10.1. Introduction to the 3–d block and transition Metals

• The elements scandium to zinc (Z = 21 to 30) are known as the 3d block of elements or 3d–block of metals because here the first of the possible d sub–shells is progressively filled (3d–block – first row of the d–blocks).

  • See rules on how to work out electron configurations

• The transition elements are group of industrially important metals mainly due to their strong inter–atomic metallic bonding giving them generally high melting/boiling points and high tensile strength.

• These–called 'transition metal characteristics' arise from an incomplete d sub–shell energy level but scandium and zinc are not true transition metals i.e. Ti to Cu are the real transition elements (reasoning later).

• Note that physically, zinc is low melting and a lower tensile strength compared to the others in the 3d block.

• Although scandium is physically typical of a transition metal e.g. high melting point and high tensile strength, chemically, scandium only forms a single and colourless triple charged ion (Sc³⁺). Therefore like zinc (only Zn²⁺), shows non of the typical characteristics of transition metal chemistry e.g. variable oxidation state, coloured complex ions, catalytic properties of the metal or ion. This is all explained in detail later.

• Therefore probably the best definition of a transition metal is an element which forms at least one ion with an incomplete d sub–shell containing at least one electron. How this relates to variable oxidation state and coloured complex ions is elaborated further in section 10.2 and the subsequent sections on the individual metals (links below) and some of the. Zinc (Zn²⁺, [Ar]3d¹⁰) and scandium (Sc³⁺, [Ar]3d⁰) cannot meet this criteria.

• The presence of the partially–filled d sub–shells of electrons gives transition elements properties which are not in general possessed by the main group elements, namely Groups 1–7 and 0, BUT, there are similarities with other metals, particularly in Groups 2, 3 and  4.

• PLEASE NOTE the following about these Transition Elements notes:

  • All the reactions are described with visual observations and full ionic equations whether redox reactions or not.

  • I have made extended use of standard electrode potentials to indicate not only the relative oxidising/reducing power of a half–cell reaction, but also to argue for the thermodynamic feasibility of a reaction.

    • However, if you have done little on using electrode potentials you should still be able to follow the reaction descriptions and the accompanying equations.

    • Electrode potentials (half–cell potentials) and how to work out E^Ø_reaction (which must be a +ve voltage for the reaction to be feasible) are all explained in Equilibria Part 7 Redox Equilibria and Electrode Potentials. How to use half–cell reactions to work out full redox equations is explained on Redox Reactions Part II section 6 Constructing redox equations.

  • In the latest Periodic Table convention, the 3d–block elements are considered the 'head elements' of Groups 3–12.

    • Groups 1–2 remain unchanged but Groups 3–7 and 0 become Groups 13–18. I tend to retain the Groups 3–7 and 0 convention for the moment but future is 13–18!

    • 'latest' Group number	3	4	5	6	7	8	9	10	11	12
      Period 4	21, Sc	22, Ti	23, V	24, Cr	25, Mn	26, Fe	27, Co	28, Ni	29, Cu	30, Zn
      Period 5	39, Y	40, Zr	41, Nb	42, Mo	43, Tc	44, Ru	45, Rh	46, Pd	47, Ag	48, Cd
      Period 6	57, La	72, Hf	73, Ta	74, W	75, Re	76, Os	77, Ir	78, Pt	79, Au	80, Hg
      Outer electrons (n = 3 to 5)	nd1(n+1)s2	nd2(n+1)s2	nd3(n+1)s2	nd5(n+1)s1	nd5(n+1)s2	nd6(n+1)s2	nd7(n+1)s2	nd8(n+1)s2	nd10(n+1)s1	nd10(n+1)s2

  • There are actually many 'vertical' chemical similarities in a 'classic' periodic table way of thinking to justify this latest 'numbering' of the Periodic Table. e.g.

    • In most cases the three elements quoted above, per vertical column, have the same outer electron configuration.

    • 'Modern Group 3': Scandium and yttrium have very similar with a relatively simple M³⁺ ion chemistry.

    • 'Modern Group 10': Nickel, palladium and platinum are good hydrogenation catalysts. They all tend to form more square planar complexes than other transition elements.

