chemical bonding l Flashcards
structure of ionic lattice
constituent ions are held in fixed positions in an ordely manner so that attraction is maximised and repulsion is minimised between oppositely charged ions
describe ionic bonds
electrostatic attraction btwn cations and anions
non directional as an ion attracts an oppositely charged ion in all directions with no preferred orientation
properties of ionic compounds
high melting/boiling point as strong ionic bonds need high amount of energy to break
soluble in polar solvent
insoluble in organic solvent
why are ionic compounds electrical conductors in molten/aqueous state but not in the solid state
solid state: ions held in fixed positions and cannot act as mobile charge carriers
molten state: ionic lattice structure breaks down and ions are free to move and act as mobile charge carriers
structure of diamond
each C atom is bonded to 4 other C atoms in a three dimensional lattice by forming strong covalent bonds
structure of graphite
each C atom is bonded to 3 other C atoms in a layered structure by forming strong covalent bonds
weak intermolecular attraction between layers
what is a metallic bond
electrostatic attraction between a lattice of positive ions and delocalised electrons
strong and non directional
what are the factors affecting strength of metallic bond
- number of valence electrons available: greater the number of valence electrons, the stronger the bonds
- cahrge of cations: higher the charge of cations, the stronger the bonds
- the size of cations: the smaller the cation, the higher the charge density, the greater the electrostatic attraction for the delocalised electrons and the stronger the bonds
how can metals have high electrical conductivity even in the solid state
the delocalised electrons, not the cations, function as charge carriers and flow towards positive terminal
how are metals malleable and ductile
a layer of positive ions can glide over another without breaking the metallic bond as metallic bonds are non-directional
describe covalent bonds
electrostatic attraction between the shared pair of electrons and positively charged nuclei
what and what are the conditions for a datice covalent bond
when the shared pair of electrons is provided by only one of the bonding atoms
one atom must have a lone pair of electrons for donation while the other atom must have a vacant low-lying orbital to accept the electron pair
how to identify central atom in a dot and cross
less electronegative element and has more unpaired electrons
how to identify side atoms in a dot and cross
more electronegative element and has fewer unpaired electrons
if H is present, it is always the side atom because it can only form 1 covalent bond
why can some elements have more than 8 valence electrons
elements of period 3 and above have vacant, low-lying orbitals that are available for the expansion of octet
rank of strength of repulsion
lp-lp > lp-bp > bp-bp
why are electron pair arranged in their specific ways
to minimise interelectronic repulsion
what is considered as one region of electron density
each lone pair
each lone electron
each single bond
each co-ordinate bond
each double bond
each triple bond
define electronegativity
measures the relative tendency of its atom to attract its shared electron-pair in a covalent bond
degree of polarity of a bond
the greater the difference in electronegativity between two atoms involved in a bond, the greater the bond dipole moment and the more polar is the bond
how do instantaneous dipole induced dipoles arise
electrons constantly moving in any particle
at any moment, electron density can be unsymmetrical and forms instantaneous dipole
dipole can induce another dipole in neighbouring particle and cause attraction between them
dipoles are weak and short-lived
how do the number of electrons affect strength of id-id interactions
more electrons, larger electron cloud size, electron cloud more easy to polarise, greater ease of formation of id-id, stronger id-id
how does surface area for molecular interaction affect strength of id-id interactions
more branching present in isomers, lesser surface area for intermolecular interactions, weaker id-id formed
straight chained hydrocarbons would have strongest id-id
how do permanent dipole-permanent dipole interactions aris
electrostatic attraction between partially positive end of one molecule and partially negative end of another molecule
present when there is a difference in electronegativity
what affects pd-pd for molecules with a similar number of electrons
for molecules that have a similar number of electrons, the molecule which has a polar character forms stronger pd-pd as compared to non-polar molecule that forms weaker id-id
how do hydrogen bonds arise
