Month: July 2015

Question: Collect the information about reactivity of VIII A group elements (noble gases) from internet or from your school library and prepare a report on their special character when compared to other elements of periodic table. (4 Marks)

Answer: ! VIII A group elements are noble gases or inert gases. As the name implies that they do not react with any other element under ordinary conditions.
This is because except He, other inert gas elements have stable ‘octet’ configuration. Even for He, the two available orbitals in 1s is filled with electrons and so it is also very inactive.

These elements can neither loose electrons nor gain electrons as they have very high ionization energy and zero electron affinity.
In recent times experiments have established that under suitable conditions heavier elements of this group can form compounds with F2, O2. Xenon (Xe) shows a tendency to loose an electron and exist as a positive ion. So it reacts with highly electronegative elements like F2 and O2 only.
e.g.: XeO3, XeO4, XeF2, XeF4, ………

Under favourable conditions Ar is able to form coordination compounds with BF3. The composition of this new compound is Ar.n.BF3. N.Barltel prepared xenon fluoroplatinate Xe (PlF6). This is the first compound prepared with an inert gas.

Incidentally, it is identified that these compounds have varied applications in different fields.

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Question: How do you appreciate the role of electronic configuration of the atoms of elements in periodic classification. (4 Marks)

Answer: H.J. Moseley (1913) analysed the X – ray pattern obtained when elements are subjected to bombardment by high energy electrons. Moseley realised that the atomic number is more fundamental characteristic of an element than its atomic weight.

This atomic number concept forced the periodic law to be changed. So the modern periodic law is ‘The physical and chemical properties of elements are the periodic function of electronic configurations of their atoms’.

According to the modern periodic law, the elements having same outer shell electronic configuration are kept in the same group. These elements have similar chemical properties.

This classification rectified all the short comings of Mendeleev’s
classification and also enabled the study of physical and chemical properties of elements easily.

I appreciate the usefulness of electronic configuration in arranging elements in periodic table so that the place of an element in this table can be easily identified.

Question: In the period 2, element X is to the right of element Y. Then find which of the elements have
I) Low nuclear charge II) Low atomic size
III) High ionization energy IV) High electronegativity
V) More metallic character (AS – 1) (4 Marks)

Answer:
I) Low nuclear charge: Element Y has low nuclear charge.
II) Low atomic size: Element Y has low atomic size.
III) High ionization energy: Element Y has high ionization energy.
IV) High electronegativity: Element Y has high electron negativity.
V) Metallic character: X has more metallic character.

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Question:
How does metallic character change when we move
I) Down a group II) Across a period. (AS – 1) (2 Marks)

Answer:
I) Down a group: Metallic character of elements increases as we move down a group.
II) Across a period: Metallic character of elements decreases as we move across a period.

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Question:
Why was the basis of classification of elements changed from the atomic mass to atomic number? (AS – 1) (4 Marks)

Answer: At the time (1829) when Dobereiner established the law of triads electrons were not discovered. With the available information of atomic weights he established the law of triads.

Even at the time of John Newlands (1865) with the same available data of atomic weights of elements he established the law of octaves. The same process continued with Mendeleev while developing the periodic table.

Mendeleev’s periodic table faced limitations and the anomalous pair of elements, and dissimilar elements placed together paved way to put the minds of the scientists on this issue.

Incidentally in 1913 Moseley’s X-ray analysis introduced the concept that the atomic number is more fundamental characteristic of an element than its atomic weight. As a result the modern periodic law is stated as ‘The properties of the elements are periodic function of their atomic number’.

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Question: Complete the following table using the periodic table. (4 Marks)