ELEMENTS OF THE FIRST TRANSITION SERIES 205Elements of the first transition series
The block of elements between Group 2 and Group 13 of the Periodic Table are known
as the transition elementsord-block elements(Sc to Zn and the elements below
them). The elements of the first transition seriesare those elements that have partly
filled d orbitals in any of their common oxidation states, which are the block of ele-
ments headed by Ti to Cu. Here, we will look mainly at the properties of the first
transition series: Ti, V, Cr, Mn, Fe, Co, Ni and Cu. These elements are typical metals
and are often referred to as the transition metals. They have very similar physical
properties. The changes in the atomic radii and first ionization energies across the
first transition series are small, because each increase in nuclear charge is well
shielded by the inner 3d electrons and only a small increased attraction is noticed by
the outer electrons in the 4s subshell. See Box 12.7.
The transition metals have the following properties in common:
1.They are hard and have high melting points, both indications that strong metallic
bonding exists within the metals.
2.They form alloys with one another and with other metals. Transition metals can
form alloys with each other because their atoms are similar in size – the atoms of
one metal can occupy positions in another metal’s lattice. (Steel is an alloy of iron
with other transition metals such as chromium.)12.7
BOX 12.6
Discovery of the noble gases
In 1888 Lord Rayleigh (1842–1915) found
that the density of nitrogen, obtained from the
air, had a higher value than that obtained for
nitrogen which had been prepared by chemical
means. Together with William Ramsay
(1852–1916), he sought an explanation. The
two scientists eventually concluded that a new
gas was present in samples of nitrogen
obtained from the air. Because Ramsay,
despite many chemical experiments, could not
get this new gas to combine with another
substance, the gas was named argon(from
the Greek ‘idle’). The gas was subsequently
found to be monoatomic, with a molar mass
of about 40 g mol^1. The two scientists
realized that there was room for another group
of elements in the Periodic Table that had
been proposed by Mendeleev and the search
began for the other elements of the group.
Helium had been named some years earlier
when unknown spectral lines were observed in
the spectrum of the sun. It had been
suspected that the lines were caused by the
presence of an unknown element. Ramsay
now managed to isolate helium from a mineral
called cleveite, and show that it had similar
properties to argon.
In 1898 Ramsay isolated three more gases in
the group from liquid air – neon, krypton and
xenon – all found to be chemically inert. Thegroup was complete a year later, when a gas
was found to be coming from samples of the
Group 2 element radium. The new inert gas
was first called ‘emanation’ (Em) and finally
radon (Rn). Ramsay received a Nobel prize for
the work in 1904.Chemists were intrigued by the apparent
unreactivity of the gases and attempts
followed to react them with other substances.
Argon was found not to react with the most
reactive element of all, fluorine, yet H. G.
Wells wrote that the Martians in the War of
the Worldsattacked Earth with a poisonous
argon compound! In the early 20th century,
when the electronic configurations of the
elements were known, the fact that the atoms
of the inert gases had completed shells of
electrons was accepted as a reason for their
lack of reactivity. However, the story was not
finished: in the early 1960s XeF 4 was
prepared. Although the ionization energies of
the outer electrons in argon were too high for
the atoms to act as electron donors in
reaction with fluorine, xenon atoms are larger
and the outer electrons are more shielded
from their nuclei. In fact, the first ionization
energy for xenon is very close to that obtained
for oxygen. Xenon and the heavier inert gases
can therefore form compounds under the right
conditions.