Page 139 - Chemistry ICSE Class IX
P. 139
The Periodic Table 127
The anomaly of higher atomic mass element being placed before
the lower atomic mass element is removed when these elements
are arranged in the order of increasing atomic number.
It shows changes in the properties of various elements while moving DĞƚĂůƐ ĂƌĞ ŽŶ ƚŚĞ ůĞŌ ƐŝĚĞ ŽĨ ƚŚĞ
across a period or down a group more clearly. ƉĞƌŝŽĚŝĐ ƚĂďůĞ͘
EXAMPLE 5.1. Nitrogen (atomic no. 7) and phosphorus (atomic no. 15) belong to
group 15 of the long form periodic table (Böhr). Write the electronic configuration
of these two elements in terms of K, L, M, N shells. Predict whether these are
metallic or nonmetallic.
Solution: 'NGEVTQPKE EQPſIWTCVKQP
Element Atomic number
K L M
Nitrogen 7 2 5
Phosphorus 15 2 8 5
Both nitrogen and phosphorus belong to Group VA of the periodic table and contain
ve electrons in their outermost shell. Therefore, both nitrogen and phosphorus are
nonmetals.
EXAMPLE 5.2. The positions of three elements, A, B and C in the periodic table
are shown below:
Group VIA Group VIIA
— —
— A
— —
B C
(a) State whether C is a metal or nonmetal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller than B in size?
(d) Which type of ion, cation or anion, will be formed by element C?
Solution: (a) Group VIIA is on the right-end of the periodic table. So, elements in
Group VIIA are nonmetals. Therefore, C is a nonmetal.
(b) In a group of nonmetals, the reactivity decreases as we go down the
group. Therefore, C is less reactive than A.
(c) B and C lie in the same period. In a period, the atomic size decreases
in going from left to right. Therefore, C is smaller than B.
(d) The element C is a nonmetal. Therefore, it forms an anion.
Illustrative Questions
Q.1. In the modern periodic table, calcium element (atomic number 20) is surrounded by elements with atomic number
12, 19, 21 and 38, respectively. Which of these have physical and chemical properties resembling with calcium?
Ans. Elements having atomic numbers 12 and 38 will show properties resembling with calcium because all these
belong to the same group (Group no. IIA).
Q.2. By considering their position in the periodic table, which one of the following elements would you expect to
have the most metallic characteristics?
Ga, Ge, As, Se, Be
Ans. Metals occupy the left side of the periodic table.
Be (beryllium) is expected to have the most metallic characteristics.
Q.3. In the modern periodic table, of the rst ten elements, which are metals?
Ans. Of the rst ten elements, the elements which are metals are Li and Be.
