In basic inorganic chemistry, which of the following species is not a cation (positively charged ion)?

Introduction / Context:
Ions are charged species formed when atoms or groups of atoms gain or lose electrons. Cations carry a positive charge, while anions carry a negative charge. Recognising common cations and anions is essential for writing chemical formulas, understanding electrolysis, and predicting reaction products. This question asks you to identify which species among the options is not a cation, i.e., which one is actually an anion.

Given Data / Assumptions:

• Aluminium ion (Al3+) is a common metal cation.
• Copper ion (Cu2+ or Cu+) is also a metal cation.
• Sulphate ion (SO4^2-) is a polyatomic ion containing sulphur and oxygen.
• Zinc ion (Zn2+) is another metal cation.
• We assume the usual oxidation states for these species.

Concept / Approach:
Cations are positively charged; they have more protons than electrons. Metal atoms like aluminium, copper, and zinc typically lose electrons to form cations such as Al3+, Cu2+, and Zn2+. The sulphate ion, SO4^2-, is a polyatomic anion with a net negative charge of minus two. It is formed when sulphur and oxygen atoms combine and gain electrons relative to their neutral state. Therefore, the only species in the list that is not a cation is the sulphate ion, which is an anion.

Step-by-Step Solution:
Step 1: Identify the charges on the metallic ions. Aluminium ion typically exists as Al3+, a positive ion (cation). Copper ion usually exists as Cu2+ or Cu+, both positive cations. Zinc ion is Zn2+, another positive cation. Step 2: Examine the sulphate ion. Sulphate is written as SO4^2-, indicating a 2- negative charge. Step 3: Compare charges to determine which is not a cation. SO4^2- is an anion (negatively charged), unlike the others, which are cations.

Verification / Alternative check:
Look at common salts containing these ions. Aluminium chloride (AlCl3), copper sulphate (CuSO4), and zinc nitrate (Zn(NO3)2) all have metal cations (Al3+, Cu2+, Zn2+) paired with anions like chloride, sulphate, or nitrate. The presence of sulphate in salts such as CuSO4 or Na2SO4 shows it pairing with positive ions, confirming that sulphate carries a negative charge. It is never written as a positive ion in basic inorganic chemistry. This standard pairing in ionic compounds confirms that sulphate is the only anionic species in the list.

Why Other Options Are Wrong:
Option A (Aluminium ion): Aluminium forms Al3+, clearly a cation used in many salts and complexes. Option B (Copper ion): Copper forms Cu+ or Cu2+, both positively charged cations in common compounds. Option D (Zinc ion): Zinc typically forms Zn2+, a standard cation in many inorganic salts.

Common Pitfalls:
Students sometimes confuse polyatomic ions like sulphate, carbonate, and nitrate with neutral molecules because they contain several atoms. Another common error is to assume that any ion mentioned without an explicit charge is a cation. To avoid this, remember that most simple metal ions are cations, while many multi atom groups containing non metals, like SO4, CO3, and NO3, are anions with negative charges. Practising with common salt formulas such as Na2SO4, CaCO3, and KNO3 reinforces this pattern.

Final Answer:
The species that is not a cation is the Sulphate ion, which is an anion (SO4^2-).