Shapes of Molecules

what are shapes of molecules determined by

number of electron pairs around the central atom

why does the largest bond angle possible exist between covalent bonds

each electron pair naturally repels

how does the presence of a lone pair reduce the angle between covalent bonds

• provide additional repulsive forces

• reduces bond angle by 2.5 degrees per lone pair

name all shapes of molecules

linear

bent

trigonal planar

triangular pyramid

tetrahedral

trigonal bipyramid

octahedral

number of bonding pairs and lone pairs in a linear shape

linear: 2 bonding pairs, 0 lone pairs

bond angle of a linear molecule

180

how many bonding and lone pairs are there in bent molecules

2 bonding pairs

2 lone pairs

bond angle of a bent/v-shaped molecule

104.5

(tetrahedral with 2 lone pairs)

number of bonding and lone pairs in a trigonal planar molecule

3 bonding pairs

0 lone pairs

bond angle of a trigonal planar

120

number of bonding and lone pairs in a triangular pyramid

3 bonding pairs

1 lone pair

bond angle of a triangular pyramid

107

number of bonding and lone pairs in a tetrahedral

4 bonding pairs

0 lone pairs

bond angle in a tetrahedral

109.5

number of bonding pairs and lone pairs in a trigonal bipyramid

5 bonding pairs

0 lone pairs

bond angle in a trigonal bipyramid

90 and 120

number of bonding and lone pairs in an octahedral

6 bonding pairs

0 lone pairs

bond angle of an octahedral

90

what is bond polarity

negative charge around a covalent bond is not evenly spread around the orbitals of the bonded atoms

define electronegativity

the power of an atom to attract a pair of electrons towards itself within a covalent bond

what does electronegativity depend on

size

nuclear charge

trend of electronegativity along a period

increases along a period as atomic radius decreases and atomic charge

trend of electronegativity down a group

decreases down a group as shielding and atomic radius increases

define a permanent dipole

• two atoms with different electronegativities bond

• more electronegative draws more of the negative charge towards itself and away from other atom

• delta negative and delta positive region created

when does a polar bond form

if two atoms with different electronegativity bond

what can polar molecules with a permanent dipole align to form

a lattice of molecules, similar to an ionic lattice

when do induced dipoles form

when the electron orbitals around a molecule are influenced by another charged particle

3 types of intermolecular forces

van der waals

permanent dipole

hydrogen bonding

describe van der waals forces

weakest type

acts as an induced dipole

what does the strength of van der waals forces depend on

Mr of the molecule/number of electrons

Shape

how does number of electrons impact strength of van der waals

more electrons = stronger molecular forces

more = increased polarisability of a molecule, making it more likely for temporary induced dipoles to form

how does electronegativity impact bonding

small electronegativity difference wil be purely covalent

very large difference = ionic

when does a polar covalent bond form

when the elements in the bond have different electronegativities

unequal distribution of electrons in the bond and produces a dipole

define symmetric molecules

all bonds are identical and no lone pairs

are symmetric molecules polar or non polar

non polar, even if individual bonds are polar

individual dipoles cancel out

how does shape (straight chain and branched) impact strength of van der waals

straight chain molecules = stronger than branches as they can pack closer together,

reduces distance over which the force acts

describe hydrogen bonding

strongest force

only form between hydrogen and the 3 most electronegative atoms: nitrogen, oxygen, fluorine

which atoms form hydrogen bonds

hydrogen with the most electronegative atoms:

nitrogen, oxygen and fluorine

how do hydrogen bonds form

the lone pair on the atoms (nitrogen, oxygen, fluorine) for a bond with a hydrogen atom from another molecule

physical properties of molecules with hydrogen bonds

higher melting and boiling points