Topic 15:
Transition Metals
In order to
develop their practical skills, students will be encouraged to carry out a
range of practical experiments related to this topic. Possible experiments to
be discussed include the stepwise reduction of vanadium(V) to vanadium(II),
investigating the reactions of copper(II) ions or chromium(III) ions, using
sodium hydroxide and ammonia solution to identify transition metal ions,
investigating autocatalysis and preparing a complex transition metal salt.
Mathematical
skills that will be developed in this topic include investigating the geometry
of different transition metal complexes.
Within this topic, students will consider the model for the filling of
electron orbitals encountered earlier in their course, and see how limitations
in that model indicate the need for more sophisticated explanations. They will
also appreciate that catalyst research is a frontier area, and one that
provides an opportunity to show how the scientific community reports and
validates new knowledge.
Edexcel A level
Chemistry (2017)
Topic 15A: Principles
of transition metal chemistry
Here are the learning objectives relating to the d
block:
15A/1: To be able
to deduce the electronic configurations of atoms and ions of the d-block
elements of Period 4 (Sc–Zn), given the atomic number and charge (if any).
15A/2: To know
that transition metals are d-block elements that form one or more stable
ions with incompletely-filled d-orbitals.
The Periodic
Table: the d block
Here is the d block:
In this topic, we begin by discussing how to work out the electron
configurations of the first row of d block elements then the vexed question
(certainly in the minds of some teachers of chemistry) as to what really
constitutes a transition metal and whether all first row d block elements are
also transition metals.
But first here are some pictures of these metals:
Determining the
electron configurations of the first row d block elements.
Starting with the atomic numbers of these d block elements, you ought to be
able to correctly build up the electron configurations of these elements.
Up to Argon Z=18, all the metals have the same electron configuration. Things only get tricky beyond that
number. For example, iron Z= 26 so as the
4s fills before the 3d that leaves 6 electrons in the 3d and an electron
configuration of :
Fe: 1s2 2s2 2p6 3s2 3p6
3d6 4s2
But look at Chromium Z=24 how do the six electrons fill the 3d and 4s?
Here we remember that half filled subshells confer an added degree of
energetic stability to the atom so chromium has 3d5 4s1
Cr: 1s2 2s2 2p6 3s2 3p6
3d5 4s1
By a similar consideration the electron configuration of copper Cu Z= 29
the extra 11 electrons fill the 3d and 4s, one in the 4s and the rest (10) in
the 3d to confer the greatest energetic stability.
Cu: 1s2 2s2 2p6 3s2 3p6
3d10 4s1
So remember that the 4s fills before the 3d and that the electrons above
the 18 fill the subshells to confer the greatest energetic stability by half
filling or filling subshells.
Definition of the
term Transition metal
You will see from the two learning objectives above from the British
Edexcel A level course that a d block element might not also be a transition
metal.
Let’s look first at the electron configurations of all the first row
d-block elements.
Here they are easy to find on the Internet.
You’ll find them in at least two representations: either as electrons in
boxes (as arrows) or with subshell electrons as superscripts.
Electrons in boxes has the advantage of showing the opposite electron spins
and keeping the shells together i.e. all third shell subshells are together.
Now which of these d block elements are said to be transition metals?
Look at the electron configurations and if the atom of the element or its
ion has an incomplete d subshell then we are going to call those elements
transition elements.
The result is that zinc is not a transition metal but it is a d block
element.
Zinc atom: 1s2 2s2
2p6 3s2 3p6 3d10 4s2
And the zinc ion Zn2+: 1s2
2s2 2p6 3s2 3p6 3d10
In both examples the 3d subshell is full with 10 electrons so the metal is
not a transition element according to the Edexcel definition.
So what is your verdict on Scandium: 1s2 2s2 2p6
3s2 3p6 3d1 4s2
when its Sc3+ ion is: 1s2 2s2 2p6
3s2 3p6
since this ion has no 3d orbital at all?
According to the definition given above it too is not a transition metal
since its ion has no 3d subshell.
Effect of half
filled orbitals:
Another thing you should see from the table of electron configurations is
the transition metals with half filled d subshells.
Note first the electron configuration of chromium:
Cr: 1s2 2s2 2p6 3s2 3p6
3d5 4s1
According to Hund’s Rule, the subshells half fill then the electrons in the
orbitals start to pair up. So chromium
is 3d5 4s1 and both are half full.
Second,
note the electron configurations of Mn2+ and Fe3+ these
are stable oxidation states of these metals because the electron configuration
in each case is 3d5 that is an energetically stable half filled 3d
subshell.
Mn: 1s2 2s2 2p6 3s2
3p6 3d5 4s2
Mn2+: 1s2 2s2 2p6 3s2
3p6 3d5
And
Fe: 1s2 2s2
2p6 3s2 3p6 3d6 4s2
Fe3+: 1s2 2s2 2p6 3s2
3p6 3d5
Third,
note the electron configuration of copper.
It has a half filled 4s orbital and a full 3d subshell.
So what
we find is that Cu+ ions are stable and colourless because the 3d
subshell is full but as soon as the Cu+ is oxidised to Cu2+
the 3d subshell becomes 3d9 and the ions are energetically stable
and blue.
Cu: 1s2
2s2 2p6 3s2 3p6 3d10 4s1
Cu+:
1s2 2s2 2p6 3s2 3p6
3d10 colourless ions
Cu2+:
1s2
2s2 2p6 3s2 3p6 3d9 blue ions
The next
post will begin to explain the individual properties of d block elements.
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