Page 135 - Chemistry ICSE Class IX

The Periodic Table 123

Illustrative Questions

Q.1. Which property of the atoms formed the basis of Mendeleev’s classi cation?
Ans. Atomic mass.
Q.2. Many scientists before Mendeleev also used atomic mass as the basis of
classi cation, but why did only Mendeleev succeed?
Ans. The secret of Mendeleev’s success was that although the classi cation was
based on atomic mass, at many places he did not follow this rule rigidly. He
laid more stress on the similarity in the properties.
Q.3. Why did Mendeleev leave many gaps in his periodic table?
Ans. Mendeleev predicted that there were many elements yet to be discovered. So,
when none of the elements known at that time t into a particular position,
he left a gap there. Later, when more elements were discovered, these were
found to t into these gaps.
Q.4. What valency will be shown by an element having atomic number 17?
Ans. The atomic number of element is 17. So, there are 17 protons in its nucleus. Its
atom, therefore, would contain 17 electrons. These electrons are distributed
in various shells as 2, 8, 7.
Thus, there are seven valence electrons. This atom would require only one
electron to complete its shell. Thus, its valency is –1.
Q.5. Magnesium forms the following compounds:
Magnesium oxide – MgO, Magnesium hydroxide – Mg(OH) , Magnesium
2
sulphate – MgSO
4
Radium belongs to the same group. Write the formulae of radium oxide, Marie and Pierre Curie
radium hydroxide and radium sulphate. discovered radium
Ans. Radium belongs to the same group as magnesium. So, it should have the
same valency. Therefore, it should form the same type of compounds. So,
the formulae of Radium oxide is RaO, Radium hydroxide is Ra(OH) and
2
Radium sulphate is RaSO .
4
ASSIGNMENT 2

Mendeleev’s periodic table
ϭ͘ ^ƚĂƚĞ ƚŚĞ ƉĞƌŝŽĚŝĐ ůĂǁ ŽŶ ǁŚŝĐŚ DĞŶĚĞůĞĞǀ͛Ɛ ƉĞƌŝŽĚŝĐ ƚĂďůĞ ǁĂƐ ďĂƐĞĚ͘
Ϯ͘ ;ĂͿ ,Žǁ ŵĂŶLJ ĞůĞŵĞŶƚƐ ǁĞƌĞ ŬŶŽǁŶ Ăƚ ƚŚĞ ƟŵĞ ŽĨ DĞŶĚĞůĞĞǀ͍
;ďͿ EĂŵĞ ŽŶĞ ĞůĞŵĞŶƚ ǁŚŝĐŚ ǁĂƐ ŶŽƚ ŬŶŽǁŶ Ăƚ ƚŚĞ ƟŵĞ ŽĨ DĞŶĚĞůĞĞǀ͕ ďƵƚ ǁŚĞŶ ĚŝƐĐŽǀĞƌĞĚ ůĂƚĞƌ ĨŽƵŶĚ ƚŽ ŚĂǀĞ ƚŚĞ ƐĂŵĞ
ƉƌŽƉĞƌƟĞƐ ĂƐ ƉƌĞĚŝĐƚĞĚ ďLJ DĞŶĚĞůĞĞǀ͘
ϯ͘ ƌĂǁ Ă ůĂLJŽƵƚ ƐŬĞƚĐŚ ŽĨ ƚŚĞ DĞŶĚĞůĞĞǀ͛Ɛ ƉĞƌŝŽĚŝĐ ƚĂďůĞ͕ ĂŶĚ ůĂďĞů ƚŚĞ ŐƌŽƵƉƐ ĂŶĚ ƉĞƌŝŽĚƐ ŝŶ ƚŚŝƐ ƚĂďůĞ͘
ϰ͘ ,LJĚƌŽŐĞŶ ĐŽƵůĚ ďĞ ƉůĂĐĞĚ ďŽƚŚ ŝŶ 'ƌŽƵƉ / ĂƐ ǁĞůů ĂƐ ŝŶ 'ƌŽƵƉ s//͘ 'ŝǀĞ ƚŚƌĞĞ ƉŽŝŶƚƐ ƚŽ ƐƵƉƉŽƌƚ ƚŚŝƐ ƐƚĂƚĞŵĞŶƚ͘

Modern Periodic Law

What is the modern periodic law tĞ ĐĂŶ ŶŽǁ ƐĞĞ ŚŽǁ ĐůŽƐĞ
DĞŶĚĞůĞĞǀ ǁĂƐ ƚŽ ƚŚĞ ŵŽĚĞƌŶ
In 1913, H.G.J. Moseley in England, proved that the more fundamental ƉĞƌŝŽĚŝĐ ůĂǁ͊ /ƚ ǁĂƐ ƚŚŝƐ ƋƵĂŶƟƚLJ͕
property of an element is its atomic number. Therefore, he suggested that the ƚŚĞ atomic number ǁŚŝĐŚ ŚĞ
DCUKU QH ENCUUKſECVKQP QH GNGOGPVU UJQWNF DG CVQOKE PWODGT. This led to ǁĂƐ ůŽŽŬŝŶŐ ĨŽƌ ĂƐ ƚŚĞ ďĂƐŝƐ ŽĨ
the modern periodic law, which states that, the properties of elements are ĐůĂƐƐŝĮĐĂƟŽŶ͘ hŶĨŽƌƚƵŶĂƚĞůLJ͕ ŝƚ ǁĂƐ
periodic function of their atomic numbers. ŶŽƚ ŬŶŽǁŶ ǁŚĞŶ ŚĞ ǁĂƐ ĂůŝǀĞ͘ ^ĞĞ
ƚŚĞ ƚLJƌĂŶŶLJ ŽĨ ƟŵĞ͊