Polarisation and Intermolecular Forces Flashcards
Define electronegativity
Electronegativity is an atom’s ability to attract the electron pair in a covalent bond
What is the most electronegative element?
Flourine
Electronegativity is measured on what?
The Pauling scale
Give examples of the most electronegative elements and their score on the Pauling scale
Flourine - 4.0 Oxygen - 3.4 Chlorine - 3.0 Nitrogen - 3.0 Carbon -2.5 Hydrogen -2.2
Can covalent bonds be polarised?
In a covalent bond between two atoms of different electronegativities, the bonding electrons will be pulled towards the more electronegative atom. This makes the bond polar
Are covalent bonds with atoms of the same element polar?
A covalent bond between two atoms of the same element is non-polar because the atoms have equal electronegativities, so the electrons are equally attracted to both nuclei
Are covalent bonds with atoms of similar electronegativities polar?
Some elements, such as carbon and hydrogen, have similar electronegativities are essentially non-polar
What happens in a polar bond?
In a polar bond, the difference in electronegativity between the two atoms causes a permanent dipole
What is a dipole?
A dipole is a difference in charge between two atoms caused by a shift in electron density in the bond
What does a greater difference in electronegativity mean in terms of a polar bond?
The greater the difference in electronegativity between the atoms, the more polar the bond
Can a whole molecule be polar?
If you have a molecule that contains polar bonds, you can end up with an uneven distribution of charge across the whole molecule. When this happens the molecule is polar
Are all molecules that contain polar bonds, polar?
Not all molecules that contain polar bonds are polar. If the polar bonds are arranged symmetrically in the molecule, then the charges cancel out and the is no permanent dipole
Polar molecules have permanent dipole-dipole forces, explain
In a substance made up of molecules that have permanent dipoles, there will be weak electrostatic forces of attraction between the S+ and S- charges on neighbouring molecules
Are intermolecular forces strong or weak?
Very weak
What are intermolecular forces?
Intermolecular forces are forces between molecules
Name three types of intermolecular forces
- van der Waals (induced dipole-dipole)
- Permanent dipole-dipole
- Hydrogen bonding
Intermolecular forces affect what properties of a compound?
The physical properties
Van der Waals forces cause all atoms to be attracted to each other. Explain
Electrons in charge clouds are always moving really quickly. At any moment, the electrons in an atom are likely to be more at one side than the other. At this moment, the atom has a temporary dipole. This dipole causes another temporary dipole and the opposite direction of the neighbouring atom. The two dipoles are attracted to each other. The second dipole causes another temporary dipole, and so on. Because the electrons are constantly moving, the dipoles are being created and destroyed all the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.
What can van der Waals forces hold molecules in?
A lattice
Explain why Iodine is solid at rooms temperature
Iodine atoms are held together in pairs by strong covalent bonds to form I2 molecules. But the molecules are held together in a molecular lattice arrangement by weak van der Waals forces
What increases the strength of van der Waals forces?
- Larger molecules have larger electron clouds, meaning stronger van der Waals forces
- Long, straight molecules can lie closer together than branched ones - the closer two molecules can get, the stronger the van der Waals forces
How do van der Waals forces affect boiling points?
Stronger van der Waals forces mean higher boiling points. This is because you need more energy to overcome the intermolecular forces so that particles can escape from the liquid surface
What is the strongest intermolecular force?
Hydrogen bonding
When does hydrogen bonding happen?
Hydrogen bonding only happens when hydrogen bonding is covalently bonded to flourine, nitrogen or oxygen
Hydrogen bonding has a huge effect on what?
The properties of substances
How does hydrogen bonding affect the melting and boiling points of substances?
Substances with hydrogen bonding have higher melting and boiling points because of the extra energy to break the hydrogen bonds
What happens when water forms ice?
When water cools to form ice, the molecules make more hydrogen bonds and arrange themselves into regular lattice structure. In this structure the water molecules are further apart on average than molecules in liquid water - this makes ice less dense than water
