Molecular forces, IMFs, VSEPR

What is VSEPR theory?

Valence Shell Electron Pair Repulsion theory: electron domains around a central atom repel each other and arrange themselves as far apart as possible.

What is a steric number?

The total number of electron domains around the central atom: bonding domains + lone-pair domains.

What counts as ONE electron domain?

Any single bond, double bond, triple bond, or lone pair counts as one electron domain.

Does a double bond count as one or two electron domains?

One electron domain.

Does a triple bond count as one or three electron domains?

One electron domain.

What is steric number 2 electron geometry?

Linear.

What is steric number 3 electron geometry?

Trigonal planar.

What is steric number 4 electron geometry?

Tetrahedral.

What is steric number 5 electron geometry?

Trigonal bipyramidal.

What is steric number 6 electron geometry?

Octahedral.

What steric numbers do you need to be able to DRAW?

Only steric numbers 2, 3 and 4.

What steric numbers do you only need to RECOGNISE?

Steric numbers 5 and 6.

What is the ideal bond angle for linear geometry?

180°.

What is the ideal bond angle for trigonal planar geometry?

120°.

What is the ideal bond angle for tetrahedral geometry?

109.5°.

Why are lone pairs important in VSEPR?

Lone pairs repel more strongly than bonding pairs and therefore compress bond angles.

What is the repulsion strength order?

Lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.

What is the difference between electron geometry and molecular geometry?

Electron geometry includes both bonding pairs and lone pairs; molecular geometry describes the positions of atoms and ignores lone pairs when naming the shape.

What is the molecular shape of a steric number 2 molecule with 0 lone pairs?

Linear.

What is the molecular shape of a steric number 3 molecule with 0 lone pairs?

Trigonal planar.

What is the molecular shape of a steric number 3 molecule with 1 lone pair?

Bent.

What is the molecular shape of a steric number 4 molecule with 0 lone pairs?

Tetrahedral.

What is the molecular shape of a steric number 4 molecule with 1 lone pair?

Trigonal pyramidal.

What is the molecular shape of a steric number 4 molecule with 2 lone pairs?

Bent.

What is the molecular shape of a steric number 5 molecule with 0 lone pairs?

Trigonal bipyramidal.

What is the molecular shape of a steric number 5 molecule with 1 lone pair?

Seesaw.

What is the molecular shape of a steric number 5 molecule with 2 lone pairs?

T-shaped.

What is the molecular shape of a steric number 5 molecule with 3 lone pairs?

Linear.

What is the molecular shape of a steric number 6 molecule with 0 lone pairs?

Octahedral.

What is the molecular shape of a steric number 6 molecule with 1 lone pair?

Square pyramidal.

What is the molecular shape of a steric number 6 molecule with 2 lone pairs?

Square planar.

What shape has a bond angle of 180°?

Linear.

What shape has an ideal bond angle of 120°?

Trigonal planar.

What shape has an ideal bond angle of 109.5°?

Tetrahedral.

What happens to bond angles when lone pairs are present?

They become smaller than the corresponding ideal electron-domain angle.

What is the approximate H–O–H angle in H₂O?

104.5°.

What is the shape of H₂O?

Bent.

What is the steric number of H₂O?

4.

How many lone pairs are on the central atom in H₂O?

2.

What is the electron geometry of H₂O?

Tetrahedral.

What is the molecular geometry of H₂O?

Bent.

What is the bond angle of CO₂?

180°.

What is the shape of CO₂?

Linear.

What is the steric number of CO₂?

2.

How many electron domains are around carbon in CO₂?

2.

Does the double bond in CO₂ count as two electron domains?

No. Each C=O double bond counts as one domain.

What is the shape of BF₃?

Trigonal planar.

What is the steric number of BF₃?

3.

What is the approximate bond angle in BF₃?

120°.

What is the shape of CH₄?

Tetrahedral.

What is the steric number of CH₄?

4.

What is the bond angle in CH₄?

109.5°.

What is the shape of NH₃?

Trigonal pyramidal.

