Page 39 - Chemistry_Form_2

Chemistry
for Secondary Schools

The following are changes in physical properties across periods:

(a) Decrease in atomic radii
Atomic radii decrease from left to right across a period due to the increasing
nuclear charge, which pulls the electrons closer to the nucleus. This trend
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contributes to designing strong and durable materials in construction and
manufacturing, as smaller atomic sizes promote stronger atomic bonding
and improved structural strength.

(b) Transition from metallic to non-metallic properties
Metals dominate the left side of the periodic table, while non-metals are
positioned on the right. On moving across a period, metallic properties
decrease, and non-metallic properties increase. This trend influences
material selection, with metals such as iron and aluminium serving as
structural materials due to their strengths, while non-metals such as carbon,
silicon, sulfur, and chlorine in plastics and rubber function as insulating
materials because of their poor conductivity.

(c) Variations in physical states
Elements exist in different states at room temperature (25 °C). Metals such
as aluminium and iron remain solid, elements such as oxygen and nitrogen
exist in gaseous form, and a few, including bromine, are liquids. This trend
is essential for safe storage and transportation. For example, gases require
pressurised cylinders, while solids remain stable in containers.

(d) Variations in melting and boiling points
Across a period, melting and boiling points generally increase at first due
to stronger metallic or covalent bonding. However, they begin to decrease
after Group IV as elements start to form molecules held together by weaker
intermolecular forces. Typically, the melting and boiling points increase
from Group I to Group IV, then decrease from Group V and stay relatively
low to Group VIII.
This trend has several practical applications in everyday life and industry.
For example, metals such as sodium (Na), magnesium (Mg), and aluminium
(Al), which lie between Groups I and III, have relatively high melting
points due to strong metallic bonding. These properties make them suitable
for use in cooking utensils, building materials, and aircraft parts where
heat resistance is important. Elements like silicon (Si) in Group IV, with
a giant covalent structure and very high melting point, are essential in
the manufacture of computer chips and solar panels. On the other hand,

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