Transition elements
THE TRANSITION ELEMENTS;
concepts
Definition:
These are elements which have partially
filled d or f shells.
Broad
defn:
-Includes elements
which have partially filled d or f shells in any of their commonly occurring
oxidation states. This includes:
1. CuII =
[Ar]3d9
2. AgII =
[Kr]4d9
Three groups of Transition elements are
recognized:
(i)The
main Transition elements or d-block elements.
(ii)The
Lanthanoid elements
(iii)The Actinoid elements.
(i)The Main Transition ® d-block elements.
(a) The First Transition series
Sc=4s23d1 =
Lightest member
Sc Ti
V Cr Mn
Fe Co Ni and Cu
Cu =
3d104s1
®Have partially filled 3d
shells either in the ground state of the free atom [All EXCEPT Cu] or in one or
more of their chemically important ions [ALL EXCEPT Sc].
Note: from
Zn = 3d104s2
until next 9 elements = NON-TRANSITION (Sr).
(b)The Second Transition
series
From
Yttrium = 5s24d1
Y Zr Nb Mo Tc Ru Rh Pd Ag
All
have partially filled 4d shells either in the free element [ALL EXCEPT Ag] or
in one or more of the chemically important ions [ALL EXCEPT Y].
(ii)The Lanthanoids:
Elements 57-71
® Starts with Lanthanum = 6s25d1.
4f shell becomes slightly
more stable than 5d shell.
Next 14 elements electrons
enter the 4f shell until Lu (Lutetium).
4f25s25p66s2 4f7
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu = 4f145d16s2.
The elements La – Lu have very similar
chemical and physical
properties.
The
Third Transition series starts with Hf = 6s25d2.
;4f146s15d10
Hf Ta W
Re Os Ir Pt
and Au
All have partially filled 5d shells in one or more of their
chemically important oxidation states as well as in the neutral atom [EXCEPT
Au].
(iii)The Actinoids:
Elements 89-103.
Starts
with Ac = 7s26d1 =
Actinium
Initially ®Not so great difference between 5f and 6d orbitals.
For Elements immediately
following Ac and their ions there may be electrons in 5f or 6d or both.
After FOUR or FIVE more
electrons have been added to the Ac configuration, the 5f orbitals
become the more stable.
Ac Th Pa U
Np Pu Am
Cm Bk Cf Es
Fm Md No Lr.
(iv) The
Transactinoid
elements = Element 104 (Rutherfordium, Rf) to 118 (Ununoctium, Uuo).
Differences
between the Three classes of Transition elements.
1.d-block elements ®Partially filled 3d, 4d or 5d orbitals.
® Electrons occupying them are largely
screened from the surroundings by the overlying shells [5s, 5p] of electrons.
® 4f orbitals deeply buried in atoms/ions
•Therefore Reciprocal interactions
of 4f electrons and surroundings are of little chemical significance.
(i)Chemistry of Lanthanoids ® Homologous.
(ii)Chemistry of d-block elements.
Erratic
and Irregular variations in chemical properties through a series of d-block
elements.
3.The Actinoids elements.
- Behaviour lies
between d-block and the Lanthanoids.
- 5f orbitals ® not so exposed as the d-orbitals and
not so well shielded as 4f-orbitals.
Factors affecting the
stability of Electronic configurations:
1.Effective nuclear charge.
•In
hydrogen (one electron atom) all the subshells of each principal shell are equienergetic.
•
•In
multi-electron atoms ®
Energies of subshell
depend on the populations of all the other levels. The s, p, d, f etc.
sub-shells split apart and drop to lower energies.
•
REASON: Steady increase in the Effective Nuclear
charge.
•Each
electron is IMPERFECTLY SHIELDED from the nuclear charge by other electrons.
•
2.Direct interactions between electrons
(Inter-electronic forces).
eg.
Special stability of half filled shells.
eg. Cr = [Ar] 4s13d5 1st Transition series.
Cu = [Ar] 4s13d10
Gadolinium
® Gd = [Xe] 4f76s25d1 Lanthanoid.
Therefore:
Inter-electronic forces and variations in
total nuclear charge play a large part in determining the configurations of
ions.
e.g.
4s orbital ® Occupied before 3d but are not always the
more stable.
Example: Elements of the First Transition
series ionize by the loss of the 4s first instead of the 3d.
e.g.
Cu = [Ar] 4s13d10
Cu2+ = [Ar] 3d9.
2.Direct interactions between electrons
(Inter-electronic forces).
eg.
Special stability of half filled shells.
eg. Cr = [Ar] 4s13d5 1st Transition series.
Cu = [Ar] 4s13d10
Gadolinium
® Gd = [Xe] 4f76s25d1 Lanthanoid.
Therefore:
Inter-electronic forces and variations in
total nuclear charge play a large part in determining the configurations of
ions.
e.g.
4s orbital ® Occupied before 3d but are not always the
more stable.
Example: Elements of the First Transition
series ionize by the loss of the 4s first instead of the 3d.
e.g.
Cu = [Ar] 4s13d10
Cu2+ = [Ar] 3d9.
THEREFORE: Stability of an electronic configuration is
determined by:
(i)Nuclear-electronic
attraction.
(ii)Shielding of one electron
by others.
(iii)Inter-electronic
repulsions.
(iv)Exchange energy of half
filled shells.
•
1.They are all metals.
2.They are almost all hard, strong,
high-melting, high-boiling metals that conduct heat and electricity well.
3.They form alloys with one another and with
other metallic elements.
4.Many of them are sufficiently electropositive
to dissolve in mineral acids although a few are “noble”
5.With very few exceptions, they exhibit variable
valence.
6.Their ions and compounds are coloured in one if not all oxidation states (with
very few exceptions).
7.Due to partially filled shells, they form
some paramagnetic compounds.
Some general observations.
1. The
most common oxidation states are +2 and +3.
2. Species that exhibit low oxidation states
can exist as discretely charged ions either in solution or in a crystalline
lattice e.g. Cr2+, Fe2+ are well known ionic species e.g. FeSO4
3. Elements in higher oxidation states are
very polarizing and can exist only in an environment of highly electronegative
atoms (oxyanions or
binary compds of
oxygen and fluorine).
eg. +6 for Cr is found in CrO42-
+6 for
Mo is found in MoO42-
+6 for
W is found in WO42-
●
4.Oxidation states less than II, except Cu are
found only with p-acid
type ligands or
in organometallic
compounds.
5. Lower oxidation states of metals = BASIC
+4
Oxidation state of metals =
AMPHOTERIC
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