    • 'Group 11': Copper, silver and gold are relatively unreactive metals in terms of corrosion. They form linear complexes like the cationic, [Ag(NH₃)₂]²⁺ or the anionic [CuCl₂]^– and [Au(CN)₂]^–. All three are extremely good conductors of heat and electricity.

    • 'Modern Group 12': Zinc and cadmium chemistry is mainly about the M²⁺ ion.

    • From modern 'Group 3 to 7' the maximum known oxidation state known (albeit in some pretty unstable compounds at times) is equal to the 'new' group number i.e. Sc/Y/La (+3) to Mn/Tc/Re (+7).

    • The discontinuity of atomic/proton number from lanthanum to hafnium on period 6 is due to the insertion of the 4f–block elements ₅₈Ce to ₇₁Lu.

• Comparison of certain properties of the 3d block of metals and other elements for Z = 1 to 38 particularly the preceding Group 1 metal potassium and the Group 2 metal calcium.

  • Periodicity plots for elements Z = 1 to 38 Look for Z = 21 (Sc) to 30 (Zn)

    •  Note: There is no direct link back to here, so use <== 'back' on browser bar.

  • Melting/boiling points: Generally higher than other elements in period 4.

  • 1st ionisation energy: The 3d block 1st ionisation energies tend to increase from left to right and fit in with the general pattern for period 4.

  • Pauling electronegativity: The 3d–block values range from a relatively low 1.3 to 1.9 and fit in with the general pattern of increasing value across period 4.

  • Atomic radius: 3d–block elements have similar values and significantly less than for potassium and calcium.

  • Electrical/thermal conductivity: The 3d–block are quite good conductors of electricity/heat and very good in the case of copper (ditto silver Ag below Cu).

  • Density: 3d–block range from 3.0 to 8.9g/cm³ and significantly more than for potassium (0.86) and calcium (1.5).

  • Periodicity plots for elements Z = 1 to 96 if you want to look for the 4d and 5d blocks!

• Other comparison points of the elements titanium to copper (true transition metals) with nearby metals.

  • Potassium (+1), calcium (+2) and scandium (+3) only have one oxidation state in compounds, whereas Ti to Cu have compounds in at least at least three oxidation states, even if some are not very stable!

10.2. Introduction – information & general characteristics of  3d block Metals Sc–Zn

Data Table 1 – summary of selected properties – concentrating only on the 3d–block

CLICK for a more detailed data table 2 summary

General Physical Characteristics

• The transition metals are the most important structural metals for industry due to their strength arising from the strong inter–atomic forces (see metal bonding and alloy structure).

• The strong bonding is due to small ionic radii and at least 3 delocalised 3d or 4s electrons contributing to the bonding which accounts for their high tensile strength, malleability (can be readily beaten into shape) and ductility (can be drawn into wire).

• They are silvery–grey solids apart from the dark orange of copper.

• They generally have high melting/boiling points and densities and readily mix with themselves or other elements to give a huge variety of alloys with a wide range of uses based on varied hardness, strength, malleability and anti–corrosion properties. 

• There is a general, but small, contraction of the atomic/ionic radii across the series as the atomic/proton number rises, i.e. an increasing positive attractive force on the outer electrons of the same sub–shells (3d and 4s).

10.2b. General Chemical Characteristics and electron configurations

21 Scandium, Sc	1s22s22p63s23p63d14s2	[Ar]3d4s
22 Titanium, Ti	1s22s22p63s23p63d24s2	[Ar]3d4s
23 Vanadium, V	1s22s22p63s23p63d34s2	[Ar]3d4s
24 Chromium, Cr	1s22s22p63s23p63d54s1	[Ar]3d4s
25 Manganese, Mn	1s22s22p63s23p63d54s2	[Ar]3d4s
26 Iron, Fe	1s22s22p63s23p63d64s2	[Ar]3d4s
27 Cobalt, Co	1s22s22p63s23p63d74s2	[Ar]3d4s
28 Nickel, Ni	1s22s22p63s23p63d84s2	[Ar]3d4s
29 Copper, Cu	1s22s22p63s23p63d104s1	[Ar]3d4s
30 Zinc, Zn	1s22s22p63s23p63d104s2	[Ar]3d4s

The chemistry is dominated by the behaviour of the 3d electrons. The 3d block corresponds to the filling of the 3d sub–shell of electrons, best appreciated by the 'box diagrams' of their electron structure.

