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Lesson 1→First Transition Series, Electronic Configurations and Economic Significance

├── Where they are in the periodic table

├── The main transition elements (d-block)

├── The four transition series

├── Economic importance of the first-series elements

├── Electron configurations of the first series

├── Rules for filling

├── Configurations of the common ions

├── Why Fe(II) is easily oxidized to Fe(III) but Mn(II) is not

├── Oxidation-state rules for the series

├── Calculating oxidation states in compounds

└── Definition of a transition element

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 1 / 13

Where they are in the periodic table

In second secondary you studied the s-block and p-block elements, which sit on the two sides of the long form of the periodic table.
Between them lie the transition elements, in the middle of the table.
They number more than 60, which is over half of all known elements.
They are divided into:

Main transition elements(d-block elements).

Inner transition elements(f-block: lanthanides and actinides) — not studied here.

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 2 / 13

The main transition elements (d-block)

In these elements the electrons enter the d sublevel one after another.
Because a d sublevel holds at most 10 electrons (five orbitals, two electrons each), the block has ten vertical columns.
The first column ends its configuration in ns² (n−1)d¹ and the last in ns² (n−1)d¹⁰, so the d sublevel is filled in sequence from left to right.
The columns are numbered from left to right IIIB (3), IVB (4), VB (5), VIB (6), VIIB (7), then group VIII, which is made of three vertical columns (8, 9, 10), then IB (11) and IIB (12).

The three columns of group VIII differ from the other B groups: their elements resemble each other horizontally (for example Fe, Co, Ni) more than they resemble the elements below them in the same column.
Groups IB and IIB (Cu/Ag/Au and Zn/Cd/Hg) are, from one point of view, not considered true transition elements (see the definition in Lesson 2).

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 3 / 13

The four transition series

Series	Sublevel being filled	Period	Starts with	Ends with
First	3d	4 (after Ca)	Sc (4s² 3d¹)	Zn (4s² 3d¹⁰)
Second	4d	5	Y (5s² 4d¹)	Cd (5s² 4d¹⁰)
Third	5d	6	La (6s² 5d¹)	Hg (6s² 4f¹⁴ 5d¹⁰)
Fourth	6d	7	(incomplete series)	—

Each complete series contains ten elements, because there are ten places in the d sublevel.
We study in detail the first transition series scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and zinc (Zn).

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 4 / 13

Economic importance of the first-series elements

Together these ten elements make up about 7 % of the mass of the Earth’s crust (iron alone is about 5 %; the others are present in very small percentages, from about 0.003 % for scandium to under 1 % for each of the rest), yet their importance is very large.

Comparison aspect	Main idea	Keywords
Scandium (Sc)	found in very small amounts spread over wide areas. A little scandium added to aluminium gives a light very hard alloy used for fighter-jet parts. It is added to mercury-vapour lamps to give light close to sunlight so it is used for night television photography	foundverysmallamounts
Titanium (Ti)	as strong as steel but less dense. Its alloys with aluminium keep their strength at high temperature (where aluminium alone softens) so they are used in aircraft and space shuttles. Because titanium is chemically inert and the body does not reject it or get poisoned by it it is used in dental implants and artificial joints. Titanium dioxide (TiO₂) is used in sun-protection cosmetics: its nanoparticles block ultraviolet rays	strongsteellessdense
Vanadium (V)	adding a little vanadium to steel gives a very hard alloy that resists corrosion used for car springs. Vanadium(V) oxide (V₂O₅) is used as a pigment in ceramics and glass as a catalyst and in making strong magnetic materials	addinglittlesteelgives
Chromium (Cr)	an active metal yet it resists attack by air. Its oxide film has a larger molecular volume than the metal atoms it forms from so it is a continuous non-porous layer that stops further reaction with oxygen. Chromium is used in plating and in steel for pipes and surgical tools. Chromium(III) oxide (Cr₂O₃) is used in pigments potassium dichromate (K₂Cr₂O₇) is an oxidizing agent	activemetalresistsattack
Manganese (Mn)	brittle so it is never used as the pure metal but in alloys or compounds. Ferromanganese is harder than steel and is used for railway tracks aluminium–manganese alloys make drink cans. MnO₂ is a strong oxidizing agent used in dry cells KMnO₄ is an antiseptic MnSO₄ is a fungicide	brittleneverusedpure
Iron (Fe)	the most important metal in heavy industry. Finely divided iron is the catalyst of the Haber–Bosch process for ammonia and of the Fischer–Tropsch conversion of water gas (CO + H₂) into fuel	mostimportantmetalheavy
Cobalt (Co)	like iron it can be magnetized so it is used for magnets it is used in modern car batteries. Cobalt has twelve radioactive isotopes cobalt-60 is the most important: it gives gamma rays of high penetrating power and is used to preserve food test the quality of industrial products detect cracks and weak welds and diagnose and treat tumours	likeironmagnetizedused
Nickel (Ni)	used in rechargeable nickel–cadmium batteries forms hard steels that resist rust and acids nickel–chromium alloys make heating coils and electric furnaces because they resist corrosion at high temperature used to plate other metals (to protect them from oxidation and give a good appearance) finely divided nickel is the catalyst for hydrogenation of oils	usedrechargeablecadmiumbatteries
Copper (Cu)	the first metal discovered by humans copper–tin alloy is bronze. Being an excellent conductor it is used for electric cables and coins. CuSO₄ is an insecticide and fungicide and is used in water purification. Fehling’s solution (a copper(II) compound) detects glucose: its blue colour changes to orange/red	firstmetaldiscoveredhumans
Zinc (Zn)	used to galvanize iron against rusting. ZnO is used in paints rubber and cosmetics ZnS is used in luminous paints and X-ray screens	usedgalvanizeironagainst

