Chapter 6 Shapes Of Molecules And IM forces Flashcards
What is the valence shell electron pair repulsion theory a model for?
Determining / predicting the Shapes of molecules/polyatomic ions
So what is the valence shell electron pair repulsion theory
1) general
2) what this means, how will bonded atoms be in a definitive shape?
- where the shape of a molecule is determined by the electron pairs surrounding the atom =
=1. electrons are negative so the pairs will repel each other such that they are arranged AS FAR APART as possible
(2. Multiple bonds = means more repulsion = smaller bind angles )
(3. Lone pairs REPEL more than bonded pairs of electrons )
- the arrangement of the electron pairs is where it’s at least repulsion, meaning bonded atoms are held in a DEFINITIVE shape due to electrons
Why are lone pairs repel stronger than bonded pair (not need to know)
- Slightly closer to central atom,
- occupies more space
(Compared with bonded pair)
= thus repels more
What is the order of repulsion , weakest to strongest, amongst pairs.
Bonded pair /bonded pair < bonded pair / lone pair < lone pair / lone pair
Remember lone pair powerful
What happens if there are 5 electron pairs around central atom
Name
Bond angle
Bipyramidal
- 120 in one plane
- 90 in other
Which ions have dative bonds (in case you are stuck)
NH4 + And HCO3+
What is electronegativity
- the ability of an atom to attract the electron pair in a covalent bond
-
What makes an atom more electronegative
How you measure
- Higher the atom number = means more protons, which means a stronger attraction
- Less shells = mean less atomic radius = means less distance needed to attract these and so stronger attraction
- the more right you go in a period the more protons and slightly lower radius as the higher nuclear charge, the radius DECREASES slightly DUE TO THEM PULLING IN ON IT.
2) Pauling scale
What will make a bond covalent , polar , or ionic basically
The electronegativity differences , if it is 0, so diatomic then it’s covalent,
- but more than that to 1.8 so say atleast 0.5 to 1.8, one atom will pull electrons closer making it negative and other slight positive so polar
- more than 1.8 it pulls so much it might aswell be ionic bind
Polar bond
Dipole
Need to know
Polar bond = a covalent bond between atoms with different electronegativies where there is a partial positive and negative charge on the atoms
Dipole: is just the name given to the separation of opposite changes between atoms over short distance
Permanent dipole
Separation of partial changes across a polar bond, arising from DIFFERENT ELECTRONEGATIVTIES ( Not random movement of electrons) is a permanent dipole . It will not change
What are three types of IM binds
What are each relative strength
PERMANET dipole dipole strong
Induce dipole dipole (London forces)weak
Hydrogen middle
Covalent strongest
How do induced dipole dipole interactions , known as London forces, come about
- at any time, the random movement of electrons means the distribution of them may by uneven in an atom
- this results in a instsneous dipole in an atom, meaning at any time an INSTANEOUS DIPOLE WILL EXIST, but it’s position changing
- the instaneous dipole will induce a dipole on a neighbouring particle based on its charge
- which can then Induce another and another
- this is all random, one moment there other not
- the attraction between induced dipoles is called an induced dipole dipole interaction
Happens in all molecules
How does the strength of London forces change?
Based on number of electrons
How do more electrons mean stronger London forces
How do these affect boiling point
The more electrons
- the LARGER the instsneous dipole, which also means larger induced dipole
- this means more amount of induced dipole dipole interactions
- and this also means the stronger ATTRACTION between the induce dipole dipole
2) more electrons mean stronger London forces, meaning more energy is needed to overcome these and thus molecules will have a higher melting snd boiling point, as it’s the intermolecular forces thst break not the covalent binds
Van der Waal forces
For both PERMANET and induced, but London specifically for induced
PERMANET dipole dipole
Boiling point?
What happens in liquid
Remmeber a dipole is where opposite partial charges are spread out in a polar bond.
When two PERMANET dipoles come close a permanent dipole dipole interaction happens between partially negative of one atom and partially positive of another.
