IB Chemistry HLTopic 2 — Models of Bonding & StructurePaper 1 & 2Higher level~9 min read
Physical Properties of Transition Elements
Potassium melts at 63°C. Vanadium, one row down and a few places along, melts at 1910°C. Both are metals with the same lattice-and-electron-sea model, so the difference has to come from how many electrons each atom is willing to throw into the sea — and transition metals are unusually generous.
📘 What you need to know
A transition element is one that has an incomplete d-subshell, or that forms at least one stable cation with an incomplete d-subshell.
That definition excludes scandium and zinc, which are d-block but not transition metals.
Sc only forms Sc3+, which is [Ar] 3d0. Zn only forms Zn2+, which is [Ar] 3d10. Neither is incomplete.
The first transition series is therefore Ti to Cu.
The 3d and 4s subshells are very close in energy, so d electrons can be delocalised into the metallic bonding as well as s electrons.
More delocalised electrons means stronger electrostatic attraction, so transition metals have high melting points and are hard and dense.
They are also excellent electrical conductors. The best three are silver, copper and gold, in that order.
What counts as a transition element
“d-block” and “transition metal” are not the same thing, and the IB expects you to know the difference. The d-block is simply a position on the periodic table: the ten columns from scandium to zinc in period 4. Transition metal is a definition about electron configuration.
Definition to learn
A transition element has an incomplete d-subshell, or forms at least one stable cation with an incomplete d-subshell.
Apply that test to the two elements at the ends of the row and both fail:
Scandium forms only Sc3+. Losing three electrons empties the d-subshell completely: [Ar] 3d0. Empty is not incomplete.
Zinc forms only Zn2+, which is [Ar] 3d10. That subshell is full, so again not incomplete.
Where the transition elements sit
Learn the two awkward cases by their ion, not by their position. It is Sc3+ and Zn2+ that fail the test, not the neutral atoms.
Why the melting points are so high
In sodium each atom contributes one electron to the sea. In an s-block metal that is all the outer shell has to give. Transition metals are different: their 3d and 4s subshells are extremely close in energy, so d electrons can join the delocalised sea alongside the s electrons.
More delocalised electrons per atom means a bigger charge difference between the cations and the sea, and therefore a much stronger electrostatic attraction. Transition metal ions are also relatively small. Both factors point the same way, and the result is melting points far above anything in groups 1 and 2.
s-block against period 4 transition metals
Beryllium is the one s-block metal that keeps up, melting at 1287°C. Its atom is tiny, so even with only two electrons the attraction is unusually strong.
🤔 Why can d electrons join the electron sea at all?
In most metals only the outermost subshell is loose enough to delocalise. In transition metals the 3d and 4s subshells sit at almost the same energy, so there is no real distinction between “outer” and “inner” for those electrons. Both sets can be pulled into the delocalised sea, and an iron atom can contribute several electrons rather than the one or two an s-block metal manages.
Electrical conductivity
The same reasoning explains conductivity. More delocalised electrons per atom means more charge carriers available to drift when a potential difference is applied, so transition metals conduct exceptionally well.
The three best conductors are silver, then copper, then gold. Silver is the best of all, but copper is what actually goes into cables — it conducts almost as well and costs a tiny fraction as much. That trade-off between performance and cost is a good example of the kind of applied reasoning the IB likes.
Property
s-block metals
Transition metals
Reason
Delocalised electrons per atom
1 or 2
often 3 or more
3d and 4s are close in energy
Melting point
low to moderate
high to very high
stronger electrostatic attraction
Hardness
soft, cuttable with a knife
hard and strong
stronger, closely packed lattice
Density
low
high
small ions packed tightly together
Electrical conductivity
good
excellent
more mobile charge carriers
🧠 The one sentence that answers most of these questions
“Transition metals delocalise both s and d electrons, so there are more delocalised electrons per atom, giving a stronger electrostatic attraction between the cations and the electron sea.” Melting point, hardness and conductivity all follow from that single line.
Worked examples
WE 1
Explain why zinc is not classified as a transition element [2]
Mark 1: quote the definition
A transition element must have an incomplete d-subshell, in the atom or in at least one stable cation.
Mark 2: apply it to zinc
Zinc forms only Zn²⁺, which has the configuration [Ar] 3d¹⁰.
That d-subshell is full, not incomplete.
No ion of zinc has an incomplete d-subshellscandium fails the same test from the other end: Sc³⁺ is 3d⁰, which is empty
WE 2
Explain why titanium (1668°C) has a much higher melting point than calcium (842°C) [3]
Mark 1: how many electrons each delocalises
Calcium contributes 2 electrons per atom. In titanium the 3d and 4s subshells are close in energy, so d electrons are delocalised too and it contributes more.
Mark 2: effect on the attraction
More delocalised electrons and a smaller cation give a stronger electrostatic attraction between the cations and the sea.
Mark 3: link to energy
More energy is needed to overcome that attraction and break the lattice.
More delocalised electrons → stronger bonding → higher melting point“both s and d electrons are delocalised” is the phrase that earns the first mark
WE 3
Silver conducts electricity better than copper, yet copper is used for household wiring. Explain.
Step 1: why both conduct well
Both are transition metals with many delocalised electrons free to move when a potential difference is applied.
Step 2: compare them honestly
Silver is the better conductor, so on performance alone it would win.
Step 3: bring in the practical constraint
Copper’s conductivity is very nearly as good but it is far cheaper and more abundant, and it is ductile enough to draw into wire.
Copper is the best balance of conductivity and costapplied questions want the trade-off named, not just the chemistry
💡 Exam tips
Quote the definition using an ion, not just the atom. That is what makes the Sc and Zn cases work.
Say incomplete d-subshell. “Partly filled” is usually accepted, but “incomplete” is the syllabus wording.
For high melting points, mention that both s and d electrons are delocalised.
Do not forget the second factor: transition metal cations are also relatively small.
Distinguish d-block (a position) from transition element (a definition) if a question uses both terms.
Remember the beryllium exception if asked whether s-block metals always melt lower.
⚠ Common mix-ups
Treating d-block and transition element as synonyms. Sc and Zn are the counterexamples.
Saying Zn has an incomplete d-subshell. Zn2+ is 3d10, which is full.
Saying Sc3+ is full. It is empty, 3d0 — also not incomplete, but for the opposite reason.
Explaining high melting points with “stronger bonds” and stopping there. Say why: more delocalised electrons.
Forgetting the electrons come from two subshells. Both 3d and 4s contribute.
Assuming every s-block metal is soft and low melting. Beryllium melts at 1287°C.
That completes The Metallic Model. Up next: The Bonding Triangle — where ionic, covalent and metallic stop being three separate boxes and become three corners of one continuous map.
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