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 5 / 13

Electron configurations of the first series

All have the core [₁₈Ar] and
4s and 3d electrons:

Element	Group	Configuration	Common oxidation states (the book underlines the most stable one)	Example compounds
₂₁Sc	IIIB	[Ar] 4s² 3d¹	+3	Sc₂O₃
₂₂Ti	IVB	[Ar] 4s² 3d²	+2, +3, +4	TiO, Ti₂O₃, TiO₂
₂₃V	VB	[Ar] 4s² 3d³	+2, +3, +4, +5	VO, V₂O₃, VO₂, V₂O₅
₂₄Cr	VIB	[Ar] 4s¹ 3d⁵	+2, +3, +6	CrO, Cr₂O₃, CrO₃
₂₅Mn	VIIB	[Ar] 4s² 3d⁵	+2, +3, +4, +6, +7	MnO, Mn₂O₃, MnO₂, K₂MnO₄, KMnO₄
₂₆Fe	VIII	[Ar] 4s² 3d⁶	+2, +3 (+6 rare)	FeO, Fe₂O₃, Na₂FeO₄
₂₇Co	VIII	[Ar] 4s² 3d⁷	+2, +3	CoCl₂, CoCl₃
₂₈Ni	VIII	[Ar] 4s² 3d⁸	+2, +3	NiO, Ni₂O₃
₂₉Cu	IB	[Ar] 4s¹ 3d¹⁰	+1, +2	Cu₂O, CuO
₃₀Zn	IIB	[Ar] 4s² 3d¹⁰	+2	ZnO

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 6 / 13

Rules for filling

Comparison aspect	Main idea	Keywords
The series starts after calcium (₂₀Ca	[Ar] 4s²). The five 3d orbitals then receive electrons one per orbital (Hund’s rule: electrons occupy empty orbitals of the same sublevel singly before pairing) up to Mn (3d⁵). After Mn electrons pair up in the orbitals until Zn (3d¹⁰)	fiveorbitalsthenreceive
Chromium and copper are exceptions.	Expected Cr: 4s² 3d⁴ and Cu: 4s² 3d⁹ but the actual configurations are Cr: 3d⁵ 4s¹ and Cu: 3d¹⁰ 4s¹. In chromium both 4s and 3d are half filled in copper 4s is half filled and 3d is completely filled. A half-filled (d⁵) or completely filled (d¹⁰) sublevel gives the atom lower energy hence extra stability. (This is a major factor though it is not the only one.)	Expectedactualconfigurations3d¹⁰

A picture of the 3d orbitals (each box is one orbital
arrows are electrons):

Sc 3d¹[↑][ ][ ][ ][ ] — Ti 3d²: [↑][↑][ ][ ][ ] — V 3d³: [↑][↑][↑][ ][ ]

Cr 3d⁵ (with 4s¹)[↑][↑][↑][↑][↑] — Mn 3d⁵ (with 4s²): [↑][↑][↑][↑][↑]

Fe 3d⁶[↑↓][↑][↑][↑][↑] — Co 3d⁷: [↑↓][↑↓][↑][↑][↑] — Ni 3d⁸: [↑↓][↑↓][↑↓][↑][↑]

Cu 3d¹⁰[↑↓][↑↓][↑↓][↑↓][↑↓] — Zn 3d¹⁰: the same.

