--- title: "First-row configuration; variable oxidation states" subject: "Engineering Chemistry" chapter: 5 key: "semester_2/engineering_chemistry:config-variable-oxidation" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/config-variable-oxidation site: BCT Engineering --- # First-row configuration; variable oxidation states Engineering Chemistry · Chapter 5: 3-d Transition Elements and their Applications - Asked in: 2072 Magh, 2+3 marks - Video: [First Row d-Block Transition Elements](https://www.youtube.com/watch?v=AoJyOLQrsBU) — Science with Dr. Johnson - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/config-variable-oxidation - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-5 ## Question Write the ground state electronic configuration of the first row transition metals. How would you explain the variable oxidation states of transition metals? ## Short answer Ground configurations: Sc 3d1 4s2, Ti 3d2 4s2, V 3d3 4s2, Cr 3d5 4s1, Mn 3d5 4s2, Fe 3d6 4s2, Co 3d7 4s2, Ni 3d8 4s2, Cu 3d10 4s1, Zn 3d10 4s2. Variable oxidation states arise because 4s and 3d electrons have similar energies, so both can be lost in bonding, giving multiple stable ions. ## Answer #### Ground state configurations (3d series) ```text Sc [Ar] 3d¹ 4s² Fe [Ar] 3d⁶ 4s² Ti [Ar] 3d² 4s² Co [Ar] 3d⁷ 4s² V [Ar] 3d³ 4s² Ni [Ar] 3d⁸ 4s² Cr [Ar] 3d⁵ 4s¹ Cu [Ar] 3d¹⁰ 4s¹ Mn [Ar] 3d⁵ 4s² Zn [Ar] 3d¹⁰ 4s² ``` Cr and Cu are exceptions: a half-filled (d⁵) or completely filled (d¹⁰) 3d subshell is extra stable, so one 4s electron moves into 3d. #### Variable oxidation states Variable oxidation states arise because the 4s and 3d electrons have very similar energies, so both 4s and some or all 3d electrons can be used in bonding. The states differ by one unit (Fe²⁺, Fe³⁺), unlike the p-block where they differ by two. ```text Ti +2 +3 +4 Fe +2 +3 V +2 +3 +4 +5 Co +2 +3 Cr +2 +3 +6 Ni +2 Mn +2 +3 +4 +6 +7 Cu +1 +2 ``` - Minimum oxidation state is usually +2 (loss of the two 4s electrons); the maximum equals the total number of 4s + 3d electrons, reached at Mn (+7). - Stability of higher states falls after Mn because the nuclear charge holds the 3d electrons more tightly. - Half-filled d⁵ (Mn²⁺, Fe³⁺) and filled d¹⁰ (Zn²⁺) are specially stable. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the ground state configuration of Cr? A. [Ar] 3d4 4s2 B. [Ar] 3d6 4s0 C. [Ar] 3d3 4s2 D. [Ar] 3d5 4s1 Answer: D. One 4s electron moves into 3d to give the extra stable half-filled d5 subshell. (Key point: Cr configuration) 2. Why are Cr and Cu exceptions to the expected filling order? A. Half-filled (d5) or completely filled (d10) 3d is extra stable, so a 4s electron moves to 3d B. 4s is higher in energy than 4p C. Their 3d orbitals are empty D. Electrons repel more in 4s than in 3d for all elements Answer: A. The extra stability of d5 and d10 drives one 4s electron into 3d. (Key point: Exceptions) 3. Why do 3d elements show variable oxidation states? A. Only the 4s electrons are available B. 4s and 3d electrons have similar energies, so both can be used in bonding C. 3d electrons are lost before 4s electrons in all cases D. They have no d electrons in compounds Answer: B. Similar energies let some or all 3d electrons join the 4s electrons in bonding. (Key point: Reason for variability) 4. Where is the maximum oxidation state reached in the 3d series, and what does it equal? A. At Zn (+12), equal to its atomic number minus 18 B. At Fe (+8), equal to its group number C. At Mn (+7), equal to the total 4s + 3d electrons D. At Sc (+7), equal to its 4s electrons Answer: C. Maximum equals the total of 4s + 3d electrons up to Mn; later it falls because the 3d electrons are held more tightly. (Key point: Maximum state) ## Related in Engineering Chemistry - [Characteristics of transition series; first-row configuration](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-characteristics-config) — Engineering Chemistry, chapter 5 - [Variable oxidation states of 3d elements](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avariable-oxidation-3d) — Engineering Chemistry, chapter 5, asked 6× - [Transition elements; oxidation states of 3d; alloys](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-ox-alloy) — Engineering Chemistry, chapter 5 - [Variable oxidation states; industrial uses of 3d elements](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avariable-ox-applications) — Engineering Chemistry, chapter 5 - [General outer electronic configuration of transition elements](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atm-general-config) — Engineering Chemistry, chapter 5 - [Transition elements; first series configuration](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Awhy-transition-elements-typical) — Engineering Chemistry, chapter 5