Bonding & Structure: Relation to Properties of Substances

What determines the amount of energy needed to change a substance's state from solid to liquid or liquid to gas?

The strength of the forces between the particles of the substance.

What is the relationship between the strength of forces between particles and a substance's melting and boiling points?

The stronger the forces between the particles, the higher the melting point and boiling point of the substance.

What structural feature of ionic compounds explains their physical properties?

They form giant ionic lattices with a regular, repeating arrangement of ions.

What type of force holds a giant ionic lattice together?

Strong electrostatic forces of attraction between oppositely charged ions.

Why do ionic compounds have very high melting and boiling points?

A large amount of energy is required to overcome the strong electrostatic forces of attraction between the ions in the giant lattice.

Why are ionic compounds solid at room temperature?

Their high melting and boiling points mean they remain in the solid state at room temperature.

What two conditions must be met for a substance to conduct electricity?

It must contain charged particles, and these particles must be free to move from place to place.

Why can ionic compounds NOT conduct electricity when solid?

The ions are held in fixed positions within the lattice and are not free to move and carry a current.

Under what two conditions CAN ionic compounds conduct electricity?

When they are melted (molten) or dissolved in water (aqueous solution).

Why does melting or dissolving an ionic compound allow it to conduct electricity?

The ions are freed from their fixed positions in the lattice and are able to move and carry an electrical charge.

In substances made of small molecules, what is overcome when the substance melts or boils?

The weak intermolecular forces between the molecules.

Why do substances composed of small molecules typically have low melting and boiling points?

Only a small amount of energy is needed to overcome the weak intermolecular forces between the molecules.

Are the strong covalent bonds within small molecules broken during melting or boiling?

No, the strong covalent bonds within the molecules are not broken during changes of state.

Why do most substances with small molecules exist as gases or liquids at room temperature?

Their low melting and boiling points mean they have already changed state at or below room temperature.

How does the size of a molecule generally affect the strength of its intermolecular forces?

As the size of the molecule increases, the strength of the intermolecular forces increases.

What is the consequence of larger molecules having stronger intermolecular forces?

Larger molecules have higher melting and boiling points because more energy is needed to overcome these forces.

Why do substances made of small molecules not conduct electricity?

The molecules have no overall electric charge, and there are no free electrons or ions to carry a current.

Why are polymers solids at room temperature?

The intermolecular forces between the large polymer chains are relatively strong, requiring more energy to overcome than in small molecules.

Why do substances with giant covalent structures have very high melting and boiling points?

A large amount of energy is needed to break the many strong covalent bonds that link all the atoms in the structure.

Why do most giant covalent structures, like diamond and silicon dioxide, not conduct electricity?

They have no free electrons or ions, as all outer electrons are held in strong covalent bonds.

What type of bonding explains the properties of metals and alloys?

Metallic bonding, which involves a giant structure of positive ions and delocalised electrons.

Why do most metals have high melting and boiling points?

A large amount of energy is required to overcome the strong metallic bonding throughout the giant structure.

Why are metals good conductors of electricity and thermal energy?

The delocalised electrons are free to move throughout the structure and can carry electrical charge or transfer thermal energy.

Why are pure metals often malleable (can be bent and shaped)?

The layers of atoms (positive ions) in the regular giant structure can slide over one another.

An _____ is a mixture containing two or more elements, where at least one is a metal.

alloy

Why are alloys harder and stronger than pure metals?

The different-sized atoms of the elements in the alloy distort the regular layers, making it more difficult for them to slide.

How does the structure of diamond account for it being extremely hard?

Each carbon atom is joined to four other carbon atoms by strong covalent bonds, forming a rigid tetrahedral network.

Why does diamond have a very high melting point?

A huge amount of energy is needed to break the millions of strong covalent bonds throughout its giant structure.

Why does diamond not conduct electricity?

All of its outer electrons are used in forming covalent bonds, so there are no delocalised electrons or free ions to carry a current.

How does graphite's structure make it soft and slippery?

Graphite consists of layers of hexagonal rings with no covalent bonds between the layers, allowing them to slide easily over one another.

Despite being soft, why does graphite have a very high melting point?

A large amount of energy is needed to break the strong covalent bonds within each layer of carbon atoms.

What feature of graphite's bonding structure allows it to conduct electricity?

Each carbon atom is bonded to only three others, leaving one delocalised electron per atom that is free to move and carry charge.

What is graphene?

A single layer of graphite, just one atom thick.

Why is graphene extremely strong and has a high melting point?

It has a large number of strong covalent bonds between its carbon atoms.

Why is graphene a good conductor of electricity?

Like graphite, it has delocalised electrons that are free to move across its surface.

What are fullerenes?

Molecules of carbon atoms with hollow shapes, often based on hexagonal rings.

Why does Buckminsterfullerene (C₆₀) have a low melting point and is slippery?

It is a simple molecular substance with weak intermolecular forces between its spherical molecules, which require little energy to overcome.

What are carbon nanotubes?

Cylindrical fullerenes, which are like a layer of graphene rolled into a tube.

Why do carbon nanotubes have high tensile strength?

They are held together by strong covalent bonds between the carbon atoms.

Why can carbon nanotubes conduct electricity?

They have delocalised electrons, similar to graphene.

How does the property of nanoparticles differ from the same material in bulk?

They have different properties due to their tiny size and very large surface area to volume ratio.

Why does the large surface area to volume ratio of nanoparticles make them effective catalysts?

It provides a much larger surface for reactions to occur on, meaning smaller quantities are needed to be effective.

Why are solids hard to compress?

Their particles are packed closely together in a regular pattern with almost no space between them.

Why do solids have a fixed shape and cannot flow?

The particles in a solid can vibrate but are held in fixed positions and cannot move from place to place.

Why are liquids difficult to compress?

Their particles are close together, leaving very little empty space to be squeezed into.

Why do liquids flow and take the shape of their container?

The particles in a liquid are free to move around and slide past each other.

Why are gases easy to compress?

The particles in a gas are very far apart, leaving large empty spaces that can be reduced.

Why do gases flow and completely fill their container?

Their particles move quickly and randomly in all directions, spreading out to occupy the entire volume.

What is the key limitation of the simple particle model regarding forces?

The simple model often assumes there are no forces between the particles, which is incorrect as these forces determine melting/boiling points.

What is the key limitation of the simple particle model regarding the shape and nature of particles?

The model represents particles as solid spheres, but in reality, they are atoms, ions, or molecules of various shapes and are not solid.

Which particles are responsible for carrying the current when molten ionic compounds conduct electricity?

The ions, which are free to move.

The melting point of MgO (2,852°C) is much higher than NaCl (801°C). Why?

The electrostatic forces between Mg²⁺ and O^2- ions are stronger than between Na⁺ and Cl^- ions due to the greater charges.

What property of silicon dioxide is due to its giant covalent structure containing millions of strong covalent bonds?

It has a very high melting and boiling point.

Why is the use of alloys often preferred over pure metals for construction?

Alloys are harder and stronger because their distorted layers are less able to slide over one another.