Electronegativity + forces

Define electronegativity

A measure of attraction of a bonded atom for the pair of electrons in a covalent bond

What is the trend of electronegativity across a period

• the larger the nuclear charge and the smaller the atomic radius, the greater the attraction for the pair or electrons in a covalent bond

• electronegativity increases

Explain the electronegativity in HCl

• Cl is more electronegative than H

• Cl atom has a greater attraction for the bonding pair of electrons than the H atom

• bonding electrons attracted to the Cl atom which forms a polar bond

• Cl has delta- (negative dipole)

• H has delta+ (positive dipole)

• charge difference is permanent dipole

Explain the electronegativity in H2 (any diatomic molecule)

• the two atoms are identical

• each H atom has an equal share of the pair of electrons in the bond

• electrons are evenly distributed

What does it mean by a symmetrical molecule

( both rules must apply)

• central atom has no lone pairs

• outer atoms are identical

• dipoles act in different directions = cancel out

• NON-POLAR MOLECULE

What does it mean by an asymmetrical molecule

• central atom has a lone pair

• outer atoms are not identical

• dipoles act in the same direction = dont cancel out

• POLAR MOLECULE

Name all IMF in highest to lowest strength

• hydrogen bonds

• permanent dipole-permanent dipole

• permanent dipole-induced dipole

• london dispersion forces (induced dipole-dipole)

What are London Dispersion Forces

• theres an uneven distributions of electrons and at any point, there will be an instantaneous dipole across the molecule

• Instantaneous dipole induces a dipole in neighbouring molecule

• small induced dipoles attract one another

• present in all covalent molecules

Explain the pattern of BPs down group 7

• BP increases

• number of electrons increase down the group

• greater instantaneous and induced dipoles, so stronger london dispersion forces between molecules

• more energy required to break the LDFs

What are permanent dipole-induced dipole interactions

• molecules with permanent dipoles approach non polar molecules which unevenly distributes the electrons in the non polar molecule

• this causes it to become slightly polar and then an attraction occurs

• molecule with permanent dipole induces a dipole in non-polar molecule

What are permanent dipole-permanent dipole forces

• permanent dipole of one molecule attracts the permanent dipole of another polar molecule to form a weak permanent dipole-dipole force

• permanent negative dipole is attracted to permanent positive dipole of other molecule

What is hydrogen bonding

• occurd between H and either N, O, F

• forms very polar bonds

Why is ice less dense than water

• ice has an open lattice with hydrogen bonds holding H2O molecules further apart

• when ice melts, H bonds collapse, allowing molecules to move closer together

Why does water have a higher MP and BP

• has hydrogen bonds between molecules which are the strongest weak IMFs

• requires a large amount of energy to break