Shapes Of Molecules And Intermolecular Forces 2.6.1 Flashcards
Electron pair repulsion theory
-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
Linear
180• 2 bonded pairs with no lone pairs
Trigonal planar
120• Three bonded pairs of electrons
Tetrahedral
109.5• 4 bonding pairs of electrons
Octahedral
90• 6 bonded pairs of electrons
Pyramidal
107• 3 bonded pairs of electrons and one lone pair repelling more
Non linear
104.5• 2 bonded pairs and two lone pairs repelling more
Electronegativity
The ability of an atom of an element to attract electrons towards itself in a covalent bond
How does electronegativity increase?
The further up and right you go in the periodic table (excluding noble gases) the more electronegative the element is.
What are the most electronegative elements?
FONCL
Electronegativity difference in purely covalent bonding?
0 (non-polar)
Electronegativity difference in a polar covalent structure?
0 to 1.8
Electronegativity difference in an ionic bonding?
Greater than 1.8, the bigger the difference in Electronegativity, the more ionic the compound will be.
What leads to a polar molecule?
An uneven distribution of charge
Factors that affect electronegativity
- Atomic radius
- Electron shielding
- Nuclear charge
How does atomic radius affect electronegativity?
As the radius of an atom increases, the bonding pairs of electrons become further from the nucleus = less attracted to the positive charge of the nucleus =lower electronegativivity
How does nuclear charge affect electronegativity?
The greater the number of protons in a nucleus, the greater the attraction to the electrons in a covalent bond = resulting in a higher electronegativity
How does electron shielding affect electronegativity?
As electron shielding increases the electron shielding decreases, as the electrons are further away from the positively charged nucleus, lowers ability to pull in electrons, decreasing the electronegativity.
How can a molecule be asymmetrical?
If the dipoles don’t cancel each other out then they are arranged asymmetrically - overall they are polar.
Strength of intermolecular forces (Weakest to Strongest)
- London forces/ Induced dipole (non-polar)
- Permanent dipole-dipole interactions (polar)
- Hydrogen forces
How do induced dipole forces arise?
- Electrons are constantly moving so the instantaneous dipole is constantly shifting
- The instantaneous dipole induces a dipole on a neighbouring molecule
- The induced dipole induces further dipoles on neighbouring molecules which then attract one another
Permanent dipole-dipole interactions
Polar molecules have permanent dipoles, will always have a negatively and positively charged end, they are attracted to each other.
Melting and boiling points of simple molecular substances
Have a low melting and boiling point- only the weak intermolecular forces break, the covalent bonds are strong and do not break
Solubility of non polar simple molecular substances
Non polar solvent can dissolve non polar substance
Solubility of polar solvent
A polar solvent can dissolve a polar solution
Why does the London forces increase as you go down the group (group 7)
Increase in size = more electrons = more London forces being established = more energy required to over come the intermolecular forces
What is hydrogen bonding formed between?
NOF (e.g water can form two hydrogen bonds because Oxygen has two lone pairs)
What are the anomalous properties in water?
-High melting and boiling points
- High surface tension
- Density of ice compared to water
Why does ice have a lower density than liquid water?
The hydrogen bonds are held together in an open lattice, holding the water molecules further apart, decreasing the density. However when ice melts the ice lattice collapses and the molecules are closer together, so liquid water is more dense
Boiling point of water
Water has a high boiling point because they have hydrogen forces above the London forces, so a higher amount of energy is needed to overcome the hydrogen bonds in water.
What lattice is iodine in and what forces are it held by?
Crystal lattice held by weak intermolecular forces. Because it is non polar, it is held together due to instantaneous induced dipole interactions (weakest) and sublimes.
What vapour is released when iodine sublimes?
Purple vapour
What can reduce the surface tension of water
Detergents
Conductivity of covalent substances
Do not contain any freely moving charged particles- unable to conduct electricity in either the solid or liquid state however some polar covalent molecules can ionise and will conduct electricity.
