4 chemical bonding Flashcards
chemical bond definition
an electrostatic force which holds two or more atoms or ions together
bond breaking and forming – exothermic or endothermic?
breaking bonds = absorbing energy = endothermic
formation = releases energy = exothermic
ionic/electrostatic bond definition
the electrostatic attraction experienced between the electric charges of a cation and an anion
dot and cross diagrams
diff symbols for the electrons of diff atoms just draw without the circle
- in order to find the charges, cross the numbers of each
eg MgCl2 – Mg2+ and Cl-
nature of ionic bonds 3
- electrostatic
- usually strong (ionic = high melting and boiling)
- non-directional (equally strong in all directions)
what does lattice energy indicate
strength of electrostatic attraction in ionic bonding
lattice energy of ionic componud definition
the enthalpy change when one mole of a solid ionic compound is seperated into gaseous ions under standard conditions
lattice energy magnitude depends on
- proportional to charges of ions
- inversely proportional to the distance
exception to lattice energy
melting point of Al2O3 is lower than that of MgO
structure of ionic compounds
giant crystal lattice structure
- with strong ionic bonds
typical properties of ionic compounds 4
- high melting and boilign points
- hard but brittle
- good conductors of electricity (molten and aqueous states)
- soluble in polar solvents, insoluble in non-polar solvents
why do ionic compounds have high melting and boiling points
large amt of thermal energy is required to overcome the strong electrostatic attractive forces between oppositely-charged ions
why are ionic compounds hard and not easily scratched
the ions are bound strongly to the lattice and are not easily displaced
why are ionic compounds brittle (easy to shatter/fracture under stress)
distortion causes ions of like charges to come close together and repel sharply
why are ionic compoudns good/bad electrical conductors in respective states
solid:
strong electrostatic attractive forces prevent the movement of charged ions – ions unable to move, cannot carry electrical charge
molten/aq:
free mobile ions present to carry the electrical charges
why are ionic compounds soluble in water and polar solvents
formation of ion-dipole attraction = release of energy = detachment of ions from crystal lattice for solvation
water is polar molecule – attacks ionic lattice and pulls it apart. ion surrounded by water molc – strongly attracted bc of electrostatic ion-dipole attraction
why are ionic compounds not soluble in non-polar solvents
non-polar sol eg hexane and benzene
- form weak forces of attraction with ions = do not pull away from the lattice = does not dissolve
definition of covalent bonding
the electrostatic attraction between a shared pair of electrons and the nuclei of the atoms being bonded
overlapping of atomic orbitals
in order to form covalent bond = atomic orbitals with unpaired electrons partially overlap
2 ways of electron sharing (covalent)
- normal covalent bonds: both atoms give 1
- coordinate (dative) covalent bonds: both electrons originiate from the same atom
what are bond pairs and lone pairs
bond pairs: shared pairs of electrons between 2 atoms
lone pairs: pairs of electrons not shared
how to draw lewis structure
LINES for each pair!!! visulaise it
octet rule definition
atoms acheive stable noble gas electronic configuration with 8 valence electrons – gain, loss, sharing of electrosn
exceptions to octet rule 2
- incomplete octet structure -
- molecules w central atoms w less than 8 e after bonding
- eg NO, NO2 (N = 5e) - expanded octet structure
- molecules w atoms w more than 8 e after bonding
- eg SF6 (usually compounds of PERIOD 3 onwards)
definition of coordinate covalent bond
the shared pair of electrons is contributed by only one of the bonded atoms
(drawn as an arrow is lewis structure)
example of dative bond
NH4+
how are pure ionic compounds formed
through the complete transder of electrons from metallic atoms to non-metallic atoms – forms cations and anions
how are pure covalent compounds formed
by EQUAL sharing of electrons between covalently bonded atoms
what do the electron clouds of an ionic bond with covalent cahracter look like
large cloud oval towards small cloud
- partial transfer of electrons
what do the electron clouds of a covalent bond with ionic character look like
one big cloud bigger on the bigger side
- unequal sharing of electrons
how do intermediate types of bonds 2
- polarisation of anion by cation (due to incomplete electron transfer)
- partial charges on covalent bond (due to diff in electroneg)
oversimplification of ionic bonding – how? 3
- electron charge assumed to be COMPLETELY transferred from one atom to another
- CHARGE of ions is assumed to be EVENLY distributed in the ions
- cations and anions are assumed to be point charges
polarity definition
the distribution of electric charge between 2 bonded atoms
polarisation definition
distortion of the anions electron cloud by a cation
polarising power definition
the ability of the cation to distort/polarise the anions electron cloud
polarisability definition
the ease with which the anion’s electron cloud can be distorted (polarised)
ionic bonds between NaCl and Al2O3
NaCl – ideal ionic bond
Al2O3 – cation is small, highly charged (high charge density) = strong attraction, distorts the cloud
- ionic bond now has covalent character bc of POLARISATION of the anion
- electron density moves to a region between the cation and anion
how to determine whats more polar
- charge density of cation + size
- compare anion size – bigger = more polarisable
how does charge and size of cation affect polarity
higher charge = higher polarising power
SMALLER size (ionic radius) = greater polarising power
how does charge and size of anion affect polarity
higher charge = greater polarisability
larger ionic radius/size = greater polarisabiliy u
ionic compound with high covalent character (very polar) will likely have (cations and anions)
- large anion (high polarisability)
- small cation (high polarising power)
either one highly charged
factors affecting bond strength
- lattice energy
- covalent character
lattice energy definition
enthalpy change when one mole of a solid ionic compound is seperated into gaseous ions under stnadard conditions
how to calculate lattice energy
charge x charge / dist + dist ???
