solids - bonding Flashcards
(24 cards)
what type of structure do metal solids adopt?
most metals have cubic/hexagonal close packed structures
how does the ionisation energy of metals link to their structures?
typically metals have low ionisation energies, able to lose valence electrons easily - the delocalised electron sea idea of metallic bonding ties into this characteristic
give one property of metals that is unexplained by the free electron model
this model doesn’t explain why conductivity of metals changes with temperature, or the existence of semi-conductors
what is band theory?
band theory extends from MO theory - it is the idea that solid metals contain millions of metal atoms whose orbitals all interact together giving many millions of MOs all with similar energies, which then all interact to form a band
- e.g. a band formed from the overlap of n metal 2s orbitals is always half filled, it contains n electrons
how does band theory explain conductivity of metals?
electrons can move freely in bands therefore can conduct charge/electricity - this is possible theoretically because of the metals partially filled band
conduction band definition
a partially filled band, this is necessary to conduct electricity
what are insulators in band theory?
compounds with filled valence bands and an empty conduction band and a large gap between the two is an insulator, as electrons cannot pass through the gap and cannot move in filled bands
what are insulators in band theory?
as the energy gap between filled valence bands and empty conduction bands becomes smaller, some electrons will have enough thermal energy to move into the conduction band + move freely, allowing the compound to partially conduct
how does band theory explain how temperature affects conduction?
as temperature increases, thermal energy of electrons increases, so more electrons have sufficient energy to move into the conduction band causing conduction to increase therefore temperature affects conduction
how does band theory explain how impurities affect conduction?
a compound which behaves as a semi-conductor when pure can have substances added to increase number of electrons coming into conduction band or to introduce a new conduction band that is lower in energy, increasing conduction
how are ionic solids held together in the ionic model?
electrostatic interactions between anions and cations - the ionic model assumes ions are hard spheres with fixed ionic radii
how can ionic radii be measured?
x-ray crystallography can accurately measure the distance between the centres of ions
this distance = cationic radii + anionic radii
(r- + r+)
why must ionic radii be measured as cationic + anionic radii?
it is difficult to determine at what point exactly the electron density of the cation stops and where the anion starts
what is the trend in relative size of anions and cations?
anions are generally larger than cations, this is because they have more electrons so repulsion is greater and effect of nuclear charge on valence electrons is smaller, increasing radius
how does ionic radii change down a group?
ionic radii increases down a group as principle QN increases as orbitals are further from the nucleus
how does charge affect cationic radii?
cations get smaller with increasing charge as ions have fewer electrons and valence electrons are at lower energy levels so effect of nuclear charge is greater and radii decreases
how does charge affect anionic radii?
anions get larger with increasing charge as ions have more electrons at higher energy levels, so the effect of nuclear charge is lesser and radii increases
how can ionic radii be used to predict structures of compounds?
as ionic structures are held together by anion/cation contacts, maximising this interaction increases the stability of the structure
- ions in a cubic environment with 8 surrounding oppositely charged neighbours are more stable than in an octahedral environment wit 6 opposite charged neighbours which are more stable than in a tetrahedral environment with 4 oppositely charged neighbours
essentially increasing coordination number = increasing stability
if cubic structure is the most stable, why does anything form anything else?
the sizes of the ions must be taken into account, a small ion cannot be in contact with 8 larger ions, and the ratio of the elements needs to be considered
what is radius ratio?
radius ratio = r+/r-
as cations are normally smaller than anions, radius ratio is usually <1
limiting radius ratio definition
the smallest value of the radius ratio for a particular coordination geometry - for a particular coordination number you can therefore calculate the smallest possible value of the ratio for which cations and anions can still be in contact
why does the radius ratio rule sometimes fail?
ions aren’t hard spheres and do not have fixed radii, this is an assumption
also structures with significant covalent interaction tend to behave differently
how does electronegativity affect bond character?
for a binary compound, the greater the electronegativity difference between elements the greater the ionic character of the bond
what is the purpose of the van arkel-kekelaar triangle?
it summarises the 3 types of bonds - all bonds can be places somewhere on the triangle between ionic, metallic and covalent
