A2/A-level Chemistry - Electronegativity

• 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...

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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)