a hydrogen atom becomes highly partially positive as it is covalently bonded to the small and highly electronegative atom with lone electron pairs, which are either F, O or N
the highly partially positive hydrogen atom can form a strong intermolecular attraction with a lone pair of electrons on an adjacent molecule
since F, O and N are small, they allow the lone pair on the other F, O and N atom to come close to the protonic H atom
the 2 critieria for hydrogen bonding
- hydrogen atom covalently bonded to either F, O or N
- lone pair of electrons on a F, O or N atom in a neighbouring molecule bearing a partially negative charge that can attract the partially positive charge on the H atom
how to determine extensiveness of hydrogen bonding
to calculate average number of hydrogen bonds per molecule,
find number of lone pairs on F/O/N
and number of H attached on F/O/N
smaller number btwn the 2 is the average no of bonds
how are physical properties affected by extensiveness of hydrogen bonding
more extensive hydrogen bonding, higher average of hydrogen bonds per molecule, more hydrogen bonds to be broken during boiling, more energy needed to be absorbed, higher boiling point
how does strength of hydrogen bonds affect physical properties
a molecule which has intermolecular hydrogen bonding will have a higher melting/boiling point as compared to molecules that have pd-pd and id-id
how does hydrogen bonding contribute to ice’s open structure
each oxygen atom in ice is tetrahedrally bonded to 4 hydrogen atoms, two by covalent bonds and two by hydrogen bonds
water molecules in ice form rigid, open, 3 dimensional networks
how does ice’s open structure affects its density
ice’s open structure prevents its molecules from getting too close to one another, occupying a larger volume for the same mass of water, hence it is less dense than water
how can intramolecular hydrogen bonding affect boiling point
H and F/O/N within the same molecule are in close proximity, allowing formation of intramolecular H bonding
less sites available for intermolecular hydrogen bonding with other molecules, so intermolecular H bonding is less extensive
since boiling overcoming intermolecular interactions, the molecules will have a lower boiling point
how can intramolecular hydrogen bonding affect solubility
H and F/O/N within the same molecule are in close proximity, allowing formation of intramolecular H bonding
less sites available for intermolecular hydrogen bonding with other molecules, so intermolecular H bonding is less extensive
when added to water, the molecule forms less H bonds with water due to intramolecular H bonding
hence the moleculae has a lower solubility
describe sigma bond
two orbitals overlap head on
electron density is concentrated between nuclei of the 2 bonding atoms, along nuclear axis
describe pi bond
two orbitals overlap side to side
has electron cloud above and below nuclear axis but zero electron density along the axis
what does a single bond consist of
a sigma bond
what does a double bond consist of
a sigma bond and a pi bond
what does a triple bond consist of
one sigma bond and 2 pi bonds
why is a sigma bond stronger than a pi bond
head to head orbital overlap in a sigma bond has a greater degree of overlap than the side to side orbital overlap in a pi bond
what is covalent bond length
distance between te nuclei of two bonded atoms
stronger the covalent bond, the shorter the bond length
how does the number of bonds btwn atoms affect bond strength
for the same bonding atoms
number of bonds increase, number of shared electrons increase, increased electrostatic attraction between bond pairs and two nuclei, increased bond strength
how does teh effectiveness of overlap of the orbitals affect bond strength
more diffuse valence orbital used in bonding, lower electron density in overlap, overlap of orbitals less effective, weaker bond
how does the difference in electronegativities of the bonding atoms affect bond strength
the greater the difference in electronegativity, the more polar the covalent bond/greater the bond polarity, the stronger the bond
how does polarising power of cation affect degree of covalent character in ionic bond
the higher the charge and the smaller the cation, the stronger its polarising power, the more teh cation can polarise the anion, the higher the degree of covalent character
how does polarisability of anion affect degree of covalent character in ionic bond
the more electrons an anion has, the larger its electron cloud is, the easier it is to polarise its electron cloud, the poalrised the anion is, the higher the degree of covalent character
why is alcl3 covalent
- al3+ has a high charge density and hence high polarising power
Al3+ distorts the electron cloud of Cl- so much that electron sharing is predominant
difference in electronegativity between al and cl is sufficiently small for alcl3 to be considered covalent
