Structure Of Atom

Step 1: Understanding Atomic Mass Unit.
The unit of atomic mass (amu) is now replaced by 'u', which stands for unified mass. The unified atomic mass unit (u) is defined as one twelfth of the mass of a carbon-12 atom.

Step 2: Analyzing the options.
(A) Unified mass: Correct. This is the standard unit used now to measure atomic mass.
(B) United mass: Incorrect. There is no such term used in atomic mass measurements.
(C) Unique mass: Incorrect. This term is not used in the context of atomic mass.
(D) Universal mass: Incorrect. This is not the correct terminology.

Step 3: Conclusion.
The correct term for 'u' is Unified mass, corresponding to option (A).

Concept:
The reduction of metal ions to their elemental form is a fundamental concept in electrochemistry. In this case, we are considering the reduction of chromium ions (Cr³⁺) to solid chromium metal (Cr_(s)).

Reduction Half-Reaction:
The balanced half-reaction for this process is:
Cr³⁺_(aq) + 3e⁻ Cr_(s)

This equation tells us that for every mole of Cr³⁺ ions reduced, 3 moles of electrons are required. Electrons act as reducing agents by supplying the negative charge needed to neutralize the positive charge on the Cr³⁺ ion, converting it to its metallic (zero oxidation state) form.

Explanation:
• Chromium in the +3 oxidation state (Cr³⁺) gains 3 electrons.
• This electron gain reduces Cr³⁺ to Cr_(s), which is the metallic form of chromium.
• The number of electrons added must match the charge of the ion being reduced to balance both mass and charge.

Conclusion:
Since the reduction of one mole of Cr³⁺ requires exactly 3 moles of electrons, the correct and balanced stoichiometric value is 3.

Therefore, the correct answer is: 3.

Solution:
To identify the oxidizing agent, let’s analyze the redox changes in the reaction:

Reaction: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)

• Fe(s) changes to Fe²⁺: it loses 2 electrons → oxidation
• Cu²⁺ changes to Cu(s): it gains 2 electrons → reduction

According to redox principles:
• The species that gets reduced is the oxidizing agent because it causes oxidation of the other.
• Since Cu²⁺ gains electrons and gets reduced, it acts as the oxidizing agent.