PHYSICAL CHEM - BONDING Flashcards
What is ionic bonding?
transfer of electrons from a metal to a non-metal
What is the structure of an ionic compound?
giant ionic lattice
What forces are present in ionic compounds?
strong electrostatic forces of attraction between oppositely charged ions
What are the properties of ionic compounds?
- high melting and boiling points
- conduct when molten/in solution (but don’t conduct when solid)
- soluble
Why do ionic compounds have high melting and boiling points?
strong electrostatic forces between oppositely charged ions require lots of energy to overcome
Why do ionic compounds conduct when molten/in solution but not when solid?
- when solid, ions are not free to move
- when molten/in solution, ions are free to move and can carry a charge through the structure
Why are ionic compounds soluble?
- water molecules are polar
- charged parts of water molecules pull ions away from the lattice causing the ionic compound to dissolve
What is covalent bonding?
sharing of electrons between 2 non-metals
What is the structure of a molecular compound?
a simple molecular structure
What forces are present in molecular compounds?
- weak intermolecular forces between molecules (permanent dipole - dipole forces, van der waals forces, hydrogen bonding)
- covalent bonds between atoms in a molecule (eg: I2)
What are the properties of a molecular compound?
- low melting and boiling points
- don’t conduct
Why do molecular compounds have low melting and boiling points?
weak intermolecular forces between molecules
require little energy to overcome
Why do molecular compounds not conduct?
no electrons free to move and carry a charge through the structure
What is diamond’s structure?
- giant covalent structure
- each carbon atom bonded to 4 others
What is graphite’s structure?
- giant covalent structure
- layers (graphene)
- each carbon atom bonded to 3 others
What type of bonding occurs in diamond and graphite?
strong covalent bonds
What are diamond’s properties?
- high melting and boiling points
- strong/hard
- doesn’t conduct
Why does diamond have high melting and boiling points?
strong covalent bonds require lots of energy to overcome
What makes diamond so strong/hard?
each carbon atom is bonded to 4 others with strong covalent bonds
Why does diamond not conduct electricity?
no delocalised electrons
What are graphite’s properties?
- high melting and boiling points
- soft and slippery
- conducts electricity
Why does graphite have high melting and boiling points?
strong covalent bonds require lots of energy to overcome
What makes graphite soft and slippery?
layers can slide over each other as they are held together by weak intermolecular forces
Why does graphite conduct electricity?
delocalised electrons are free to move and can carry a charge through the structure
What is metallic bonding?
bonding between metal atoms
What is a the structure of a metal?
- +ve metal ions
- sea of delocalised electrons
What type of bonding occurs in metals?
metallic bonding
What are the properties of metals?
- high melting and boiling points
- conduct electricity
- malleable
Why do metals have high melting and boiling points?
- strong metallic bonds
- strong electrostatic forces of attraction between +ve metal ions and delocalised electrons
Why do metals conduct electricity?
delocalised electrons are free to move and can carry a charge through the structure
Why are metals malleable?
layers can slide over each other
What is electronegativity?
the relative ability of an atom to attract electrons in a covalent bond
What are the factors affecting electronegativity?
- nuclear charge
- atomic radius
- shielding
Describe the trend in electronegativity down a group
decreases
Explain the trend in electronegativity down a group
electronegativity decreases as:
- nuclear charge increases
- shielding increases
- atomic radius increases
Describe the trend in electronegativity across a period
increases
Explain the trend in electronegativity across a period
electronegativity increases as:
- nuclear charge increases
- shielding remains constant
- atomic radius decreases
What is the most electronegative element?
fluorine
What is the name of the numerical scale of electronegativities?
Pauling’s scale
How can you determine the type of bonding in a molecule from the electronegativity of the elements?
- if both atoms have a similar electronegativity, they both attract electrons with the same strength therefore the bond is covalent
- if there is a large difference in electronegativities then the bond is ionic
What type of bond is present when the difference in electronegativities of the atoms is less than 0.5?
covalent bond
What type of bond is present when the difference in electronegativities of the atoms is between 0.5 and 1.7?
polar covalent bond
What type of bond is present when the difference in electronegativities of the atoms is greater than 1.7?
ionic bond
What does the shape of a molecule depend on?
electron pairs around the central atom (bonding and lone pairs)
Why do the electron pairs around the central atom affect the shape of the molecule
electrons are all -vely charged so they repel (lone pair charge clouds repel more than bonding pair charge clouds)
Order these from smallest to biggest:
- bonding pair - bonding pair angles
- lone pair - lone pair angles
- lone pair - bonding pair angles
- bonding pair - bonding pair angles
- lone pair - bonding pair angles
- lone pair - lone pair angles
How can the no. of electron pairs in a molecule be used to determine the shape of the molecule?