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 7 / 13

Configurations of the common ions

In forming positive ions the atom loses the 4s electrons first, and only then 3d electrons (the 4s and 3d sublevels are close in energy, and once the 3d sublevel starts to fill, the 4s electrons are the outer, easiest to remove).
Examples:

Ion	Configuration	Remarks
Sc³⁺	[Ar] (3d⁰)	has the stable argon structure the reason why Sc forms only +3 and never Sc²⁺ (losing only the two 4s electrons would leave 3d¹ which is easily lost)
Ti⁴⁺	[Ar] 3d⁰	in TiO₂
V⁵⁺	[Ar] 3d⁰	in V₂O₅
Cr³⁺	[Ar] 3d³	3 unpaired electrons
Mn²⁺	[Ar] 3d⁵	half-filled, stable
Fe²⁺	[Ar] 3d⁶	4 unpaired electrons
Fe³⁺	[Ar] 3d⁵	half-filled, 5 unpaired electrons
Co²⁺	[Ar] 3d⁷	3 unpaired electrons
Ni²⁺	[Ar] 3d⁸	2 unpaired electrons
Cu⁺	[Ar] 3d¹⁰	filled sublevel
Cu²⁺	[Ar] 3d⁹	1 unpaired electron (so Cu is a transition element)
Zn²⁺	[Ar] 3d¹⁰	filled sublevel

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 8 / 13

Why Fe(II) is easily oxidized to Fe(III) but Mn(II) is not

Comparison aspect	Main idea	Keywords
₂₆Fe	[Ar] 3d⁶ 4s². Fe²⁺ is 3d⁶ Fe³⁺ is 3d⁵ — half-filled more stable. So Fe²⁺ readily loses one more electron to give the more stable Fe³⁺ (the reaction goes towards the more stable ion)	halffilledstablereadily
₂₅Mn	[Ar] 3d⁵ 4s². Mn²⁺ is already 3d⁵ (half-filled stable). Mn³⁺ would be 3d⁴ (less stable). So Mn²⁺ is not readily oxidized to Mn³⁺	alreadyhalffilledstable

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 9 / 13

Oxidation-state rules for the series

All elements of the series (except Sc) show the +2 state, formed by loss of the two 4s electrons first. Higher states are reached by losing 3d electrons in turn.
The highest oxidation state rises from +3 (Sc) to +7 in Mn (group VIIB), then falls gradually to +2 in Zn. The maximum state generally does not exceed the group number (except group IB: Cu, Ag, Au).
it does not reach the maximum corresponding to removing all eight electrons of 4s and 3d. States above +3 are unimportant for iron. Its +2 state comes from losing the two 4s electrons and its +3 state corresponds to a half-filled 3d⁵ sublevel (stable).
Transition elements show several oxidation states, while representative metals show one. The reason is that the 4s and 3d sublevels are close in energy, so electrons are removed from both in sequence and the successive ionization energies rise only gradually. (For vanadium, 4s² 3d³, the successive ionization energies of the five outer electrons increase smoothly, giving V²⁺ up to V⁵⁺; the sixth ionization energy shows a very sharp jump, because that electron would have to be taken from the stable argon core, so +5 is the maximum.)

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 10 / 13

Calculating oxidation states in compounds

The algebraic sum of the oxidation numbers equals the charge on the species (zero for a neutral compound); oxygen is −2, alkali metals +1.
Worked example 1. Find the oxidation number of the transition metal in (a) K₂Cr₂O₇, (b) KMnO₄, (c) K₂MnO₄, (d) V₂O₅, (e) Fe₃O₄.
(a) 2(+1) + 2x + 7(−2) = 0 → 2x = 12 → Cr = +6.
(b) +1 + x + 4(−2) = 0 → Mn = +7.
(c) 2(+1) + x + 4(−2) = 0 → Mn = +6.
(d) 2x + 5(−2) = 0 → V = +5.
(e) 3x + 4(−2) = 0 → x = +8/3 on average; the formula is the mixed oxide FeO·Fe₂O₃, containing one Fe(II) and two Fe(III) (+2 + 2×3 = +8 for three atoms).
In (a), (b) and (d) the metal has lost all its 4s and 3d valence electrons (it is d⁰), so those compounds are diamagnetic and do not themselves absorb visible light through d–d transitions; they are often strongly oxidizing since the metal is in its highest state.

