(a) List any three observations which posed a challenge to
Mendeleev’s Periodic Law.
(b) How does the metallic character of elements vary on moving from
(i) left to right in a period,
(ii) from top to bottom in a group
of the Modern Periodic Table ?
Give reason for your answer.
Answers
Answer:
a.1.Position of hydrogen .
2.position of isotopes
3.position of whole number of atomic masses like cobalt and nickel.
b. 1 . metallic character goes on decreasing while going to left to right.
2. metallic character goes on increasing while going from top to bottom
Explanation:
Reasons.
a . Mendeleev's periodic table based upon atomic mass number.
b.1.metallic character decrease from left to right because of increase in nuclear attraction.
2.metallic character increase from top to bottom due to decrease in nuclear attractions as atomic radius increase i.e. metallic character increase
Hope it help you
a) (i) Position of hydrogen : Position of hydrogen in the periodic table is uncertain. It has been placed in 1A group with alkali metals, but certain properties of hydrogen resemble those of halogens. So, it may be placed in the group of halogens as well.
(ii) Position of isotopes : Isotopes are the atoms of the same element having different atomic masses. Therefore, according to Mendeleev’s classification these should be placed at different places depending upon their atomic masses. For example, hydrogen isotopes with atomic masses 1, 2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties.
(iii) Anomalous pairs of elements : In certain pairs of elements, the increasing order of atomic masses was not obeyed. In these, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses.
b) (i) decreaces , REASON : as effective nuclear charge acting on valence shells electrons increases across a period the tendency to lose electron hence decreases
(ii) increases , reason : the effective nuclear charge acting on valence electron decreases and hence the are able to lose electrons easily .
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