2.2 covalent model part 2 Flashcards
what does bond polarity arise from
difference in electronegativity of the bonded atoms
what does molecular polarity depend on
bond polarity and molecular geometry
electronegativity
relative measure of how an atom attracts a pair of electrons in a covalent bond
what happens to pair of electron if electronegativity of both atoms is the same
the electron pair is equally attracted to their positive nucleii, bond will be symetrical, even distribution of negative charge
what happens to pair of electrons if electronegativity of atoms is not equal
pair of electrons is more strongly attracted to more electronegative atom, bond is polar, meaning there is an unequal distribution of charge between two atoms, there is a partial negative charge on more elctronegative atom, and partial positive to the less electronegative atom
the degree of polarisation depends on
the difference in electronegativity between the two atoms in the covalent bond
describe trend in electronegativity
increases up periodic table and increases from left to right
explain trend in distance of elements and polarity
next to each other probably not polar, seperation of 1 element then bond is polar, seperation of 2 or 3 elements then bond will be strong polar, seperation of 4 likely not covalent bond
differnece in electronegativity greater than 0 but less than 1.8
polar bond
difference in electronegativity bigger than 1.8
ionic bond
why is it that even if molecule has polar bonds the molecule as a whole might not be polar
for molecule to be polar its shape must be such that the centres of positive and negative cannot by symmetrical arranged, if charges are symmetrical over whole molecule they can cancel each other out, resulting in the molecule itself being non-polar.
a molecule with distinct aysemetrical regions of positive and negative charge are said to have
a dipole
how is a molecular dipole shown
using an arrow poiting towards the region of negative charge
intramolecular force
forces resulting from strong bonds within the molecule or structure, both giant and simple covalent molecular have these
to break these bonds often recquire high energy input
intermolecular forces
the force of attraction between neighbouring molecules or atoms are weak
what are the 3 main typs of intermolecular forces of attraction
london dispersion forces
dipole dipole forces
hydrogen bonding
from wekest->strongest
london dispersion forces
present in all atoms or molecules that have electrons
the movement of electrons in the orbitals of atoms or molecules gives rise to temporary dipole, when this is formed it can induce the formation of a dipole in neighbouring atoms or molecules electrostatic attraction between the temporary and induced dipoles results in a weak attractive force to arise= london force
van derwaals forc
combnation of both london and dipole dipole forces
steps for formation of london forces
- in an atom or molecule movement of electrons results in the formation of a spontaneous temporary dipole
2.close proximity to another moelcule induces a dipole to form in the other molecule the elctron cloud becomes distorted and the molecule is polarized
3.temporary and induced dipoles attract each other forming a weak temporaray london force
what are the factors which influence formation of london forces
happens more frequently in lower temperatures as atoms or molecules move more slowly increasing frequency of force formation
what happens in non polar molecules in terms of intermolecular forces
in non polar molecules london forces may be the only attractive force between molecules. Since the force is the weakest of the intermoleculcular forces, non polar molecules are often gases or liquids and slids with relitevly low melting points
what 3 factors affect the strength of london forces
1.increasing number of electrons
2.increasing the volume of the electron cloud
3.increasing the contact area between molecules
explain why increasing number of electrons icnreases strnegth of london forces
the greater the molar mass the bigger the number of electrons and the weaker will be the attraction for the electrons for the nucleus and the ease with which the electron cloud can be polarized (induced dipole)
explain why increasing volume of the electron cloud increases strength of london forces
the greater the volume of the electron cloud the weaker will be the attraction of an electron for the nucleus and the ease with which the electron cloud can be polarised (induced dipole)
explain why increasing contact area between molecules icnreases the strength of london forces
london forces is only effective over short range since the molecules or atoms need to be in close enough proximity for the temporary dipole to induce a dipole in neighbouring molecules or atoms. The means that the greater the contact area of the molecules that can come close together the greater the london force
why do noble gases have really low melting points
weak london dispersion forces the only have london forces only need weak eneergy to break bon, no intramolecular polar bond
explain the trend in melting point down the group of nobel gases
bigger the number of electrons bigger molar mass down the group, easier distortion to produce induced dipoles hence strength of london forces increases, increasing melting point
explain why the melting points of halogens is higher than the noble gas in same period
they are diatomic, meaning double the molar mass, increased strength london force, increased surface area between separate molecules more electrons easier to distort
explain trend in melting point down the halogen group
increases in m.p down the group due to increase in mass and number of electron and volume of electron cloud, hence strength of london forces increases
explain the dipole dipole force
must be polar molecule
this occurs because of the electrostatic attraction between moecules with a permanent dipole
because this is an attraction between permenant dipoles the force is much stronger than london forces so the m.p of such susbtances with a mass similar to molecules with only london forces is higher
all molecules which have dipole- dpiole forces also have london forces
explain hydrogen Bonding
this occurs in molecules than contai hydrogen covalently bonded to nitrogen, oxygen or fluorine. All these three elements have non-bonding lone pairs of electrons that can interact with a hydrogen that has a permenant dipole, partial positive chagre. In addition all 3 elements are small and so are able to interact closely with the hydrogen atom.
what is meant by sigma bond
overlap of atomic orbitals along the internuclear axis
atomic orbitals overlap head to head
what is a pi orbital
when atomic orbitals overlap sideways p-p
