Among the following substances, which one is a chemical compound rather than an element or a mixture: mercury, ozone, air, or ammonia?

Introduction / Context:
Distinguishing between elements, compounds, and mixtures is a core learning outcome in introductory chemistry. Elements consist of only one kind of atom, compounds are formed when atoms of different elements combine in fixed ratios, and mixtures contain two or more substances physically mixed. This question presents several familiar substances and asks which one is a compound, providing a good test of your ability to classify matter correctly.

Given Data / Assumptions:

• Mercury is a silvery liquid metal at room temperature.
• Ozone is a form of oxygen present in the atmosphere, with formula O3.
• Air is the gaseous material around us containing nitrogen, oxygen, and other gases.
• Ammonia is a pungent gas with formula NH3, used in fertilisers and cleaning agents.
• Argon is a noble gas listed as an extra option to reinforce classification.

Concept / Approach:
A compound must contain atoms of at least two different elements combined in a fixed ratio and represented by a chemical formula that includes more than one element symbol. Ammonia fits this definition because its formula is NH3, showing nitrogen and hydrogen in a fixed ratio of one nitrogen to three hydrogen atoms. Mercury is a pure metal element, symbol Hg. Ozone is an allotrope of oxygen, made of only oxygen atoms, so it is still classified as an element. Air is a mixture of different gases and has no fixed formula. Argon is another element, a noble gas. Therefore, ammonia is the only compound among the listed substances.

Step-by-Step Solution:
Step 1: Identify mercury as a metallic element with symbol Hg and atomic number 80. Step 2: Recognise ozone as O3, a form of elemental oxygen with three oxygen atoms, still an element because it contains only one type of atom. Step 3: Recall that air is a mixture of nitrogen, oxygen, and other gases in variable proportions, not a single pure substance. Step 4: Recall that ammonia has the formula NH3, which includes nitrogen and hydrogen atoms combined chemically in a fixed ratio. Step 5: Conclude that ammonia is the compound among the options.

Verification / Alternative check:
Check whether each substance can be broken down into simpler substances by chemical means only. Ammonia can be decomposed into nitrogen and hydrogen, confirming that it is a compound of two elements. Mercury cannot be broken down into simpler substances by chemical methods because it is an element. Ozone, though chemically different from O2, is still a form of elemental oxygen and can only be converted into other forms of oxygen. Air can be separated into its components by physical methods such as liquefaction and fractional distillation, which is typical of mixtures. Argon is a noble gas element that does not form a simpler substance by chemical means.

Why Other Options Are Wrong:
Mercury is an element, not a compound, because it consists of only one type of atom. Ozone, O3, is an allotrope of oxygen and is therefore also an element, not a compound. Air is a mixture of several gases and does not have a fixed composition or formula, so it is neither an element nor a compound. Argon is a noble gas element. These substances do not meet the definition of a compound containing more than one element in fixed proportion.

Common Pitfalls:
A frequent mistake is to think that any substance with more than one atom in its formula, such as O3, must be a compound. The correct rule is that a compound must contain atoms of different elements. Another confusion is between air and compounds such as ammonia, because both may be colourless gases at room temperature. To avoid these errors, always look at the chemical formula and ask whether more than one element symbol is present and whether the composition is fixed.

Final Answer:
Among the given substances, the compound is ammonia (NH3).