Consider the diagrams of four different substances below: P represents a mono-atomic element, Q a diatomic element, R a compound, and S a mixture. Identify pure substances, a diatomic element, a mono-

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2.4 Consider the diagrams of four different substances below:

• P: Represents a mono-atomic element (individual identical atoms).
• Q: Represents a diatomic element (identical molecules, each with two identical atoms).
• R: Represents a compound (identical molecules, each with two different types of atoms).
• S: Represents a mixture of two different elements (individual small atoms and diatomic larger atoms).

2.4.1 Identify each of the following:

• a) Pure substances.

  • Pure substances are elements or compounds.
  • Answer: $\boxed{\text{P, Q, R}}$

• b) A diatomic element.

  • A diatomic element consists of molecules made of two identical atoms.
  • Answer: $\boxed{\text{Q}}$

• c) A mono-atomic element.

  • A mono-atomic element consists of individual identical atoms.
  • Answer: $\boxed{\text{P}}$

• d) A mixture.

  • A mixture contains different types of particles not chemically bonded together.
  • Answer: $\boxed{\text{S}}$

• e) A compound consisting of two different elements.

  • A compound consists of molecules made of two or more different types of atoms chemically bonded.
  • Answer: $\boxed{\text{R}}$

2.4.2 Which one of P, Q or R represents each of the following the best?

• a) Carbon monoxide (CO)

  • CO is a compound made of one carbon atom and one oxygen atom. Diagram R shows molecules made of two different types of atoms.
  • Answer: $\boxed{\text{R}}$

• b) Helium atoms

  • Helium (He) is a noble gas and exists as individual atoms. Diagram P shows individual atoms.
  • Answer: $\boxed{\text{P}}$

• c) Oxygen gas molecules

  • Oxygen gas (O$_2$) is a diatomic element. Diagram Q shows diatomic molecules of the same type of atom.
  • Answer: $\boxed{\text{Q}}$

2.4.3 For diagram S, which one of the following statements is TRUE?

• Diagram S shows individual small atoms and diatomic molecules of larger atoms, indicating two different elements mixed together.
• A: It is a mixture.
• B: It is a pure substance.
• C: It is an element.
• D: It is a compound.
• The correct statement is that it is a mixture.
• Answer: $\boxed{\text{A}}$

2.5 Consider the diagram:
The diagram shows 5 identical molecules. Each molecule consists of one central dark atom bonded to four lighter atoms.

2.5.1 How many molecules can you identify in the diagram?

• Count the distinct groups of atoms. There are 5 such groups.
• Answer: $\boxed{\text{5}}$

2.5.2 How many atoms are there in one molecule?

• Each molecule has 1 central dark atom and 4 lighter atoms.
• Total atoms per molecule = $1 + 4 = 5$.
• Answer: $\boxed{\text{5}}$

2.5.3 How many different elements are represented in the diagram?

• There are two types of atoms (dark and light), representing two different elements.
• Answer: $\boxed{\text{2}}$

2.5.4 How many atoms are there in TOTAL represented in the diagram?

• There are 5 molecules, and each molecule has 5 atoms.
• Total atoms = $5 \text{ molecules} \times 5 \text{ atoms/molecule} = 25 \text{ atoms}$.
• Answer: $\boxed{\text{25}}$

2.5.5 Choose the correct answer: (Based on the structure shown, one central atom bonded to four others)

• The diagram represents a molecule with one central atom bonded to four other atoms. This structure is characteristic of methane (CH$_4$).
• Answer: $\boxed{\text{CH}_4}$