Bonding and structures

Explain why substances with giant ionic structures are often brittle but metallic structures are malleable.

Because shifting layers bring like-charged ions together, they repel one another and break, making ionic formations brittle. On the other hand, delocalized electrons in metallic structures make them malleable, enabling atoms to move past one another without rupturing bonds. Thus, metals may be twisted and molded, while ionic combinations break.

Explain why giant molecular structures have higher melting points than simple molecular structures.

Larger molecules typically have greater boiling points as their relative molecular mass increases. This is because there are higher forces of attraction between larger molecules, or molecules with more mass. Larger molecules have more attractions that need to be overcome, and even if these forces are small, they must be overcome if the substance is to boil.

Diamond and graphite are two forms of carbon with giant molecular structures. Explain why graphite conducts electricity but diamond does not.

Electricity is conducted by graphite. There is only one free electron per carbon atom because each one is connected to three others. Between the layers are these unbound (delocalized) electrons. They carry charge and are free to move about the building.

Diamond is not electrically conductive. Each carbon atom is surrounded by four covalent bonds that hold all of its outer shell electrons. There are therefore no particles that are free to carry a charge.

The effect of alloying copper with zinc on the strength of the alloy is shown in the table below

i. Describe and explain the change in tensile strength as the percentage of zinc increases from 0% to 40%.

ii. State the name of the alloy of copper with zinc.

i. In an alloy of copper and zinc, the alloy becomes stronger when the percentage of zinc increases from 0% to 40% while it is easily noticed that the tensile strength of pure Cu is 2.3 and of pure Zn is 1.4. Thus, zinc ions are larger than copper ions and the presence of different-sized metal ions makes the arrangement of the lattice less regular. This stops the layers of ions from sliding over each other so easily when a force is applied.

ii. Brass is an alloy of copper and zinc.

Crystals of sodium chloride have a lattice structure.

a. Describe a sodium chloride lattice

b. Explain the following properties of sodium chloride.

i. Sodium chloride has a high melting point.

ii. Sodium chloride conducts electricity when molten but not when solid.

iii. Sodium chloride is hard but brittle.

a. Sodium chloride forms an ionic lattice due to the electrostatic attraction between sodium ions (Na⁺) and chloride ions (Cl⁻). This arrangement leads to a highly organized three-dimensional structure.

b.

i. Sodium chloride has a high melting point due to the strong ionic bonds that need to be overcome for the solid to transition to a liquid state.

ii. While NaCl does not conduct electricity in its solid state, it does conduct when dissolved in water or melted, as the ions are free to move.

iii. Ionic compounds, such as NaCl, are brittle. When stress is applied, the ions shift, and like-charged ions come into alignment, resulting in repulsion that leads to breaking rather than bending.

The diagram shows some allotropes of carbon.

i. Give the name of this  allotrope, which has the formula C₆₀.

ii. Explain in terms of structure and bonding why this structure is gaseous at 800 °C but diamond is not.

i. buckminsterfullerene

ii. This structure has a relatively low sublimation point: it turn directly from the solid to the vapour state when heated to about 600 ^oC. Thus, it is in the gaseous state at 800 ^oC. This is because there are van de Waals’ forces between each buckminsterfullerene molecules whereas in diamond, there is not this phenomenon occurring because there is a strong covalent bonding throughout the whole structure leading a lot of energy to be needed to break these strong bonds and separate atoms. Therefore, this structure is gaseous at 800 °C but diamond is not.

The structure below shows an allotrope of carbon in the form of tubes.

i. Give the name of this allotrope of carbon.

ii. Describe the similarities and differences between this structure and graphite.

i. This allotrope of carbon is fullerenes of hexagonally arranged carbon atoms like a single layer of graphite bent into the form of a cylinder.

ii.

The similarities:

• Both have high electrical conductivity because electrons are delocalised and are able to move along when a voltage is applied
• Both have high melting points because there is a strong covalent bonding throughout the structure

The difference:

This structure has a very high tensile strength when a force is applied along the long axis of the cylinder. They can be up to 100 times stronger than steel of the same thickness, whereas graphite does not.

Which solid has a giant covalent lattice structure?

A. Calcium fluoride

B. Nickel

C. Silicon (IV) dioxide

D. Sulfur

The answer is C.

Silicon dioxide or silica is the main component of sand. Both are non-metals bonded together with covalent bonds

A is incorrect because calcium fluoride is an ionic compound.

B is incorrect because nickel is a metal with metallic bonding.

D is incorrect because sulfur is found as a cyclic S₈ molecule

Which chemical has the highest melting point?

A. Sulfur 

B. Calcium oxide 

C. Lithium 

D. Sodium fluoride

The answer is B.

Ionic compounds usually have higher melting points than metallic and covalent structures.

A is incorrect because sulfur is a covalent material

C is incorrect because lithium is a metallic material

D is incorrect because the ions in sodium fluoride have a smaller charge than those in calcium oxide leading to weaker force of attraction between the ions which requires less energy to overcome

Which diagram represents part of a giant covalent lattice structure?

A. 1, 2 and 3

B. 1 and 2

C. 1 and 3

D. 3 only

The answer is C.

The structure in option 1 is graphite which has strong covalent bonds with 3 other carbon atoms, making it create a giant layered structure with delocalised electrons. Thus, it has a giant covalent lattice structure.

The structure in option 2 is the structure of sodium chloride, a giant ionic structure.

The structure in option 3 is represented by the diamond which has strong covalent bonds with 4 other carbon atoms, making it generate a regular tetrahedral structure with no free electrons. Thus, it has a giant covalent lattice structure.