Page 151 - Chemistry ICSE Class IX
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The Periodic Table 139
Density. Alkaline earth metals have densities much higher than that
of alkali metals.
Melting and boiling points. Alkaline earth metals have low, but
much higher melting and boiling points than alkali metals.
'NGEVTQPKE EQPſIWTCVKQP Alkaline earth metals have only two
electrons in their outermost shell. Thus, these metals consist of two
valence electrons.
Chemical reactivity. Alkaline earth metals are also reactive metals.
However, these metals are less reactive than alkali metals. Alkaline
earth metals react with oxygen on heating, and with water (both cold
and hot) they liberate hydrogen.
Occurrence. Alkaline earth metals do not occur free in nature because
of their higher reactivity.
Metallic character. Alkaline earth metals are fairly strong metallic
(or electropositive) in nature. However, these are less metallic than
alkali metals. The metallic character of these metals increases down
the group.
Halogens (Group 17)
Elements. F, Cl, Br, I
Physical state. Diatomic gases.
No. of valence electrons. Seven.
Atomic size. Smallest in their respective period.
Conductivity. Nonconductors of electricity.
Reactivity. Highly reactive nonmetals.
Zero Group (Group 18)
Occurrence. Zero group elements (or noble gases), except radon,
are found in atmosphere. They make up about 1% of the earth’s
atmosphere.
Physical state. Under normal conditions, all zero group elements are
monoatomic, colourless, and odourless gases.
Density. The density of these elements increases with increase in
atomic mass.
Melting and boiling points. They have low melting and boiling points
as compared to the elements of other groups.
'NGEVTQPKE EQPſIWTCVKQP The outermost shell of these elements is
completely lled, i.e. they have eight electrons in their valence shell
except helium (which has two electrons in its valence shell).
Chemical reactivity. As zero group elements have completely lled
valence shell, the atoms of these elements do not have tendency to
gain or lose electron and form chemical bonds with other atoms.
Nature of compounds. These elements are quite soluble in water. They
also form compounds with oxygen and uorine but under extreme
conditions.
