Chemistry, Third edition

VARIATION OF PROPERTIES OF ELEMENTS WITHIN GROUPS 213

BOX 12.11

Mendeleev’s periodic table

Dimitri Mendeleev (1834–1907), a Siberian
chemist, was not the first scientist to notice
similarities between some groups of elements,
but he was the first to organize the elements
into a Periodic Table and use the concept to
successfully predict the properties of then
undiscovered, elements.

Convinced that the atomic weights (now
atomic masses) of the elements were related
to their properties and anxious to present
some sort of ‘ordering’ system to the
chemistry of the elements he published his
Periodic Law:

Elements placed according to the value of
their atomic weights present a clear
periodicity of their properties.

Periodicity means ‘reoccurring at regular
intervals’. The order of atomic weights (now
called atomic masses) did not always preserve
the law and Mendeleev had to deviate from a
strict order where elements were obviously

misplaced. Iodine, for example, obviously

belonged with the halogens even though the

order that Mendeleev worked with did not

predict this. Mendeleev realized that some

elements had yet to be discovered, so he left

gaps for those elements and dared to forecast

some of their properties. Later, when these

elements were discovered, some of his

predictions were found to be astonishingly

accurate and led to acceptance of the law and

great excitement amongst the scientific

community. Consider his predictions, for

example, for the element gallium, which he

named ‘Eka-aluminium’:

Predicted in 1871 discovered 1875

Eka-aluminium Gallium

Atomic weight 68 atomic weight 69.9

Atomic volume 11.5 atomic volume 11.7

Specific gravity 6.0 specific gravity 5.96

The classification of elements in the Periodic

Table is one of the greatest contributions

made to modern chemistry. Today’s improved

Periodic Table positions the elements in order

of atomic number rather than atomic weight

because scientists have realized that, and to

some extent why, the properties of elements

depend on their atomic number (which gives

us the number and arrangements of the

electrons in their atoms).

The hunt for new elements continues. To date,

many elements between 93 and 118 have been

synthesized artificially by nuclear reactions. The

heaviest elements are very unstable and cannot

be of any practical use, but scientists predicted

that an ‘island of stability’ exists at around

element 114 (one atom of this element was

reported in January 1999, and it does seem to

be more stable than other similarly heavy

elements). Attempts have already been made

to predict the properties of, as yet,

undiscovered elements using Periodic trends.

Dmitri Mendeleev, a Russian chemist and
professor at the University of St Petersburg.
He was known to cut his hair only once a year,
in the Spring!

Group

Period I II III IV V VI VII VIII

1H

2 LiBeBCNOF

3NaMgAlSiPSCl

K Ca * Ti V Cr Mn Fe Co Ni

4 CuZn* * AsSe Br

Rb Sr Y Zr Nb Mo * Ru Rh Pd

5 AgCdIn SnSbTeI

The asterisk * marked spaces for elements which had yet to be discovered. They were scandium,

gallium, germanium and technetium.