Chemistry XII chapter 1

Question 1.4: Give the reasons for following.

a. The d and f block elements are called transition elements.

Answer: They are called transition elements due to following reasons.

i. They are present between s and p-block elements of the periodic table.

ii. Their properties are in transition between s and p block elements.

b. Lanthanide contraction controls the size of elements of 6th and 7th period.

Answer: As we move from one element to next on its right in a period of elements in periodic table, valence shell remains same and nuclear charge increasse by 1 unit in each element. So, attraction between valence electron and nucleus also increases. As a result, atomic size gradually decreases in the period. However, this reduction in atomic size is significant in 6th and 7th period of elements. It is known as Lanthanide contraction. It controls the size of 6th and 7th period of elements.

C. The melting and boiling points of elements increases from left to the right upto the middle of s and p-block elements and then decrease onward.
Answer: The melting and and boiling points of elements depend upon their number of binding electrons. Greater the the number of binding electron, higher the melting and boiling points. The number of binding electrons increases from left to right across the period of elements in the periodic table and is maximum four up to 4th group. For the example, Carbon has four binding electrons and has giant structure. So therefore, it has high melting and boiling points. Next

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