2.2- covalent model Flashcards
covalent bonding
electrostatic attraction between a shared pair of electrons and positively charged nuclei
whats the definition of the covalent bond iself
electrostatic attraction of the positive nucleii for the region of negatively charged jigh electron density
whats the region of high electron density
space between the atoms where a pair of electrons are shared
atoms forming a covalent bond will achieve a lower energy state by…
sharing electrons and have a full valence shell. This can be doen by either forming single double or triple bond
octet rule
filling of the valence shell to achieve 8 electrons, this rule can in certain circumstances be broken
electronegativity
relative measure of attraction of an atom for a pair of electrons (in a covalent bond)
effective nucleus charge determines
the net positive charge pulling these electrons towards the nucleus
electronegativity and ionization energy
whats the hybrid diagram
the one with the lines
lewis diagram
one with the dots
2 electron domains
= linear, andgle= 180
eg. co2
3 electron domains
= triognal planar
angle= 120
eg. c2h4 or bf3
3 electron domains
1 lone pair
angle= 104.5
because stronger repulsion from lone pairs, forcing bonding pairs closer together
=bent linear
eg.so2
4 electron domains
=tetrahedral
angle= 109.5
eg. ch4
4 electron domains
1 lone pair
=trigonal pyramidal
angle= 107 due to extra repulsion from lone pair
eg. Nh3
4 electron domains
2 lone pairs
=bent
angle= 104.5 even greater repulsion
eg. Oh2
repuslion diagram explain
biggest repulsion is between two pairs oflone pair
intermedaite= between lone pai and bonding pair
least between 2 bonding pairs
domain geometry
= total number of domains, negatively charged areas with electrons ncluding line pairs
molecular geometry
only bonding pair areas
electronegativity
ability of atom to draw pair of electrons towards itself in a covalent bond
higher value in pauleen scale means
more electronegativ element it is in diatomic molecules electron density shared equally between 2 atoms eg. h2
AS BOTH ATOMS HAVE SAME ELECTRONEGATIVITY VALUE= EQUAL ATTRACTION FOR BODNING PAIR OF ELECTRONS LEADING TO FORMATION OF A COVALENT BOND which is non polar
polar bond
when 2 atoms in a covalent bond have different electronegstivity, covalnt bond= polar, electrons are drawn to more electronegative atom, electron distributions is aysemmetric
the less electronegative atom si partially positive charge and more is partially negative harge
bigger the differnece in electronegativity between 2 atoms higher polarity of covalent bond
dipole moment
measure of how polar a bond is , arrow points to partilly negative charge
intramoleculer vc inter
intra within molecules
inter between seperate moelcules
what are the 3 main intermoleculear forces
london (dispersion) force
dipole dipole attraction
dipole induced dipole
hydrogen bonding
london dispersion force
temporary dipole lasts very shortly as electron always moving
greater number of electrons= greater likehood of forming this tempoerary dipole
greater surface area= more contact adjacent moleculer, greater ability to induce a dipole in adjacent molecule, more london forces
dipole dipole attraction
temporary dipole exist in all molecules but also have permenant dipo;e
molecules with permanent dipole are also attrcted to each other by permenant dipole dipole bonding which slightly increases the strength of intermolecules attration= slightly higher boiling point than molecules with only london forces
only happens in polar molecules
dipole induced dipole attraction
some mixtures have both polar and non polar molecules
permenant dipole of a polar molecule can cause a temporary seperation of charge on non polar molecuke
force acts in addition to london and dipole dipole force
hydrogen bonding
strongest intermlecular force
special type of permenant dipole dipole bonding
to take place species has to have an Oxygen and a nitrogen or fluroine (very electronegative) atom with an available lone pair of electrons and hydrogen attached to it
bond is very polarised
hydrogen becomes so positively charged it can form bond with lond pair of and O, N, F atom in another molecule
Van der waals force
include; london force, dipole induced dipole, dipole dipole,
forces occur between molecules (intermolecularly and within molecule (intramolecularly)
5 domains
electron domain= trigonal bipyramidal
moelcular geometry=trigonal bipyramidal
angles=120,90
5 domains
1 lone pair
electron domain= trigonal bipyramidal
moelcular geometry=see saw
angles= <120, <90
5 domains
2 lone pairs
electron domain= trigonal bipyramidal
moelcular geometry=T shaped
angles= <90
5 domains
3 lone pairs
electron domain= trigonal bipyramidal
moelcular geometry=linear
angle=180
6 electron domains
electron domain= octahedral
moelcular geometry=octahedral
angles= 90
6 electron domains
1 lone pair
electron domain= octahedral
moelcular geometry=square based pyramid
angle= <90