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Definition of a transition element

A transition element is an element whose d or f orbitals contain electrons but are not completely filled, either in its atomic state or in one of its oxidation states.
Coinage metals Cu, Ag, Au (3d¹⁰4s¹, 4d¹⁰5s¹, 5d¹⁰6s¹) have a full d sublevel in the atom, but in the +2 (Cu) or +3 (Au) states d becomes d⁹ or d⁸ (incomplete), so they are transition elements.
Zn, Cd, Hg (d¹⁰ in the atom and in the +2 ion) never have an incomplete d sublevel, so they are not transition elements.
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Part 13

📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

Lesson 1 – First Transition Series, Electronic Configurations and Economic Significance
Detailed Academic Lesson Overview:
Transition elements are defined as elements where the (d) or (f) subshell contains unpaired electrons in their atomic or common oxidation states.
The First Transition Series (Sc21 to Zn30) fills the 3d sublevel and possesses strategic industrial significance in metallurgy and catalysis.

🌳 Hierarchy Classification & Conceptual Roadmap

First Transition Series (3d)

Comparison aspect	Main idea	Keywords
•	├── 1. Electronic Configurations (4s² 3d¹ to 4s² 3d¹⁰)	ElectronicConfigurations3d¹⁰
│ ├── Anomalous Configurations	24Cr ([Ar] 4s¹ 3d⁵) and 29Cu ([Ar] 4s¹ 3d¹⁰)	24Cr29Cu3d¹⁰
│ └── Variable Oxidation States	Loses 4s electrons first then 3d progressively	Loseselectronsfirstthen

├── 2. Economic Significance & Metallurgy

│ ├── Scandium (Sc)Sc-Al alloy for MiG aircraft, mercury-vapor stadium lamps

│ ├── Titanium (Ti)High-strength alloys with Aluminum for spacecraft, dental implants

│ ├── Vanadium (V)Steel alloys for car springs; V2O5 catalyst in sulfuric acid contact process

│ └── Chromium (Cr)Leather tanning and metal plating due to protective passive oxide layer

└── 3. Iron, Cobalt, Nickel, Copper & Zinc

Comparison aspect	Main idea	Keywords
├── Iron (Fe) & Cobalt (Co)	Magnets rechargeable batteries Co-60 radiation source	Magnetsrechargeablebatteriesradiation
├── Nickel (Ni)	Ni-Cd rechargeable batteries hydrogenation catalyst electroplating	rechargeablebatterieshydrogenationcatalyst
├── Copper (Cu)	Electrical wiring bronze alloy (Cu + Sn) Fehling's reagent for glucose	Electricalwiringbronzealloy
└── Zinc (Zn)	Galvanization to prevent iron corrosion ZnO in paints ZnS in luminous screens	Galvanizationpreventironcorrosion
•	📋 Master Teacher's Blackboard Rules & Exam Secrets	MasterTeacherBlackboardRules
RULE 1	Sc possesses only one oxidation state (+3) forming [Ar] with empty 3d subshell	possessesoxidationstateforming
RULE 2	and Hg are NOT considered transition elements because their (d) sublevel is completely filled (d¹⁰) in both atomic state and only oxidation state (+2)	consideredtransitionelementsbecause
RULE 3	Anomalous configurations of 24Cr and 29Cu arise because half-filled (d⁵) and completely filled (d¹⁰) sublevels confer extra thermodynamic stability	Anomalousconfigurations24Cr29Cu
EXAM TRAP	The maximum oxidation state (+7 in Manganese) never exceeds the group number (Group VIIB) but Group 1B (coinage metals Cu Au) exhibits oxidation states exceeding their group number (+2 and +3)	maximumoxidationstateManganese
📊 Detailed Comparison Matrix	Economic Importance and Catalyst Applications of First Transition Series	EconomicImportanceCatalystApplications

Element	Key Industrial Alloy / Compound	Catalytic / Practical Application
Scandium (Sc)	Sc-Al alloy (Lightweight & extreme hardness)	MiG fighter jet frames & stadium lamps
Titanium (Ti)	Ti-Al alloy (Resists extreme heat)	Spacecraft & artificial joint/dental implants
Vanadium (V)	Ferro-vanadium steel	Car spring manufacture & V₂O₅ in Contact Process
Chromium (Cr)	Nichrome alloy (Ni-Cr) & Stainless steel	Heating coils & leather tanning / corrosion barrier
Iron (Fe)	Steel (Fe-C) & Sponge iron	Haber-Bosch ammonia synthesis & Fischer-Tropsch

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance · 13 / 13

Curriculum

Chemistry

Chapter 1 - Transition Elements and Iron Metallurgy

Chapter 2 - Chemical Analysis

Chapter 3 - Chemical Equilibrium

Chapter 4 - Electrochemistry

Chapter 5 - Organic Chemistry

More lessons

Lesson 1: First Transition Series, Electronic Configurations and Economic Significance