A-Level Chemistry - Topic 2 - Bonding and Structure

What is an ionic bond?

the strong electrostatic attraction between two oppositely charged ions

What are the two things affecting the strength of an ionic bond?

ionic charged

ionic radii

How does the melting point of ions with a charge of 1 compared to ions with a charge of 2?

ions with a charge of 2 have a higher melting point

What is charge density?

the ratio of the charge of an ion to its volume

How does ionic radii affect the melting point of an ionic compound?

smaller ions = pack closer together than larger ions

smaller + closely packed ions have stronger ionic bonding

What does the size of an ion depend on?

its electron shells

and atomic number

How does the ionic radius change as you go down a group?

it increases

Why does the ionic radius increase as you go down a group?

the ionic radius increases as the atomic number increases

(due to the extra added electron shells)

What are isoelectronic ions?

ions of different atoms with the same number of electrons

How does the ionic radius of a set of isoelectronic ions change as the atomic number increases?

the ionic radius of a set of isoelectronic ions decreases

What are ionic crystals?

giant lattices of ions

Why are ionic lattices called "giant"?

because it's made up of the same basic unit repeated over and over again

What is the shape of sodium chloride's lattice?

cube shaped

What does the theory of ionic bonding fit?

the evidence from physical properties

What are the main physical properties of ionic compounds?

high melting points

soluble in water (but not in non-polar solvents)

don't conduct electricity when solid but do when molton/dissolved/aqueous

can't be shaped due to repulsion = shatter

What are ionic compounds not soluble in?

non-polar solvents

What happens when you electrolyse a green solution of copper(II) chromate(VI)?

filter paper turns blue at the cathode

turns yellow at the anode

What colour are copper(II) ions in solution?

blue

What colour are chromate(VI) ions in solition?

yellow

Why is Copper(II) chromate(VI) green?

contains both copper(II) ions - blue - and chromate(VI) ions - yellow

What else is present (in terms of attraction/repulsion) when covalent bonding occurs?

positive nuclei attracted to the area of electron density between the 2 nuclei

but also a repulsion (2 positively charged nuclei repel each other, same with electrons)

to maintain a covalent bond = have to be a balance between these forces

What is the "bond length"?

the distance between 2 nuclei is the distance where the attractive and repulsive forces balance each other

= the bond length

How does the bond enthalpy change in terms of electron density?

higher the electron density between the nuclei (the more electrons in the bond)

= the stronger the attraction between the atoms

= the higher the bond enthalpy

= the shorter the length of the bond

What is bond enthalpy?

energy required to break a bond

How does a C=C bond compare to a C-C bond?

C=C = greater bond enthalpy and shorter than a C-C bond

What is a dative covalent bond also called?

a coordinate bond

What is a dative covalent bond?

where an atom donates both electrons to a bond

How is NH4+ formed?

by a dative covalent bond

it forms when the nitrogen atom in an ammonia molecule donates a pair of electrons to a proton (H+)

How is dative covalent bonding represented?

as an arrow pointing AWAY from the donor atom

What is an example of a stable covalent compound where the central atom doesn't have a full outer shell?

AlCl3

since Al only has 6 electrons in its outer shell

What do 2 AlCl3 compounds come together to form?

Al2Cl6

one Cl in each of the two AlCl3 molecules donate a lone pair to the Al on the other molecule

= form 2 dative covalent bonds, allowing Al to have a full outer shell

What does the shape of a molecule depend on?

the number of pairs of electrons in the outer shell of the central atom

What will electron pairs do to each other?

repel each other as much as they can

What also affects how much electron pair repel each other?

the type of electron pair

Which repels each other more: lone pairs or bonding pairs?

lone pars repel more than bonding pairs

Where are the greatest angles in molecules?

between lone pairs of electrons

= bond angles between bonding pairs are reduced

Why are bonding pairs reduced?

since they are pushed together by lone pair repulsion

What is the order of repulsion in terms of electrons?

LP/LP (biggest)

LP/BP

BP/BP (smallest)

What is electron repulsion theory?

electron pairs repel each = they
position themselves as far apart as possible

all bonding electron pairs repel each other
equally

lone pairs offer more repulsion than bonded
pairs

What is the bond angle for methane?

no lone pairs

all bond angles are 109.5

What is the bond angle for ammonia?

1 lone pair of electrons

all 3 bond angles are 107

What is the bond angle for water?

2 lone pairs = reduce the bond angle even more

bond angle is 104.5

How are 3D molecular diagrams drawn?

solid wedges = pointing out of the page

broken lines = bonds pointing into the page

How can you predict the shape of a molecule?

1. find the central atom

2. work out the number of electrons in the outer shell of it

3. work out how many electron are shared with the central atom

4. add up the electrons and divide by 2 (to find the number of electron pairs on the central atom)

5. compare the number of electron pairs with the number of bonds to find the number of lone pairs

6. use the number of electron pairs + lone pairs + bonding centres around the central atom to work out the shape of the molecule

What is a "bonding centre"?

the atoms bonded to the central atom

What is the bonding angle and name for 2 electron pairs around a central atom?

180

linear molecules

What is the bonding angle and name for 3 electron pairs around a central atom with no lone pairs?

120

trigonal planar

What is the bonding angle and name for 4 electron pairs around a central atom with no lone pairs?

109.5

tetrahedral

What is the bonding angle and name for 4 electron pairs around a central atom with 1 lone pair?

107

trigonal pyramidal

What is the bonding angle and name for 4 electron pairs around a central atom with 2 lone pairs?

104.5

nonlinear/"bent"

What is the bonding angle and name for 5 electron pairs around a central atom with no lone pairs?