What is the steric number of NH₃?

4.

How many lone pairs are on N in NH₃?

1.

What is the approximate H–N–H angle in NH₃?

107°.

What is the electron geometry of NH₃?

Tetrahedral.

What is the shape of BeCl₂?

Linear.

What is the steric number of BeCl₂?

2.

What is the shape of SO₂?

Bent.

What is the steric number of SO₂?

3.

How many lone pairs are on the central S atom in SO₂?

1.

Is SO₂ linear or bent?

Bent.

What is the shape of CCl₄?

Tetrahedral.

Is CCl₄ symmetrical?

Yes. Its four identical C–Cl bonds are arranged symmetrically in a tetrahedral structure.

Is CCl₄ polar or nonpolar?

Nonpolar.

Why is CCl₄ nonpolar despite having polar C–Cl bonds?

The four bond dipoles cancel because of the symmetrical tetrahedral arrangement.

Is H₂O polar or nonpolar?

Polar.

Why is H₂O polar?

It has polar O–H bonds and a bent shape, so the bond dipoles do not cancel.

Is CO₂ polar or nonpolar?

Nonpolar.

Why is CO₂ nonpolar despite having polar C=O bonds?

It is linear and symmetrical, so the two equal and opposite dipoles cancel.

Is NH₃ polar or nonpolar?

Polar.

Why is NH₃ polar?

Its trigonal pyramidal shape and lone pair prevent the N–H dipoles from cancelling.

What determines whether a covalent bond is polar?

The difference in electronegativity between the bonded atoms.

What does a dipole arrow point toward?

The more electronegative atom.

What does the crossed end of a dipole arrow represent?

The less electronegative atom.

What does it mean if the net dipole moment is Ø?

All bond dipoles cancel, so the molecule is nonpolar.

Does having polar bonds automatically make a molecule polar?

No. A molecule can contain polar bonds but be nonpolar if the dipoles cancel.

What two major factors determine molecular polarity?

Bond polarity and molecular geometry/symmetry.

What does molecular symmetry have to do with polarity?

Symmetrical arrangements can cause equal bond dipoles to cancel.

Can a molecule with polar bonds be nonpolar?

Yes. Examples include CO₂ and CCl₄.

Can a molecule with no polar bonds be polar?

No.

What does “all polar bonds the same?” mean?

It asks whether the molecule's polar bonds are equivalent in type/orientation; it does not mean the bonds are nonpolar.

What should you put for “all polar bonds the same?” if there are no polar bonds?

Usually Y if the worksheet forces Y/N, although logically N/A would be more precise.

Why can HCN have a net dipole?

The H–C and C≡N bonds are different polar bonds and the molecule is linear but not symmetrical, so their dipoles do not cancel.

What is the shape of HCN?

Linear.

What is the steric number of HCN's central carbon?

2.

Is HCN polar?

Yes.

What is the shape of PH₂Br?

Trigonal pyramidal.

What is the steric number of P in PH₂Br?

4.

How many lone pairs are on P in PH₂Br?

1.

Are all polar bonds in PH₂Br the same?

No. The P–H bonds and P–Br bond are different.

Is PH₂Br polar?

Yes.

Why is PH₂Br polar?

It is trigonal pyramidal and contains different polar bonds, so the dipoles do not cancel.

What is the shape of H₂O₂?

Nonlinear/nonplanar about the O atoms; each O has a bent arrangement due to its lone pairs.

Is H₂O₂ polar?

Yes. Its O–H bond dipoles do not cancel completely.

What is the difference between bond polarity and molecular polarity?

Bond polarity describes an individual bond; molecular polarity describes the overall distribution of charge after all bond dipoles are considered.

What are the three intermolecular forces you need to know?

Dispersion forces, dipole–dipole forces, and hydrogen bonding.

What is the weakest intermolecular force generally discussed in this course?

Dispersion forces.

What causes dispersion forces?

Temporary fluctuations in electron distribution create instantaneous dipoles that induce dipoles in nearby particles.