Unit 2 Practice: Medium

Answer: MgO contains ions with larger charge magnitudes.

Explanation: NaCl contains Na⁺ and Cl⁻.

MgO contains Mg²⁺ and O²⁻.

Coulombic attraction increases as ionic charge increases.

Because MgO has 2+ and 2− ions, its electrostatic attractions are much stronger.

Answer: MgO

Explanation: Melting an ionic solid requires disrupting strong attractions within its lattice.

Mg²⁺ and O²⁻ attract much more strongly than Na⁺ and Cl⁻.

More energy is therefore required to melt MgO.

Answer: The ions can move when molten.

Explanation: In solid NaCl, Na⁺ and Cl⁻ are locked into fixed lattice positions.

When melted, the ions become mobile.

Moving charged particles can carry electrical current.

Answer: Metallic bonding allows layers of atoms to shift while remaining attracted to delocalized electrons.

Explanation: Metallic bonding is not based on rigid directional bonds between specific pairs of atoms.

When metal atoms shift position, the delocalized electron system can continue holding the structure together.

This allows metals to be shaped without immediately breaking.

Answer: A substitutional alloy has atoms replacing some metal atoms in lattice positions.

An interstitial alloy has smaller atoms occupying spaces between metal atoms.

Explanation: Substitutional alloys work best when the atoms have reasonably similar sizes.

Interstitial alloys require much smaller atoms that can fit into the gaps of the metal lattice.

Answer:

O=C=O

Each oxygen has two lone pairs.

Explanation: There are 16 total valence electrons.

Using two C=O double bonds gives carbon and both oxygen atoms complete octets while keeping formal charges at zero.

Answer: Nitrogen forms four single N—H bonds and has no lone pairs. The entire structure has a +1 charge.

Explanation: Count electrons:

N = 5 4 H = 4 +1 charge means subtract 1.

Total:

8 valence electrons

All eight are used in four N—H bonds.

Answer: C≡N⁻, with one lone pair on each atom in the complete Lewis structure.

Explanation: CN⁻ has:

4 + 5 + 1 = 10 valence electrons

A triple bond gives both atoms octets when the remaining electrons are placed as lone pairs.

The overall formal charge is −1.

Answer: Typically:

N = +1

double-bonded O = 0

each singly bonded O = −1

Explanation: The charges add to:

+1 + 0 − 1 − 1 = −1

which matches the overall nitrate charge.

There are three equivalent resonance structures.

Answer: More than one valid Lewis structure can represent the electron distribution of a species.

Explanation: The atoms do not rapidly switch between resonance structures.

Instead, the real electron distribution is a resonance hybrid, with electrons delocalized across multiple atoms.

Answer: 4/3 ≈ 1.33

Explanation: Each resonance structure contains:

one double bond = bond order 2 two single bonds = 1 + 1

Total bond order:

4

Spread across three equivalent N—O bonds:

4/3 ≈ 1.33

Answer: Trigonal planar

Explanation: Boron has three bonding groups and no lone pairs.

Three electron groups arrange approximately 120° apart.

Answer: Bent

Explanation: Sulfur has three main electron groups around it in the VSEPR model:

two bonding regions one lone pair

The electron geometry is trigonal planar.

The molecular geometry is bent.

Answer: Trigonal bipyramidal

Explanation: Phosphorus has five bonding groups and no lone pairs.

Five electron groups produce trigonal bipyramidal geometry.

Answer: Octahedral

Explanation: Sulfur has six bonding groups and no lone pairs.

Six electron groups arrange octahedrally.

Answer: Linear

Explanation: Xe has five electron groups:

2 bonding groups 3 lone pairs

Its electron geometry is trigonal bipyramidal.

The three lone pairs preferentially occupy equatorial positions, leaving the two Xe—F bonds opposite each other.

Therefore the molecular geometry is linear.

Answer: Square planar

Explanation: Xe has six electron groups:

4 bonding groups 2 lone pairs

The electron geometry is octahedral.

The two lone pairs occupy opposite positions, leaving four F atoms in one square plane.

Answer: Nonpolar

Explanation: Each C=O bond is polar.

However, CO₂ is linear and symmetric.

The two equal bond dipoles point in opposite directions and cancel.

Therefore the molecule has no net dipole.

Answer: Polar

Explanation: The N—H bonds are polar, and NH₃ has trigonal pyramidal geometry.

Because the molecule is not symmetric enough for the dipoles to cancel, it has a net molecular dipole.

Answer: sp²

Explanation: Each carbon has three electron groups:

two C—H sigma bonds one C—C bonding region

The C=C double bond counts as one electron group for hybridization.

Three groups correspond to sp².