Chem Chapter 6: Shapes of molecules

do electrons pairs repel or attract?

repel so that they are arranged as far apart as possible

how is the shape of a molecule or ion determined

by the electron pairs surrounding a central atom

how does the arrangement of the electron pairs affect the shape

it minimises repulsion and thus holds the bonded atoms in a definite shape

different numbers of electron pairs result in…

different shapes

is ch4 symmetrical or non symmetrical?

symmetrical with 4 covalent c-h bonds

how many bonded pairs of electrons surround the central carbon atom in CH4

4

how do the 4 electron pairs in ch4 act

repel one another as far apart as possible

shape of ch4

tetrahedral shape

4 equal H-C-H bond angles of 109.5 degrees

solid line

bond in the plane of the paper

solid wedge

comes out of the plane of the paper

dotted wedge

goes into the plane of the paper

which one is closer to the central atom? a lone pair or bonded pair?

lone pair of electrons

it occupies more space than a bonded pair

resulting in a lone pair repelling more strongly than a bonding pair

relative repulsions

bonded pair bonded pair < bonded pair lone pair < lone pair lone pair

how is ammonia and water different to methane

the electron pairs are a mixture of bonded pairs and lone pairs

the 4 electron pairs around the central atom...

repel one another as far apart as possible into a tetrahedral arrangement

lone pairs repel more strongly than bonded pairs therefore...

lone pairs repel bonded pairs slightly closer together, decreasing the bond angle

what does the bond angle mean

the angle between the bonded pairs of electrons

the bond angle is reduced by...

2.5 degrees for each lone pair

how are molecules containing multiple bonds treated?

each multiple bond is treated as a bonding region

principles of electron - pair repulsion theory ( can be applied to any number of electron pairs surrounding the central atom )

electron pairs around the central atom repel each other as far apart as possible, the greater the number of electron pairs, the smaller the bond angle, lone pairs of electrons repel more strongly than bonded pairs of electrons

the electron pairs repel one another so that they are...

so that they are arranged as far apart as possible

the greater the number of electron pairs...

the smaller the bond angle

lone pairs of electrons repel...

more strongly than bonded pairs of electrons

in a covalent bond, what are the nuclei of the bonded atoms attracted to?

the shared pair of electrons

in molecules of elements, eg H2, 02, N2 and CL2, the atoms are the same element and...

the bonded electron pair is shared evenly

when may the shared pair of electrons in a covalent bond experience more attraction from one of the bonded atoms than the other?

when the bonded atoms are different elements because: the nuclear charges are different, the atoms may be different sizes, the shared pair of electrons may be closer to one nucleus than the other

what is electronegativity

the attraction of a bonded atom for the pair of electrons in a covalent bond

how is electronegativity measured?

using the pauling scale

across the period table...

the nuclear charge increases

the atomic radius decreases

a large pauling value indicates...

that atoms of the same element are very electronegative

the noble gases are not included in the pauling scale because...

they tend not to form compounds

the non metals nitrogen, oxygen,fluorine and chlorine have the ---- electronegative atoms

most

the group 1 metals have the ----- electronegative atoms

least

electronegativity difference of 0

covalent

electronegativity difference of 0-1.8

polar covalent

electronegativity difference of greater than 1.8

ionic

if the electronegativity difference is large, one bonded atom will have much greater attraction...

for the shared pair than the other bonded atom

the more electronegative atom will have gained control of the electrons and the bond will now be...

ionic rather than covalent

in a non-polar bond, the bonded electron pair is...

shared equally between the bonded atoms

A bond will be non-polar when:

the bonded atoms are the same or the bonded atoms have the same or similar electronegativity

what is a pure covalent bond

in molecules of elements such as H, O and Cl, the bonded atoms come from the same element and the electron pair is shared equally

are carbon and hydrogen compounds polar or non polar

non polar because they have very similar electronegativities

hydrocarbon liquids are non - polar solvents and...

do not mix with water

what happens in a polar bond

the bonded electron pair is shared unequally between the bonded atoms

when is a polar covalent bond created

when the bonded atoms are different and have different electronegativity values

what does the delta sign mean

small

what is a dipole

the separation of opposite charges

what is a permanent dipole

when a dipole in a polar covalent bond does not change, to distinguish it from an induced dipole

the atom with the larger electronegativity value has a...

delta negative charge

the atom with the smaller electronegativity value has the

delta positive charge

for molecules with more than 2 atoms, there may be...

