Page 136 - Chemistry ICSE Class IX

124 ICSE Chemistry – 9

How did atomic number prove a better basis of
classification
By choosing atomic number as the basis of classi cation, the anomalies
in the original Mendeleev’s periodic table got removed, as described
below:
ƒ Inversion in Mendeleev’s table. The anomaly of having the element
of higher atomic mass before the element with lower atomic mass,
is now removed because the element having higher atomic mass
had the lower atomic number in such cases. For example, the atomic
number of Ar is 18 and that of K is 19, although the atomic mass of
Ar is higher than that of K.
ƒ Position of isotopes. Although isotopes of an element have different
atomic masses, they have same atomic number. Hence, according
to the atomic number as the basis of classi cation, these should be
allotted the same place in the table.
H. G. J. Moseley
Modern Long Form Periodic Table

Taking atomic number as the basis of classi cation of elements, Böhr
constructed a periodic table known as the Long Form Periodic Table,
in 1920.
In 1984, IUPAC (International Union of Pure and Applied Chemistry)
recommended that the groups in the long form periodic table be numbered
from the left, as 1 to 18. This form of long form periodic table is known as
Long Form Periodic Table (IUPAC) 1984.
The long form periodic table relates the position of an element to its
electronic con guration.

What are the features of the long form periodic table
dŚĞ ůŽŶŐ ĨŽƌŵ ƉĞƌŝŽĚŝĐ ƚĂďůĞ ;/hW (Böhr 1920)
ϭϵϴϰͿ ĐŽŶƐŝƐƚƐ ŽĨ ϭϴ ǀĞƌƟĐĂů ĐŽůƵŵŶƐ
ĐĂůůĞĚ ŐƌŽƵƉƐ͘ dŚƵƐ͕ there are 18
groups in long form periodic table͘ The general features of the long form periodic table (Böhr 1920) are:
dŚĞƐĞ ŐƌŽƵƉƐ ĂƌĞ ŶƵŵďĞƌĞĚ ϭ ƚŽ ϭϴ
ĨƌŽŵ ƚŚĞ ůĞŌ͕ ƵƐŝŶŐ Arabic numerals͘ Groups
dŚĞ ďĂƐŝĐ ƐƚƌƵĐƚƵƌĞ ŽĨ ƚŚĞ ůŽŶŐ ĨŽƌŵ ƒ Böhr’s long form periodic table consists of nine groups — Group I
ƉĞƌŝŽĚŝĐ ƚĂďůĞ ĐŽŶƐƚƌƵĐƚĞĚ ďLJ ƂŚƌ
ŝƐ ƌĞƚĂŝŶĞĚ ŝŶ ƚŚĞ /hW ůŽŶŐ ĨŽƌŵ to VIII and Group 0. Groups I to VII are divided into subgroups A
ƉĞƌŝŽĚŝĐ ƚĂďůĞ͘ and B. In this periodic table, the groups are numbered (in the Roman
numerals) from the left as IA, IIA, IIIB, IVB, VB, VIB, VIIB, VIII (three
columns), IB, IIB, IIIA, IVA, VA, VIA, VIIA and 0.

Thus, there are nine groups, 16 sub-groups and 18 columns in the
long form periodic table (Böhr 1920).

ƒ The periodic table is divided into four blocks based on the electronic
con guration.
Š s-block. This block consists of groups IA and IIA.

Š p-block. This block consists of groups IIIA to VIIA and 0.
The elements belonging to s- and p-blocks are called main group
elements (or representative elements).