4 bond pairs can be
- as far apart as poss.
- as they repel equally
How do HCl and HF differ in appearance when drawn?
- bigger arrow showing dipole in HF ∵ bigger dipole
- draw lone pairs on for F for H bonding
- ∵ lone pair= one of the conditions for H bonding!!!
What does VSEPR mean?
- e- pairs repel
- move as far apart in space as poss.
- MINIMISE REPULSION
l-l > l-b > b-b
Describe briefly strucutre of diamond
- rigid
- tetrahedral
- lattice
Describe briefly structure of graphite
- planar
- hexagonal layered structure
- layers held by weak IMF
What’s the shape with 2 lone pairs and 3 bond pairs?
T-shape/ trigonal planar
Formula of hydrogencarbonate ion
HCO3-
Formula of nitrate ion
NO3-
2 single bonds + one double bond
Give 2 examples of polar molecules
- Urea
- lots of polar bonds
- Ethanol
∵ has both polar & non-polar ends\
∴ can dissolve both polar & non-polar substances
