What are the elements, compounds, diatomic elements, and 1:1 ratio compounds? Also, complete the decomposition of two water molecules.

Here are the answers to your questions:

QUESTION 3: Intermediate

3.1) The substances that are elements are:

• A, B, D

3.2) The substances that are compounds are:

• C

3.3) A substance that is a diatomic element is:

• A, B

3.4) A compound where atoms are bonded in a fixed ratio of 1:1 is:

• C

QUESTION 4: Intermediate

The diagram shows the decomposition of two water molecules ($2\text{H}_2\text{O}$). Water decomposes into hydrogen gas ($2\text{H}_2$) and oxygen gas ($\text{O}_2$).
The balanced equation is:

$$2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$$

The diagram already shows one $\text{O}_2$ molecule (two large circles bonded) and one $\text{H}_2$ molecule (two small circles bonded). To complete the decomposition of two water molecules, a second $\text{H}_2$ molecule is needed.

The missing molecule to be drawn is:

• Two small circles bonded together (representing a hydrogen molecule, $\text{H}_2$).

Here is how you would draw it to complete the diagram:

$$\text{O} - \text{H} \quad \text{O} - \text{H} \quad \rightarrow \quad \text{O} - \text{O} \quad \text{H} - \text{H} \quad \boxed{\text{H} - \text{H}}$$

(Where the large circle is O and small circle is H)