Each half–arrow is an electron, which tend to singly occupy the sub–orbitals as much as possible to minimise repulsion (Hund's Rule of maximum multiplicity). 

The outer electrons of the element are either in the 3d or 4s sub–shell. The 4s sub–shell is initially filled by potassium [Ar]4s¹ and calcium [Ar]4s².

The electron arrangement for each element from Sc to Zn is also given at the start of each individual metal section in terms of s, p and d notation.

All 10 elements, Sc to Zn are 3d block elements (the filling of the 3d sub–shell) BUT a true transition element is one in which there is an incomplete d sub–shell holding at least one electron in one or more chemically stable ions (Ti to Cu). For 3d block metals this means at least one stable ion with the configuration within the range [Ar]3d¹ e.g. Ti³⁺ to [Ar]3d⁹ e.g. Cu²⁺ and so excludes scandium and zinc. Zinc only forms Zn²⁺, [Ar]3d¹⁰ and scandium only forms Sc³⁺, [Ar]3d⁰, so neither can meet this criteria for a true transition metal. See theory of colour in transition metal complexes.

There are two apparent anomalies in the electron configuration sequence from left to right as the 3d sub–shell energy level is filled:

Cr is not 3d⁴4s² and Cu is not 3d⁹4s²

because inner half–filled or fully–filled filled 3d sub–shells seem to be a little lower in energy, marginally more stable.

The total number of outer 3d/4s electrons is equal to the maximum oxidation state from Sc(+3) to Mn(+7) and there are many stable compounds exhibiting these maximum oxidation states.  After Mn there is significantly less stability of species with the metal in oxidation states above +3 for Fe and Co, and above +2 for Ni, Cu and Zn.

The four 'classic' chemical characteristics (but NOT unique to transition metals) are ...

(1) Complex formation: Appendix 2  offers an introduction as well as numerous examples 'en route' particularly from Ti to Cu.

(2) Formation of coloured ions: Appendix 4  offers an introduction to the origin of the colour in transition metal complex ions as well as examples 'en route' from colourless 'non–transition' Sc³⁺ complexes, coloured Ti^{II, III, IV} to Cu^II 'true transition' complexes and finally colourless 'non–transition' Zn²⁺ complexes at the end of the 3d–block.

(3) Variable oxidation state – variable valency:

• From Sc to Mn the maximum oxidation state is determined by the total maximum number of 3d and 4s electrons. After that, things get very complicated but the maximum tends to fall down to +2 for zinc after +3 for Fe and Co (there are some higher oxidation state species, but not that common and not that stable in aqueous media).

  • See Table of examples of compounds/complexes for different oxidation states of the 3d–block elements.

• The relative ease of oxidation state change for Ti to Cu AND the maximum oxidation state formed by Sc to Mn, is partly explained by considering the ionisation energies involved and a comparison with Group 1, 2 and 3 metals helps too.

  • In the sequences below the atoms and ionised species are all in the gaseous state as is the convention for ionization energy data.

  • The energies (kJmol^–1) required to remove the next most loosely bond electron to give the next more highly charged ion (the next higher oxidation state) are shown as a sequence.

  • Only for the first example, potassium, are the full formal equations shown.

  • The successive ionisation energy sequences for Group 1 (potassium), Group 2 (calcium), the 3d–block (e.g. titanium) and Group 3 (gallium) are now considered for period 4.

  • Gp1: K_(g) == +418 ==> K⁺_(g) == +3070 ==> K²⁺_(g) 

    • they would be formally written as:

      • for the 1st ionisation energy: K_(g) – e^– ==> K⁺_(g)  

      • and for the 2nd ionisation energy: K⁺_(g) – e^– ==> K²⁺_(g)  

  • Gp2: Ca_(g) == +590 ==> Ca⁺_(g) == +1150 ==> Ca²⁺_(g) == +4940 ==> Ca³⁺_(g) 