2 or more polar bonds

the dipoles may reinforce one another to produce a larger dipole over the whole molecule or...

cancel out if the dipoles act in opposite directions

what are intermolecular forces

weak interactions between dipoles of different molecules

Intermolecular forces fall into 3 main categories:

induced dipole - dipole interactions ( London forces ) permanent dipole - dipole interactions hydrogen bonding

what are intermolecular forces largely responsible for

physical properties such as melting and boiling points

what are covalent bonds responsible for

they determine the identity and chemical reactions of molecules

What are London forces

weak intermolecular forces that exist between all molecules, whether polar or non polar they act between induced dipoles in different molecules

are induced dipoles temporary or permanent

temporary, in the next instant of time the induced dipoles may disappear, only for the whole process to take place amongst other molecules

how are induced dipoles resulted

from interactions of electrons between molecules

london forces the more electrons in each molecule:

the larger the instantaneous and induced dipoles the greater the induced dipole - dipole interactions the stronger the attractive forces between molecules

larger numbers of electrons mean larger induced dipoles so...

more energy is then needed to overcome the intermolecular forces, increasing the boiling point

permanent dipole - dipole interactions act between...

the permanent dipoles in different polar molecules

what is a simple molecular substance made up of

simple molecules - small units containing a definite number of atoms with a definite molecular formula such as neon,hydrogen,h20 and co2

in a solid state what do simple molecules form

a regular structure called a simple molecular lattice

in the simple molecular lattice:

the molecules are held in place by weak imf the atoms within each molecule are bonded together strongly by covalent bonds

all simple molecular substances are ---------- bonded

covalently

at room temp, simple molecular substances may exist as

solids, liquids or gases

all simple molecular substances can be solidified into simple molecular lattices by...

reducing the temperature

simple molecular substances have ---- melting and boiling points

low

when a simple molecular lattice is broken apart during melting

only the weak imf break the covalent bonds are strong and do not break

Covalent substances with simple molecular structures fall into 2 categories...

polar and non polar

what happens when a simple molecular compound is added to a non polar solvent

imf form between the molecules and the solvent

in solvents the interactions weaken the imf in the simple molecular lattice so...

the imf break and the compound dissolves

non polar simple molecular substances tend to be ----- in non polar solvents

soluble

when a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules so

the intermolecular bonding within the polar solvent is too strong to be broken, therefore simple molecular substances tend to be insoluble in polar solvents

polar covalent substances may dissolve in polar solvents because...

the polar solute molecules and the polar solvent molcules can attract each compound

what do some biological molecules contain

hydrophobic and hydrophilic parts

the hydrophilic part will be...

polar and contain electronegative atoms ( usually oxygen )that can interact with water

the hydrophobic part will be...

non - polar and is comprised of a carbon chain

why are simple molecular structures non conductors of electricity

no charged particles can move so there is nothing to complete an electrical circuit

What is a hydrogen bond

a special type of permanent dipole - dipole interaction between molecules

what does a hydrogen bond contain

an electronegative atom with a lone pair of electrons eg oxygen, nitrogen, fluorine a hydrogen atom attached to an electronegative atom eg H-O, H-N, H-F

the hydrogen bond acts between a lone pair of electrons on an electronegative atom in one molecule and...

a hydrogen atom in a different molecule

what is the strongest type of intermolecular attraction

hydrogen bonds

how is a hydrogen bond shown in a diagram

by a dashed line

ice is ---- dense than water

less

why is ice less dense than water?

hydrogen bonds hold water molecules apart in an open lattice structure, the water molecules in ice are further apart than in water, solid ice is less dense than liquid water and floats

what is a structural feature in water that decreases the density of water on freezing

the holes in the open lattice structure

what happens when ice melts?

the ice lattice collapses and the molecules move closer together, so water is denser than solid ice

why does water have much higher melting and boiling points than would be expected from just London forces

a high quantity of energy is needed to break the hydrogen bonds in water

what happens when the ice lattice breaks

the rigid arrangement of hydrogen bonds in ice is broken

what happens when water boils

the hydrogen bonds break completely

how is the double helix structure of DNA held together

by hydrogen bonds which enable a single DNA strand to create a perfect copy of itself in replication

4 bases of dna

adenine A, thmine T, cytosine C, guanine G

how do A and T pair in DNA

by forming 2 hydrogen bonds

how do C and G pair in dna

by forming 3 hydrogen bonds

adenine and guanine are both...

purine bases with 2 ringed structures

thymine and cytosine are both...

pyrimidine bases with single ringed structures

hydrogen bonding in the double helix can only take place between...

a pure and a pyrimidine base

why do bases have to fit together in DNA?

so that a hydrogen atom from one molecule and an electronegative atom ( O or N ) from the other molecule are aligned correctly to maximise hydrogen bonding