Shapes of molecules and intermoleculr forces

What does molecular shape depend on?

Number and arrangement of electron pairs surrounding central atom

Bonding pairs = forming covalent pairs with other atoms

Lone pairs =. Not involved in bonding and remain on central atom

What is the electron pair repulsion theory?

Molecule adopts a shape that minimises repulsion between electron pairs due to negative charge

1. Electron pairs repel eachother

2. Lone pairs cause more repulsion than bonding pairs because they are closer

3. Electron pairs arrange themselves as far apart as possible to minimise repulsion

What order does repulsion decrease in?

Lone pair - lone pair > lone pair - bonding pair > bonding pair - bonding pair

Consequently , lone pairs occupy more space, affecting molecules shape when compressing bond angles

Linear shape

2 electron pairs

2 bonding pairs, 0 lone pairs

Trigonal planar

3 electron pairs

3 bonding pairs , 0 lone pairs

Tetrahedral

4 bonding pairs

109.5

Trigonal Pydramidal

4 electron pairs

3 bonding, 1 lone pairs

Bent / linear

2 bonding pairs, 2 lone pairs

Trigonal bipyramidal:

5 electron pairs

Octahedral

6 bonding pairs

Shapes of species with multiple bonds:

Treat each multiple bond as if it is a single electron pair

What is electronegativity?

Ability of atoms to attract shared electron pair in a covalent bond towards itself

Fluorine is most electronegative , then oxygen , nitrogen and chlorine

What is Pauling scale

Electronegativity measured on Pauling scale, higher value = more electronegative

What factors affect electronegativity?

Atomic radius = smaller atom are more electronegative because their electrons are closer t nucleus, stonger electrostatic attraction

Nuclear charge = higher positive charge in nucleus increases strength of electrostatic attraction between nucleus and electrons

Shielding = electrons in inner shells weaken electrostatic attraction between nucleus and outer electrons

Trends in electronegativity across a period

Increase

Nuclear charge increases, pulls electronics closer and decreases atomic radius + results in stronger electrostatic attraction between nucleus and outer shells

Trends in electronegativity down a group

Decreases

Down a group, nuclear charge increases, atomic radius increases more, number of inner shell electron shielding also increases

Increased distance between nucleus and outer electrons + greater shielding weakens electrostatic atttraction

What are polar bonds and how are they formed?

In a covalent bond between atoms with different electronegativity els, bonding electrons are more attracted to electronegative atom - the unequal sharing makes bond polar

Polar bonds have permanent dipole (seperation of positive and negative charges within polar molecule, resulting from uneven distribution of electrons)

More electronegative atom has negative charge and less electronegative atom has positive charge, bigger difference in en, more polar the bond

How is the polarity of a molecule dependant on how its polar bonds are arrange in 3D?

Charge is unevenly distributed in polar molecules

CCL4 = non- polar = symmetric arrangement causes dipoles to cancel out, even though it has polar bonds

CHCL3 = polar = asymmetric arrangemtn doesn’t allow dipoles to cancel out, lead to a net dipole

Difference in electronegativity between C-Cl and C-H causes dipoles to not fully cancel out

How does electronegativity predict bond type?

En difference is 0, like diatomic molecules - H2, O2 = pure covalent bond as electrons shared equally

En difference increases, bond is more polar covalent e,g H-Cl

En difference is very large, bond is ionic = more en atom takes bonding electrons from less en atom NaCl

What are intermolecular forces? + 3 types

Attractive forces that exits between molecules, these are weaker than covalent bonds that hold atoms together within molecules

1. Induced dipole dipole forces (london forces)

2. Permanent dipole - dipole

3. Hydrogen bonding

Summarise characteristics of the 3 intermolecular forces

What are london forces and how do they occur?

Present between all atoms and molecules

1. Electrons in atom constantly move, at one point there are more electrons on one side than other = temporary dipole

2. This temporary dipole induces an opposite dipole in a neighbouring atom, cause weak electrostatic attraction between atoms

3. Induced dipole induce further dipoles in nearby particles

4. Dipoles constantly forming and disappearing as electrons move, overall effect is net attraction between atoms

What factors affect the strength of induced dipole dipole forces?

Size - larger atoms and molecule have more electrons that can become polarised, stronger temporary dipoles

SA = larger SA has stronger induced dipole dipole forces as more electron aloud is exposed for interactions

So stronger london forces = higher BP

Can london forces be used to hold molecules together in a lattice structure?

Yes

Solid iodine = molecules held together by covalent bonds

I2 molecules attracted to eachother by london forces, form a molecular lattice

London forces also explain existence of noble gas liquids / solids, as it allows them to condense into liquid and solids at very low temps

What are permanent dipoles and how do they occur?

Electrostatic attractions between partial positive end of polar molecule and partial negative end of another

1. H-Cl bond is polar due to greater electronegativity of chlorine

2. H atom has a partial positive charge and Cl has partial negative

3. HCL molecules align so the delta positive charge on H is attracted to delta negative chlorine

Methanal, CH2O has higher boiling point than ethane C2H6 - explain using intermolecular forces

Methanal is polar due to carbonyl group C=O (has permanent dipole bond and london forces)

Ethane is non polar , only has london forces

More enegry required to overcome stronger intermolecular forces in Methanal, lead to higher BP

What is hydrogen bonding and its requirements?

Type of permanent dipole dipole force that occurs when hydrogen is bonded to highly electronegative element fluorine, oxygen or nitrogen

1. Molecule must contain a H atom bonded to F O N

2. Must be a lone pair of electrons on F O N to interact with H

How do hydrogen bonds form?

H-F, H-O, H-N bonds are highly polar due to large electronegativity difference between H and elements - leads to partial positive charge on H and partial negative on FON atom

Small size of H allows it to get close to lone pair of electron on adjacent FON

Lone pairs on FON are regions of high electron density (high partial negative charge)

Positively charged H is strongly attracted to negatively charged lone pair, form H bond between molecules

How to draw hydrogen bond?

How does hydrogen bonding impact properties of substance?

Greater solubility in water = substances that form h bonds in water (ethanol) are soluble

Higher MP / BP = extra energy needed to overcome strong H bond forces

How does hydrogen bonding explain the anomalous properties of water and ice?

Ice is less dense than water = in ice, water molecules arranged in a 3D lattice held by H bonds, when melting, these H bonds break H bonds are relatively longer than covalent bonds, so ice is less dense

Water and ice have high MP / BP = compared to other molecules of similar size, due to H bonds which require more energy to break

How do intermolecular forces explain properties of simple molecular substances?

Low MP / BP = weak intermolecular forces dont need a lot of energy to overcome, often liquid/gas at room temp

Solubility in water = polar molecules interact with water molecules, non polar molecules (only have london forces) are insoluble

Electrical conductivity = covalent compounds do not conduct electricity, regardless of polarity