Element Cr goes from Cr --> Cr³⁺ element Fe goes from Fe³⁺ --> Fe²⁺ oxidation reactions are when the species gives out electrons and its oxidation number increases. reduction reactions are when the species take in electrons and its oxidation number reduces. Cr : oxidation number of Cr - 0 and Cr³⁺ - +3 Theres an increase in oxidation number from 0 to +3. Therefore its an oxidation reaction. Cr gets oxidised. Fe : oxidation number of Fe³⁺ - +3 and Fe²⁺ - +2 There's a decrease in oxidation number of Fe from +3 to +2, therefore its a reduction reaction and Fe gets reduced.
Cr reduces Fe - Cr is the reducing agent Fe oxidises Cr - Fe is the oxidising agent
Elements with complete electron shells are most likely noble gases which have 8 valence electrons in their outer shell. So the letter farthest to the right should be the correct answer. Noble gases are found in group 18