120 and 90

trigonal bipyramidal

What is the bonding angle and name for 6 electron pairs around a central atom with 0 lone pairs?

90

octahedral

What is the bonding angle and name for 6 electron pairs around a central atom with 2 lone pairs?

90

square planar

What is the shape of diamond (and silicon, too)?

each carbon atom bonded to 4 others

in a tetrahedral arrangement

What is the shape of Silicon(IV) dioxide?

similar but different lattice arrangement to diamond

What is a tetrahedron?

What do the properties of giant structures provide evidence for?

covalent bonding

What are the properties of giant covalent structures?

high melting points

very hard

good thermal conductors

insoluble

cant conduct electricity

Why are giant covalent structures good thermal conductors?

since vibrations travel easily through the stiff lattices

Why are giant covalent structures insoluble?

covalent bonds = atoms are more attracted to their neighbours in the lattice than to solvent molecules

since they're insoluble in polar solvents = displays they don't contain ions

What is an example of a polar solvent?

water

How many other carbon atoms is graphite bonded to?

3 other = delocalised electrons

Why do metals have high melting + boiling points?

because of the strong metallic bonding

(number of delocalised electrons, the charge, the size of the metal ion and the lattice structure affect the m.p)

Why are metals malleable + ductile?

since there's no bonds holding specific ions together

+ the layers of positive metal ions are separated by layers of electrons

= the layers of metal ions can slide over each other without disrupting the attraction between the positive ions and electrons

Why are metals good thermal conductors?

since they can pass kinetic energy to each other

What could impurities in metals do and why?

dramatically reduce electrical conductivity

by reducing the number of electrons that are free to move and carry charge

the electrons transfer to the impurities = forming anions

Why are metals insoluble and what is the exception to this?

because of the strength of the metallic bonds

EXCEPTION: liquid metals

What is electronegativity?

the ability of an atom to attract the bonding electrons in a covalent bond

How is electronegativity measured?

pauling scale (0-4)

What is the most electronegative element?

flourine (4.0)

What are other examples of very electronegative elements?

oxygen

chlorine

nitrogen

What are the values of the least electronegative elements?

around 0.7

What are the factors that contribute to having such a high electronegativity?

higher nuclear charges

smaller atomic radii

How does electronegativity change across periods and down groups?

across periods: increasing electronegativity

down groups: decreasing electronegativity

Where do the bonding electrons sit within covalent bonding?

in orbitals between the two nuclei

What happens if both atoms have similar/identical electronegativities?

the bonding electrons with sit roughly sit midway between the 3 nuclei

= the bond will be non-polar

What happens when the bonding atoms have very similar electronegativities?

the bonds between them are essentially non-polar

What happens to the bonding electrons when there's a relatively strong electronegativity?

the bonding electrons will be pulled more towards the electronegative atom

= causes the electrons to be spread unevenly

= there will be a charge across the bond

= the bond is said to be polar

EACH ATOM HAS A PARTIAL CHARGE (ONE ATOM IS SLIGHTLY POSITIVE AND THE OTHER IS SLIGHTLY NEGATIVE)

Within a polar bond, what does the difference in electronegativity cause?

a dipole

What is a dipole?

a difference in charge between the 2 atoms caused by a shift in electron density in the bond

What does "𝛿" mean?

slightly

What are the 2 extremes when talking about bonding?

purely covalent (electronegativity = 0)

= bonding electrons are arranged completely evenly within the bond

completely ionic

Where do most compounds lie?

in the middle of the 2 bonding extremes

= often got ionic AND covalent properties

What does a higher difference in electronegativity mean?

the more ionic in character the bonding becomes

When are bonds polar?

if the difference in electronegativity is > 0.4

What does whether a molecule is polar or not depend on?

its shape

+ the polarity of its bonds

What does a polar molecule have?

an overall dipole

What is an overall dipole within a polar molecule?

a dipole caused by the presence of a permanent charge across the molecule

How can you tell if a molecule with several polar bonds is a polar molecule?

if the polar bonds are arranged so that they point in opposite directions

= they're point each other out

= non-polar overall

if the polar bonds all point in roughly the same direction

= then the molecule will be polar

What are intermolecular forces?

forces between molecules

much weaker than covalent/ionic/metallic bonds

What are the three types of intermolecular bonds?

london forces (instantaneous dipole-induced dipole bonds)

permanent dipole-permanent dipole bonds

hydrogen bonding

What is the strongest type of intermolecular bond?

hydrogen bonding

What are london forces also called?

instantaneous dipole-induced dipole bonds

What do london forces cause?

all atoms and molecules to be attracted to each other

Why do london forces occur?

1. electrons in charge clouds are always moving really quickly so at any moment the electrons in an atom are likely to be more to one side than the other

= temporary/instantaneous dipole occurs

2. this dipole can induce another temporary dipole in the opposite direction = these 2 dipoles are attracted to each other

= DOMINO EFFECT

3. since electrons are constantly moving = dipoles are being created + destroyed all the time

= despite the dipoles keep changing, the overall effect = atoms attracted to each other

What can london forces hold molecules together in?

a lattice

e.g. Iodine = I2 molecules held via covalent bonds = molecules then held together in a molecular lattice by weak london forces

= called the simple molecular structure

Why aren't all london forces the same strength?

larger molecules = larger electron clouds = stronger london forces

What does a molecule with a greater surface area have and why?

stronger london forces

= since they have a bigger EXPOSED electron cloud

What do you need to overcome when boiling something?

intermolecular forces

= so that the particles can escape from the liquid surface

What do stronger london forces mean?

a higher b.p.