Non Polar and Polar Molecules

Shapes of Molecules

Define a dipole.

A separation of charge caused by a difference in electronegativity, producing a δ+ end and a δ− end.

Why does a dipole form in a covalent bond?

The bonding electrons are attracted more strongly towards the more electronegative atom, causing a separation of charge.

Which atom in a polar bond becomes δ−?

The more electronegative atom.

Which atom in a polar bond becomes δ+?

The less electronegative atom.

What does δ+ represent?

A slight positive charge.

What does δ− represent?

A slight negative charge.

What determines whether a covalent bond is polar?

The difference in electronegativity between the two bonded atoms.

What happens to bond polarity as the electronegativity difference increases?

The bond becomes more polar.

What determines the overall polarity of a molecule?

The polarity of its bonds and the shape of the molecule.

Why is molecular shape important when determining polarity?

The shape determines whether individual bond dipoles cancel out or reinforce one another.

When is a molecule non-polar despite containing polar bonds?

When the bond dipoles completely cancel because of the molecule's symmetrical shape.

When is a molecule polar?

When its bond dipoles do not completely cancel, giving an overall dipole.

What is meant by an overall dipole?

A net separation of charge across the whole molecule.

How can you determine whether a molecule is polar?

Identify the polar bonds, determine the molecular shape, then decide whether the bond dipoles cancel.

Why are H₂ and Cl₂ non-polar?

The two atoms in each molecule are identical and have the same electronegativity, so the bonding electrons are shared equally.

Are bonds between identical atoms polar?

No, because there is no electronegativity difference.

Why is HCl polar?

Chlorine is more electronegative than hydrogen, so the bonding electrons are attracted towards chlorine, producing Hδ+–Clδ−.

What are the partial charges in HCl?

H is δ+ and Cl is δ−.

Why does the HCl molecule have an overall dipole?

It contains one polar bond, so there are no other bond dipoles to cancel it.

Is CO₂ polar or non-polar?

Non-polar.

Why is CO₂ non-polar even though its C=O bonds are polar?

CO₂ is linear and symmetrical, so the two equal C=O bond dipoles act in opposite directions and cancel.

What shape allows the dipoles in CO₂ to cancel?

Linear.

What is the bond angle in CO₂?

180°.

Is BCl₃ polar or non-polar?

Non-polar.

Why is BCl₃ non-polar even though its B–Cl bonds are polar?

It is trigonal planar and symmetrical, so the three bond dipoles cancel.

What shape allows the dipoles in BCl₃ to cancel?

Trigonal planar.

Is CCl₄ polar or non-polar?

Non-polar.

Why is CCl₄ non-polar even though all four C–Cl bonds are polar?

It is a symmetrical tetrahedral molecule, so the bond dipoles cancel.

What shape is CCl₄?

Tetrahedral.

Is CHCl₃ polar or non-polar?

Polar.

Why is CHCl₃ polar?

Its tetrahedral arrangement contains different surrounding atoms, so the bond dipoles do not cancel and the molecule has an overall dipole.

Can a tetrahedral molecule be polar?

Yes. If the surrounding atoms are different so that the bond dipoles do not cancel, the molecule can be polar.

Can a tetrahedral molecule be non-polar?

Yes. If it is symmetrical and its bond dipoles cancel, it is non-polar.

Is NH₃ polar or non-polar?

Polar.

Why is NH₃ polar?

It is trigonal pyramidal, so the three polar N–H bond dipoles reinforce rather than cancel, producing an overall dipole.

What shape is NH₃?

Trigonal pyramidal.

How does the lone pair in NH₃ contribute to its polarity?

It makes the molecule asymmetrical, so the bond dipoles do not cancel.

Is H₂O polar or non-polar?

Polar.

Why is H₂O polar?

It is V-shaped, so the two polar O–H bond dipoles do not cancel and instead reinforce one another.

What shape is H₂O?

V-shaped.

Why don't the O–H dipoles in water cancel?

The molecule is V-shaped rather than linear, so the dipoles do not act in exactly opposite directions.

Does having polar bonds always mean that a molecule is polar?

No. Polar bond dipoles can cancel if the molecule is symmetrical.

Does a symmetrical molecule containing identical surrounding atoms tend to be polar or non-polar?

Non-polar, because its bond dipoles usually cancel.

Does an asymmetrical molecule containing polar bonds tend to be polar or non-polar?

Polar, because its bond dipoles do not completely cancel.

What is the polarity of a linear molecule with two identical polar bonds pointing in opposite directions?

Non-polar because the dipoles cancel.

What is the polarity of a symmetrical trigonal planar molecule with three identical polar bonds?

Non-polar because the dipoles cancel.

What is the polarity of a symmetrical tetrahedral molecule with four identical polar bonds?

Non-polar because the dipoles cancel.

Why are trigonal pyramidal molecules such as NH₃ often polar?

Their asymmetrical shape means the bond dipoles do not cancel.

Why are V-shaped molecules such as H₂O often polar?

Their angled shape prevents the bond dipoles from cancelling.

Define dipole moment.

A measure of the polarity of a bond or molecule based on the magnitude of the separated charges and the distance between them.

What is the unit of dipole moment?

Debye, symbol D.

What does a dipole moment of zero indicate?

The molecule has no overall dipole and is non-polar.

What does a larger dipole moment generally indicate?

A more polar molecule.

Why is cis-dichloroethene polar?

The C–Cl bond dipoles act partly in the same direction and do not cancel, producing an overall dipole.

Why is trans-dichloroethene non-polar?

The molecule is symmetrical and the C–Cl bond dipoles act in opposite directions and cancel.

How can stereoisomers have different polarities?

Their different spatial arrangements can cause their bond dipoles either to cancel or to reinforce one another.

What two things should you always consider when deciding whether a molecule is polar?

Bond polarity and molecular shape.

Give an exam method for deciding whether a molecule is polar.

Identify electronegativity differences and bond dipoles, determine the molecular shape, then state whether the dipoles cancel or give an overall dipole.