- Electronegativity is taught in A2/A-level Chemistry unit 3.1.3
- electronegativity is described as a measure of the tendency of an atom to attract a bonding pair of electrons
- electronegativity difference between bonding atoms can explain differences in strength of intermolecular forces among molecules
Electronegativity
Definition:
describe the way in which the negative charge is distributed in a molecule
can be measured by Pauling’s scale (range from 0~4)
Factors:
1. nuclear charge (number of protons)
--> more protons = higher electronegativity
2. distance between nucleus & outermost shell electrons (atomic radius)
--> greater radius = lower electronegativity
3. shielding effect (numbers and types of orbitals occupied)
--> more occupied shells = lower electronegativity
Trends:
- electronegativity increases across the period (from left to right)
- ---> Reason: number of electrons increase
- electronegativity decreases down the group
- --> Reason: number of shell & nuclear distance increase
- most electronegative atom N,O,F (top right corner)
Bond Polarity
Definition:
unequal distribution of electrons among atoms in a covalent bond
- e.g. polar HF: one pair of polar H-F bond where H has electronegativity of 2.1 and F with 4.0
- polar H2O: two symmetrical H-O polar bond and a pair of lone pair electrons
- HOWEVER, molecules with polar bond can be non-polar
- e.g. non-polar CO2: two symmetric C-O polar bonds that cancel out each other
- shape and symmetry dependent
Intermolecular forces between molecules
- non-bonding interaction
- weaker than covalent bonds
- can be easily broken under high temperature
- can co-exist at the same time
- Three major types: Van Der Waals Forces (weakest) < Permanent dipole-dipole interaction < Hydrogen bonding (strongest)
Van Der Waals forces (/London dispersion forces)
induced electrical interactions between two or more molecules
due to temporary shift of electrons density among atoms
exists in all atoms/molecules
distance and size dependent
- e.g. homonuclear/heteronuclear molecules(I2, O2, C6H14, HCl...)
Permanent Dipole-dipole interactions
attractive forces between the positive end of one polar molecule and the negative end of another polar molecule
due to electronegativity difference among atoms (permanent effect)
- e.g. HCl, CH3OH...
Hydrogen bonding (strongest)
- special type of dipole-dipole attraction
- H atom covalently bonded to a VERY electronegative atom (e.g. N,O,F)
- the attraction between hydrogen on one molecule to another electronegative atom on another molecule
- HF, H2O, CH3OH...
Image References:
- Electronegativity Trends in Periodic Table - https://periodictableguide.com/all-periodic-trends-in-periodic-table/
- Polarity in molecules - https://brewminate.com/032218-26-biology-chemistry-evolution/
- Intermolecular Forces Examples - https://www.researchgate.net/figure/Different-intermolecular-interaction-types-and-their-energies-Taken-from_fig26_309589044
Drafted by Yoyo (Chemistry)