Page 156 - Chemistry ICSE Class IX
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144 ICSE Chemistry – 9
Let us say it Again
Dobereiner’s triads. The atomic mass of the middle element of a Dobereiner’s triad is equal to the arithmetic mean of
the atomic masses of the other two elements.
Newlands’ law of octaves. The properties of each element resembled those of the eighth element before it and of the
eighth element following it.
OR
The properties of the elements were repeated at every eighth element like the eighth note of an octave in music.
Mendeleev’s periodic law. Mendeleev’s periodic law states that “the properties of the elements are a periodic function
of their atomic masses.”
Moseley found that the atomic number is a more fundamental property than atomic mass.
Modern periodic law. The modern periodic law states that “the properties of the elements are the periodic function of
atomic number.”
Long form periodic table. The original long form periodic table was designed by Böhr in 1920.
Periods. The horizontal rows of elements in the periodic table are called periods.
Groups. The vertical columns of elements in the periodic table are called groups.
Elements belonging to the same group have the same number of valence electrons, show the same valency, and show
similar chemical behaviour.
In the long form periodic table (Böhr), the groups are numbered from the left as IA, IIA, IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIB, IIIA,
IVA, VA, VIA, VIIA and 0.
The number of elements in each period of the long form of the periodic table is equal to the number of electrons in the
corresponding electron shell.
Periodicity in a group. In a group, the atomic size or atomic radius and metallic character increase, while the
electronegativity and ionisation energy decrease while going from top to bottom of the group.
Periodicity in a period. In a period, the number of valence electrons, electronegativity and ionisation energy increase,
while atomic radius (or size), metallic character and the basic nature of the oxides decrease while going from left
to right.