  • 3d–block: e.g. Ti_(g) == +661 ==> Ti⁺ == +1310 ==> Ti²⁺ == +2720 ==> Ti³⁺ == +4170 ==> Ti⁴⁺ == +9620 ==> Ti⁵⁺

  • Gp3: Ga_(g) == +577 ==> Ga⁺ == +1980 ==> Ga²⁺ == +2960 ==> Ga³⁺ == +6190 ==> Ga⁴⁺ 

• So, for Groups 1, 2 and 3, the ionisation energy dramatically rises after the outer shell of s or p electrons are removed, i.e. a very stable electronic noble gas structure ([Ar], 1s²2s²2p⁶3s²3p⁶) is left. This gives a maximum positive stable oxidation state equal to the group number. The energy required (very endothermic) to make Na²⁺, Ca³⁺ and Ga⁴⁺ is too high to be compensated by exothermic bond formation with other elements like oxygen or chlorine etc.

  • Also note that intermediate lower oxidation states Ca⁺ and Ga²⁺ (and  Ga⁺?)are not very stable either.

  • I'm afraid ionisation energies and electron arrangements are not the only factors to be considered, you also need to study the Born Haber Cycle in some detail to prove this, but not here and not usually on a pre–university course!

• For the transition metals, at first, the successive ionisation energies rise relatively gradually, due to the 3d/4s electron levels being of similar energy. When all the outer s and d electrons are removed to leave an [Ar] core, there is, as with Groups 1–3 etc., a dramatic rise as an electron must be removed from the inner very stable noble gas (argon) core.

  • Therefore Ti has a maximum oxidation state of +4, but +2 and +3 species are also formed, but NOT +5 compounds.

  • This does mean however, across the 3d–block, there is the potential for very high oxidation states if there are enough 3s and 3d electrons that can be energetically favourably removed or become involved in stable bonding e.g. Mn has a maximum oxidation state of +7 by 'removing  *  or 'sharing' the outer 3d⁵4s² electrons. (see data table).

    • Similarly you can argue that the maximum oxidation states for vanadium would be +5 and chromium +6, as is indeed is the case!

    • After manganese, things get complicated and there is a general decrease from Mn (+7) to Zn (+2) in the maximum possible higher oxidation states, and many higher oxidation state compounds of Fe, Co, Ni and Cu are unstable and uncommon.

  •  *  Of course e.g. in manganese (VII) compounds, 7 electrons are not removed to give an Mn⁷⁺ ion, but all 7 outer electrons are involved in the bonding and, unlike calcium and gallium, true transition metals form many stable compounds of the 'intermediate' oxidation states e.g. manganese forms +2, +3, +4, +6, +7 oxidation sate compounds.

  •  This is due to closeness of the energies of the 3d sub–shell electrons and the stabilising influence of ligand molecules like water or ammonia and ligand ions like chloride or cyanide. Vacant 3d orbitals (and 4s/4p orbitals too) can accept pairs of electrons to for stable dative covalent bonds.

    • I'm afraid arguments for the characteristic variable oxidation states of transition metals based on ionisation energies and the similar energies of the 3d orbitals is a bit limited, but better than nothing!

    • I have done detailed notes on oxidation state/oxidation number and redox reactions – a lot of which come up in transition metal chemistry.

(4) Catalytic activity by the elements and their compounds:

• Many examples are quoted in the two pages of detailed notes and the theory of heterogeneous and homogeneous catalysis is outlined in Appendix 6.

Scandium * Titanium * Vanadium * Chromium * Manganese * Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum

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Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Scandium * Titanium * Vanadium * Chromium * Manganese * Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum

Introduction 3d–block Transition Metals * Appendix 1. Hydrated salts, acidity of hexa–aqua ions * Appendix 2. Complexes & ligands * Appendix 3. Complexes and isomerism * Appendix 4. Electron configuration & colour theory * Appendix 5. Redox equations, feasibility, E^ø * Appendix 6. Catalysis * Appendix 7. Redox equations * Appendix 8. Stability Constants and entropy changes * Appendix 9. Colorimetric analysis and complex ion formula * Appendix 10 3d block – extended data * Appendix 11 Some 3d–block compounds, complexes, oxidation states & electrode potentials * Appendix 12 Hydroxide complex precipitate 'pictures', formulae and equations