effect of covalent character on strength of ionic bond
incr strength
eg. AgCl experimental lattice e is higher than theoretical bc theoretical considers it as purely ionic BUT its actually ionic with covalent character
properties of ionic compounds with covalent compounds 2
- lower melting point
- may be soluble in organic solvents
process of stating reasons for determining which is more covalent
state which is more covalent and which is more ionic
- size of cation
- polarising power of cation
- extent of distortion of anion electron cloud
- extent of covalent character eg BeCl2 > MgCl2
define electronegativity
the relative attraction an atoms has for the shared pair of electrons in a covalent bond
across period and down group electronegativity
across - incr
down - decr
where are the most electronegateive elements
top right hand corner
why does electronegativity incr across the period
atoms smaller = decr dist between elec and nuclei
charge incr but shielding effect constant
electrostatic attraction incr
why does electronegativity decr down a group
larger atoms = incr dist = decr electrostatic attraction
assumtions in the model of coavlent bonding
assumes that the electron pair is shared equally between the 2 atoms
polarity of identical atoms
NON POLAR – eg H2, CL2
how to explain a polar covalent bond 3
- electrons shared unequally – diff in electronegative values
- valence electron density distorts towards higher electroneg = polarisation
- polar bond – partial electrical charges due to uneven dist of e
what is a dipole
2 equal and opposite charges seperated over a distance
why is there dipole moment
the more electroneg atom with higher charge density – will have a partial negative charge
2 things needed for a covalent molecule to be polar
- at leas tone polar bond
- dipole moment must not cancel out (needs net dipole moment) (the arow with the cross)
factors affecting covalent bond strenght 3
- bond length (incr length, decr strenth)
- bond multiplicity (incr multiple bonds = incr strength)
- bond polarity (incr polarity = incr ionic character = incr strentgh)
how does incr bond length affect a covalent bond strength
longer length = nuclei further from shared pair of electrons
exception: F2 (weak bc realtively large electron + internuclear repulsions + small overlap of bondign orbitals)
how are shapes of covalent molecules predicted
valence shell electron pair repulsion theory
VSEPR
- any atom bonded to more than one other atom = central atom
- each bond/lone pair = electron domain
how to calculate number of lone pairs
(no. of e in valence shell - no. of e in bondign pairs) / 2
how do electron pairs arrange themselves (in terms of repulsion)
to MINIMISE repulsion
order of strength of repulsion
lone-lone > lone-bond > bond - bond
explain streght of repulsion
bonded pair held more tightly by TWO nuclei, lone pair held only by ONE atom = electron cloud more diffuse
steps to determine shapes of molecules and ions (VSEPR) 5
- draw dot and cross
- count no of electron pairs around the central atom (single/double/triple/dative all counted as 1 pair)
- determine basic shape (by total no of e pairs aroudn central)
- determine actual shape (exact no of lone and bond pairs)
- draw structure
0 lone pairs, 2 electron domains
linear, 180
0 lone pairs, 3 electron domains
trigonal planar, 120
1 lone pair, 3 electron domains
v shaped / bent, <120 (trigonal planar)
0 lone pair, 4 electron domains
tetrahedral, 109.5
1 lone pair, 4 electron domains
trigonal pyramidal, <109.5 (tetrahedral)
2 lone pairs, 4 electron domaisn
v shaped/ bent, «109.5 (tetrahedral)
0 lone pairs, 5 electron domains
trigonal bipyramid, 90, 120
1 lone pair, 5 electron domains
see-saw, trigonal bipyramid (lone pairs replace equatorial electron pairs)
2 lone pairs, 5 electron domains
T-shaped, trigonal bipyramid (lone pairs replace equatorial electron pairs)
what happends when there are lone pairs in a trigonal bipyramid structure
they will replace the equatorial electron pairs first to minimise repulsion
0 lone pair, 6 electron domains
octahedral, 90
1 lone pair, 6 electron domaisn
square pyramidal, octahedral
2 lone pairs, 6 electron domains
square planar, octrahedral
behaviour of lone pairs in an octahedral geomertry
1st lone pair: any position as they are all equivalent
2nd lone pair: position opposite
