Page 138 - Chemistry ICSE Class IX

126 ICSE Chemistry – 9
Š Fourth and Fifth periods contain 18 elements each (atomic nos: 19
to 36 and 37 to 54, respectively), and are called Long periods.
Š Sixth period contains 32 elements (atomic nos: 55 to 86), and is
called the Longest period.

Š Seventh period (atomic no: 87 onwards) is also complete now.
ƒ Each period begins with the element that has only one electron in a
new main shell.

What are the characteristics of a group
A vertical column in the periodic table is called a group. There are nine (9)
groups in the Böhr’s long form periodic table. These are numbered as IA,
ůů ĞůĞŵĞŶƚƐ ŝŶ Ă ŐƌŽƵƉ ĐŽŶƚĂŝŶ ƚŚĞ IIA, IIIB, IVB, VB, VIB, VIIB, VIII (three columns), IB, IIB, IIIA, IVA, VA, VIA,
ƐĂŵĞ ŶƵŵďĞƌ ŽĨ valence electrons͘ VIIA and 0 starting from the left. On the extreme left of the periodic table is
ͻ Group IA. On the extreme right end is the Group 0.
ĂĐŚ ƉĞƌŝŽĚ ƐƚĂƌƚƐ ǁŝƚŚ ĂŶ ĞůĞŵĞŶƚ ŝŶ
ǁŚŝĐŚ Ă ŶĞǁ ƐŚĞůů ƐƚĂƌƚƐ ŐĞƫŶŐ ĮůůĞĚ All elements in any group of the periodic table show
ǁŝƚŚ ĂŶ ĞůĞĐƚƌŽŶ͕ ĂŶĚ ĞŶĚƐ ƵƉ ǁŝƚŚ Ă Š similar properties due to similar outer electronic con guration,
ŶŽďůĞ ŐĂƐ ǁŚŝĐŚ ŚĂƐ Ăůů ƚŚĞ ŝŶŶĞƌ ĂŶĚ
ŽƵƚĞƌ ƐŚĞůůƐ ĐŽŵƉůĞƚĞůLJ ĮůůĞĚ͘ and
ͻ Š a gradual change in the properties due to decreasing attraction
ůů ĞůĞŵĞŶƚƐ ŝŶ Ă ƉĞƌŝŽĚ ĐŽŶƚĂŝŶ ƚŚĞ between the nucleus and the valence electrons as we go down the
ƐĂŵĞ ŶƵŵďĞƌ ŽĨ ƐŚĞůůƐ͘ group.

What are the characteristics of a period

A horizontal row of elements in the periodic table is called a period. There
are seven periods in the periodic table. Each period starts with Group IA
(alkali metal group), and ends with Group 0 (noble elements group).
ƒ The ſTUV RGTKQF elements (H and He) have only one shell – K-shell.
K-shell starts getting lled at H (K : 1), and is completely lled at He
(K : 2).

ƒ The second period elements (Li, Be, B, C, N, O, F and Ne) have two
shells – K and L shells. The L-shell starts getting lled with Li (K, L :
2, 1), and is completely lled with Ne (K, L : 2, 8).

ƒ Third period elements have three shells – K, L and M shells. It starts
with Na (K, L, M : 2, 8, 1), and ends with Ar (K, L, M : 2, 8, 8).
Similarly, the fourth period elements have four shells - K, L, M and
N, and so on.

What are the merits of long form periodic table

The long form periodic table has the following merits:
ƒ Long form of the periodic table is based upon atomic numbers which
is a more fundamental property of the atom.
ƒ It relates the position of an element in the periodic table with its
electronic con guration.
ƒ It removes all the anomalies and drawbacks of the Mendeleev’s
table based on increasing atomic mass, by changing the basis of
classi cation to atomic number. For example,
Š all the isotopes of an element are now placed in the same position
because they all have the same atomic number.