Shapes of molecules and intermolecular forces Flashcards
Explain the electron pair repulsion theory
Electrons surrounding an atom determine the shape of the atom
Electron pairs repel one another so that they are arranges as far apart as possible
Arrangement of electrons minimises repulsion and creates a definite shape
What does a solid line represent?
Represents a bond on the plane of the paper
What does a solid wedge represent?
Represents a bond coming out the plane of the paper
What does a dotted wedge represent?
Represents a bond going into the plane of the paper
Where are lone pairs located in relation to central atom?
Closer to the central atom as occupies more space
What electrons repel more strongly?
Lone pairs
Explain a tetrahedral shape
4 bonded pairs
No lone pairs
Bond angle 109.5
Explain a pyramidal shape
3 bonded pairs
One lone pair
Bond angle 107
Explain a non-linear shape
2 bonded pairs
2 lone pairs
Bond angle 104.5
Explain a linear shape
2 bonded regions
No lone pairs
Bond angle 180
Explain a trigonal planar shape
3 bonded regions
No lone pairs
Bond angle 120
Explain an octahedral shape
6 bonded regions
No lone pairs
Bond angle 90
What shape are carbonate and nitrate ions?
Trigonal planar
What shape are sulphate ions?
Tetrahedral
Define ‘electronegativity’
A measure of attraction of a bonded atom for the pair of electrons in a covalent bond
What are the most electronegative elements?
Fluorine
Oxygen
Nitrogen
Chlorine
What scale is electronegativity measured on?
Pauling electronegativity scale
How does electronegativity affect whether the bond is ionic or covalent?
The greater the difference electronegativity value is the more likely it is to be ionic because one bonded atom will have a much greater attraction for shared pair of electrons
What does it mean for a bond to be non-polar?
The bonded electron pair is shared equally between the bonded atoms
When do non-polar bonds occur?
The bonded atoms are the same
The bonded atoms are the same or have a similar electronegativity value
What is a pure covalent bond?
The bonded atoms come from the same element and the bond is shared equally
What is a polar bond?
The bonded electron air is shared unequally between the atoms
What is polar covalent bond?
Bonded atoms are different and there is a difference in electronegativity values
Define ‘dipole’
Separation of opposite charge
When does a permanent dipole occur?
A dipole in a polar covalent bond that doesn’t change
Define ‘intermolecular forces’
The weak interactions between dipoles of different molecules
What are the 3 types of intermolecular force?
London Forces
Permanent dipole-dipole
Hydrogen bonding
What properties do different types of bond affect?
Intermolecular Forces= physical properties
Covalent bonds= chemical properties
Describe process of London forces forming
Movement of electrons produce a changing dipole
An instantaneous dipole will form but location is constantly changing
Instantaneous dipole induces a dipole in neighbouring molecule
Induced dipole induces further dipoles in neighbouring molecules
How does change in number of electrons affect strength of London forces?
The more electrons:
the larger the instantaneous and induced dipoles
the greater the induced dipole-dipole interactions
the stronger the attractive forces between molecules
Where do permanent dipole dipoles form?
Between permanent dipoles in different polar molecules
What is a simple molecular lattice made up of?
Simple molecules( small units containing a definite number of atoms with a definite formulae)
How are molecules held together in a simple molecular lattice?
The molecules are held in place by weak intermolecular forces
The atoms within the molecules are held in place by strong covalent bonds
What are the boiling points of simple molecular substances like?
Low, the weak London forces between molecules can be broken at very low levels of energy. Covalent bonds are NOT broken.
Describe the solubility of non polar simple molecular substances in non-polar solvents
Intermolecular forces form between the molecules and the solvent
Interactions weaken the IF in the simple molecular lattice
IF forces break and compound dissolves
Describe solubility of simple molecular substances in polar substances
Little interaction between molecules in lattice and solutes as IF forces between the polar solvent is too strong.
Describe electrical conductivity of simple molecular substances
Not conductive as no mobile charged particles
Where do hydrogen bonds form?
A hydrogen atom attached to an electronegative atom
Why is solid water less dense than liquid water?
Hydrogen bonds hold water molecules apart in an open lattice structure when solid
The water molecules in ice are further apart than in water
Solid ice is less dense so floats
Why does water have an anomalously high melting and boiling point?
Due to the presence of hydrogen bonds
