444 THE LANTHANIDES AND ACT1NIDESlinium marks the half-way stage when filling of the inner sub-level is
half complete. It is known that this represents a particularly stable
electronic configuration—hence gadolinium forms only the ions
Gd3+ (by loss of three outer electrons) and shows no tendency to
add or lose electrons in the half-filled inner level. This behaviour
may be compared with the element before gadolinium, europium,
Eu, which exhibits an oxidation state of two as well as three, and the
element following, terbium, which exhibits states of -1-3 and +4.
In the actinides, the element curium, Cm, is probably the one
which has its inner sub-shell half-filled; and in the great majority of
its compounds curium is tripositive, whereas the preceding elements
up to americium. exhibit many oxidation states, for example -1-2,l 3 _|-4 + (^5) an(j + (^5) ancj berkelium, after curium, exhibits states of
- 3 and +4. Here then is another resemblance of the two series.
The many possible oxidation states of the actinides up to ameri-
cium make the chemistry of their compounds rather extensive and
complicated. Taking plutonium as an example, it exhibits oxidation
states of + 3, +4, +5 and -f 6, four being the most stable oxidation
state. These states are all known in solution, for example Pum as
Pu^3 + , and PuIV as PuOf+. PuO|+ is analogous to UOf + , which is
the stable uranium ion in solution. Each oxidation state is charac-
terised by a different colour, for example PuO^ + is pink, but change
of oxidation state and disproportionation can occur very readily
between the various states. The chemistry in solution is also com-
plicated by the ease of complex formation. However, plutonium can
also form compounds such as oxides, carbides, nitrides and anhyd-
rous halides which do not involve reactions in solution. Hence for
example, it forms a violet fluoride, PuF 3 , and a brown fluoride,
PuF 4 ; a monoxide, PuO (probably an interstitial compound), and
a stable dioxide, PuO 2. The dioxide was the first compound of an
artificial element to be separated in a weighable amount and the
first to be identified by X-ray diffraction methods.
THE ELEMENTS BEYOND THE ACTINIDES
Element 103, lawrencium, completes the actinides. Following this
series, the transition elements should continue with the filling of the
6d orbitals. There is evidence for an element 104 (eka-hafnium); it
is believed to form a chloride MC1 4 , similar to that of hafnium. Less
positive evidence exists for elements 105 and 106; attempts (so far
unsuccessful) have been made to synthesise element 114 (eka-lead),
because on theoretical grounds the nucleus of this element may be
stable to decay by spontaneous fusion (as indeed is lead). "Super-