Permanent dipole dipole also have London’s forces, so there boiling point will be HIGHER
3) CONSTANTLY BREAK AND REFORM
Simple molecular substances
What and properties
Made up of simple moleuculed (small units with definite number of atoms), form regular structure called simple molecular lattice.
- these held by weak IM bonds
- but string covalent between atoms
2; properties include = low melting and boiling point
Solubility of non polar+ non polar simple molecular substance
Simple + non polar
= IM forces are made between the molecules and the solvent
- these interactions WEAKEN the IM forces already in simple molecular substance
= thus the IM forces break and the compound DISSOLVES
THEREFORE NON POLAR SOLVENT IS SOLUBLE IN NON POLAR SIMPLE MOLECULAR
Solubility of simple molecular and polar
When simple molecular + polar added
= little interaction between molecules in lattice and solvent molecules
- thus IM bonding remains too strong in molecular substance to break
= thus simple molecular is not soluble in POLAR substances
Polar substances in polar solvents ?
Will dissolve
-polar solute molecules and the solvent molecules WILL attract each other, thus they can dissolve
- it’s like ions dissolving in water, sugar is polar with OH so interacts with water OH and dissolve
Hydrophilic will be polar and contain something electronegative like oxygen thst can interact with water in biological molecules..,
Solubility summary
Non polar substance with non polar simple molecular = yes
Polar with non polar molecular = no
Polar with polar = yes
Like dissolves like
What are hydrogen bonds
What are they between
How shown hydrogen bond and what is the shape around it ?
- an especially strong PERMANENT dipole dipole interaction
2) occur between
- an electronegative (so bit partially negative ) atom with a lone pair of electrons (oxygen, nitrogen fluorine )
- a hydrogen atom attached to an electronegative atom (H-O)
3) dashed line + linear + strongest IM bond
Why?
- h needs to be next to electronegative atom so overall it’s positive
- lone pairs need to be in electronegative atom so it can go positive to negative
Common hydrogen bond (don’t need to know)
Water, ammonia Hydrogen fluoride
Anamolous properties of water = density Why?
What happens when it melts then
Solid ice denser then liquid water when normally this isn’t true.
- hydrogen bonds hold water molecules at a fixed sirs cells apart in an OPEN LATTICE STRUCTURE.
- here the maximum close distance it can get is at 4°= maximum density
- Binds extend outwards, holding molecules apart and forms TETRAHEDRAL LATTTICE OF HOLES, decreasing density
- water molecules in ice are further apart thus than in water , so solid ice less dense and floats
2) when ice melts, this tetrahedral lattice of holes in ice collapses allowing molecules to move closer together. So density increases
What use is ice being less dense for us
Ice floats in lakes, forming INSULATING layer for animals below where temperatures are normally constant
- and also preventing water from freezing allows a habitat for aquatic animals
Anamalous properties : high melting and boiling
Has higher boiling and melting because
- along with LONDON FORCES, they have hydrogen (which is stronger permanent permanent dipole)
- thus a lot of energy needed to break hydrogen bonds , so higher melting boiling .
Without it it would be -75 °lol
Why water can have these properties and not other hydrogen bonded molecules
Explain the trends of graph
One reason why water has higher boiling point than HF and NH3 is because it can form 2 hydrogen binds due to two lone pairs, and be bonded with two lone pairs as has two hydrogens. The others all have some uneven distribution of lone to hydrogen bonds.
This graph, the first all hydrogen bonded, so higher boiling point, then not so they drop, but then they have more electrons = higher boiling point due to larger instanaeous dipoles, stronger + greater induced etc
Other anamlous properties = surface tension, viscosity (don’t need to know)
SHC, viscosity, surface tension
DNA AT how many bonds CG how many
Why always paired correctly
AT = 2 CG = 3
Because of chemical structure and shape these 4 bases are always paired correctly
Electrical conductivity with IM forces
In simple molecular, as they all IM forces and no mobile charges, it csn’t conduct electricity