- work out what the central atom is
- work out no. of electrons in outer shell of central atom
- +1 for every atom that the central atom is bonded to (if its an ion after this step +1 for each -ve charge/-1 for each +ve charge)
- ÷ by 2 to find no. of electron pairs around the central atom
- compare no. of electron pairs to the no. of bonds to find no. of lone pairs and no. of bonding pairs
- deduce shape
What do wedges represent?
bonds going out of the page
What do broken lines represent?
bonds going into the page
What is the shape and bond angle of a molecule with no lone pairs and 2 bonding pairs?
- linear
- 180°
What is the shape and bond angle of a molecule with no lone pairs and 3 bonding pairs?
- trigonal planar
- 120°
What is the shape and bond angle of a molecule with 1 lone pair and 2 bonding pairs?
- bent/non-linear
- 117.5°
What is the shape and bond angle of a molecule with no lone pairs and 4 bonding pairs?
- tetrahedral
- 109.5°
What is the shape and bond angle of a molecule with 1 lone pair and 3 bonding pairs?
- trigonal pyramidal
- 107°
What is the shape and bond angle of a molecule with 2 lone pairs and 2 bonding pairs?
- bent/non-linear
- 104.5°
What is the shape and bond angle of a molecule with no lone pairs and 5 bonding pairs?
- trigonal bipyramidal
- 90° and 120°
What is the shape and bond angle of a molecule with 1 lone pair and 4 bonding pairs?
- seesaw
- 87° and 102°
What is the shape and bond angle of a molecule with 2 lone pairs and 3 bonding pairs?
- T-shaped
- 88°
What is the shape and bond angle of a molecule with no lone pairs and 6 bonding pairs?
- octahedral
- 90° and 180°
What is the shape and bond angle of a molecule with 2 lone pairs and 4 bonding pairs?
- square planar
- 90°
Why are some covalent bonds polar?
difference in electronegativities so bonding electrons more strongly attracted to more electronegative atom
Why are covalent bonds between atoms of the same element always non-polar?
no difference in electronegativity so electrons are equally attracted to both atoms
What are the 3 types of intermolecular force?
- permanent dipole - dipole forces
- van der waals forces
- hydrogen bonding
Order the 3 types of intermolecular force from strongest to weakest
- H bonds
- permanent dipole - dipole forces
- van der waals forces
What is a dipole?
difference in charge between 2 atoms
What does δ+ mean?
slightly +ve charge
What does δ- mean?
slightly -ve charge
What causes molecules to be polar?
if a molecule has polar bonds it may result in an uneven distribution of charge throughout the molecule causing it to be polar
Why do the charges cancel out sometimes in molecules with polar bonds?
if the bonds are arranged symmetrically then the charges cancel out
What occurs between molecules with permanent dipoles?
weak electrostatic forces of attraction between δ+ and δ- charges on neighbouring molecules
How do temporary dipoles occur?
- electrons are constantly moving
- uneven distribution of electrons causes δ+ and δ- charges in the molecule causing a temporary dipole
How do van der waals forces occur between molecules?
- temporary dipole in 1 molecule can induce another temporary dipole in the opposite direction on a neighbouring atom
- the 2 dipoles are then attracted to each other
Why are temporary dipoles constantly being created and destroyed?
because electrons are constantly moving
How does the size of a molecule affect the strength of van der waals forces?
the larger the molecule the stronger the van der waals forces because larger molecules have larger electron clouds
How does the strength of van der waals forces affect melting and boiling points?
the stronger the van der waals forces, the higher the melting and boiling points as more energy is required to overcome the forces
What elements does hydrogen have to be covalently bonded to in order to be able to form a H bond and why?
- fluorine
- oxygen
- nitrogen
- F, O and N are all very electronegative so they draw bonding electrons further away from H atom causing the H atom to be δ+
- H atoms form weak bonds with lone pairs on the F/O/N atoms of other molecules
Why do substances with H bonds have higher melting and boiling points than other molecular substances?
H bonds are the strongest type of IMF
What are the 2 anomalous properties of ice?
- relatively high meting and boiling points due to H bonding
- less dense than water as H2O molecules are further apart in the lattice than in liquid water