6 electron domains 2 lone pairs
electron domain= octahedral
moelcular geometry=square planar
angle= 90
formal charge
number of valence electrons- 1/2(number of bonding electrons) - number of non bonding electrons
preferred lewis diagram
one in which differnece in formal charge of atoms is closest to 0
what are the two main types of molecular orbtials
sigma and pi
SP3 hybridisation
1 2s
3 2p
SP2 hybridisation
1 2s
2 2p
SP hybridisation
1 2s
1 2p
covalent bond
electrostatic attraction between a shared pair of electrons and positively charged nuclei or electrostatic attraction of the positive nuclei to the region of negatively charged high electron density
occurs between atoms that both have a medium to high electronegativity
what is the cotet rule
the tendency of atoms to gain a valence shell with a total of 8 electrons
what is a dative bond
a covalent bond in which both electrons of the shared pair originate from the same atom
what keeps atoms together in a covalent bond
the positive nucleis electrostatic attraction to the region of negative charged high electron density
atoms forming covalent bonds will acheiv a lower energy state because
they share electrons and have a full valence shell, this can be done by forming sinle double or even triple bonds
what is the filling of the valence shell to achieve 8 electrons called
octet rule
hybrid diagram
lines
lewis diagram
dots and crosse
electronegativity
relative measure of attraction of an atom for a pair of electrons (in a covalent bond)
what does effective nuclear charg determines
electronegativity and ionisation energy
hydroxium ion
H3O +
small portion of water molecules are dissasociated into protons H+ and hydroxide ions OH-. The protons actually exist as hydroxonium ions, the proton forming a dative bond with an undissociated water molecule
why does a quadruple bond not exist
because the huge strain involved in making the fourth bond prohibits it
describe trends seen by number of bonds
as number of bonds increases, there is an increase in numbers of negative electron pairs which draws them closer to the positive nucleii, decreasing bond length
bigger bond energy as increase in number of bonds,
what is bond energy
the measure of the strnegth of the electrostatic force of attracton between the carbon atoms and refers to the energy recquires to break the bond and seperate atoms
what are weaker and why, sigma or pi bonds
pi bonds are weaker bonds than sigma bonds because the electrostatic attraction is off the axis
as pi bonds are weaker…
they are more reactive enabling the compound to undergo a wide range of chemical reactions by addition
ethane
single carbon bonds
only a sigma bond
ethene
double carbon bonds
one sigma bond
and one pi bond abve and below sigma
ethyne
triple carbon bonds
one sigma
two pi bonds, above and below, infront and behind
what happens due to hybridisation
makes bond shorter, more stable and hence more difficult to break
what is the shape of covalent molecules and ions determined by
repulsion between electron pairs and numbers of regions of high electron density surrounding an atom termed electron domains
4 electron domains and 2 bonding pairs
domain geometry= tetrahedral
molecular geometry= diatomic
angle= 180
j
6 bonding electrons and
3 lone pairs
electron domain= octahedral
molecular geometry= T-shape
angle=<90
6 bonding electrons
4 lone pairs
electron domain= octahedral
molecular geometry= linear
angle=180
formal charge
can be used to decide which lewis structure is preferred from several, the formal charge is the charge an atom would have if all atoms in the molecule had the same electronegativity
formal charge formula
(number of valence electrons)- 1/2(number of bonding electrons)- (number of non-bonding electrons)
in order for an atom to form covalent bonds from electrons in different sub- orbitals two steps need to occur
electrons in their low energy sub-orbitals (ground state) pass through a temporary phase (excited state) where vacant p sub-orbitas become singly occupied by promotion of an s electron (a similar process occur during expansion of an energy level- expansion of the octet)
- single electron in the different sub- orbitals become equivalent in a process called hybridisation forming new hybrid orbital
SP3
electron domain geometry
tetrahedral domain geometry
SP2
domain geometry
trigonal planar domain geometry
SP
linear domain geometry
delocolization
is a phenomenon that occurs in molecules with 2 or more double bonds that are in close proximity to each other. A double bond involves the hybridization of electrons to form sigma and pi bonds. A pi bond results from the overlap p orbitals.
electron in pi bonds become delocolised accross whole structure
if a molecule contains 2 or more pi orbitals
they can overlap to form a delocalised pi bond
resonance energy
due to the lower energy state of the actual structure the expected enthalpy change is higher than the actual enthalpy energy recquired
