Chemical Bonding : Short questions

Question 6.12: Why is ionization measured when atom is gaseous state?

Answer: Ionization energy is measured when an electron is removed from valence shell of an atom. In gaseous atom, the valence electron is under influence of single nucleus. On the other hand, when atom is closer to another atom of the same or different kind, then outer electron is under influence of more than one nucleus. Then it is impossible to measure exact force of attraction between valence electrons and nucleus within an atom.

Question 6.13: Why the ionization energies decrease down the group although the nuclear charges increase?

Answer: There are two factors which cause decrease in the ionization energies from top to bottom in the periodic table.

Increase in shielding effect.

Addition a new shell in each element down the group.

These two factors decrease the force of attraction between the nucleus and outermost electrons so ionization energy decreases down the group.

Question 6.14: Ionization energy is index of the metallic character. Why?

Answer: The elements with low ionization energies are called metal and those having high ionization energies are called non-metals. The elements which have intermediate values are called metalloid. In the periodic table, as ionization energies of elements increase from left to right so metallic character of elements decreases. In the same way, down the group as ionization energies decrease, metallic character of elements increases.

Question 6.15: How do successive ionization energies help in guessing the valency of an element?

Answer: Energy gap between two successive ionization energies is small when both electrons are removed from same shell. However, large energy gap occurs between two successive ionization energies, when second electron is removed from lower shell. If energy gap between first and second is very large, then valency of element is one. Similarly when energy gap between two and third I.Es. is very large then valency the element is two. For example first second and third I.E. values for Mg are 738 Kj/mol, 1450 Kj/mol and 7730 Kj/mol respectively as difference between second and third are very large so its valency is 2.

Question 6.16: Why second I.E. of an element is always greater then first one? Answer: First electron is removed from a neutral atom so first I.E. is low. The second electron is removed from uni-positive ion so second I.E. is very high. Reason is that second electron is relatively closer to the nucleus as compared to first and hence more force of attraction between second electron and nucleus.

Question 6.17: Why the ionization energies of III-A group elements are less as compared to II-A, although the values should increase from the left to the right in a period?

Answer: Although the atomic sizes of III–A group elements are smaller than those of II–A group element in their own periods but Ionization energies of III–A group elements are lower. This can be explained on the basis of electronic distribution in valence subshell. The II–A group elements have relatively stable electronic configuration due to completely filled s-sub shell. So relatively high energy is needed to remove an electron from its completely filled stable orbital.

Question 6.18: Why do ionization energies of groups III–A and VI–A show abnormal trend?

Answer: This can be explained on the basis of electronic distribution in valence subshell. The II–A group elements have relatively stable electronic configuration due to completely filled s-sub shell. So relatively high energy is needed to remove an electron from its completely filled stable orbital. Similarly, the ionization energies of VI–A group elements are lower than ionization energies of V–A group elements. This is due to fact that, V–A group elements have relatively stable electronic configuration due to half filled p-sub shell. From top to bottom in a group, the atomic radii increase due to addition of new shell. Although the positive charge on the nucleus increases down the group but its effect is neutralized by a corresponding shielding effect of inner shell electrons.

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