Chemistry - electronegativity and bond polarity

electronegativity definition

measure of the ability of an atom to attract the bonding pair of electrons in a covalent bond

bonding pair of electrons in the covalent bond are either:

equally shared (usually 2 atoms of the same element), or unequally shared (so the bond is polar)

if the electron pair is closer to Y then…

Y is more electronegative

relationship between polarity and difference in electronegativity

the larger the difference in electronegativity between the two atoms, the greater the degree of polarity

what will a polar bond possess

a dipole

general rule

if there is at least a 0.4 difference in electronegativity then the bond is polar

trends in electronegativity

decreases down a group, increases across a period, noble gases do not have electronegativity

why does electronegativity decrease down a group?

atomic radius increases so the distance between the positive nuclear charge and the outer electrons involved in bonding increases

why does electronegativity increase across a period?

atomic radius decreases, so the distance between the positive nuclear charge and the outer electrons involved in bonding decreases

why do noble gases not have electronegativity

do not usually form covalent bonds/molecules

how does electronegativity differences affect the length of bonds

large differences give shorter bonds than predicted from atomic radii

pure covalent bond e.g. H2, O2, N2

both bonding atoms have the same electronegativity, so the bonding pair of electrons is relatively equally shared

polar covalent bond e.g. HCl

due to difference in electronegativity of the bonding atoms, the bonding pair of electrons is displaced towards the more electronegative atom resulting in a dipole, known as polarisation - bond is polar

why is CO2 non-polar

its dipoles cancel each other out as it is a symmetrical molecule so there is no net dipole movement

why is water polar

dipoles do not cancel each other out, is asymmetrical, there is a net dipole movement

what way does a dipole go

from positive to negative

examples of non polar molecules

BF3 and CCl4

examples of polar molecules

HCl and NF3