Chemistry Class 11 NCERT Solutions: Chapter 3 Classification of Elements and Periodicity Part 3
Q: 16. Among the second period elements the actual ionization enthalpies are in the order
(i) Be has higher
(ii) O has lower
(i) During the process of ionization, the electron to be removed from beryllium atom is a 2s-electron, whereas the electron to be removed from boron atom is a 2p-electron. Now, 2s-electrons are more strongly attached to the nucleus than 2p-electrons. Therefore, more energy is required to remove a 2s-electron of beryllium than that required to remove a 2p-electron of boron. Hence, beryllium has higher
(ii) In nitrogen, the three 2p-electrons of nitrogen occupy three different atomic orbitals. However, in oxygen, two of the four 2p-electrons of oxygen occupy the same 2p-orbital. This results in increased electron-electron repulsion in oxygen atom. As a result, the energy required to remove the fourth 2p-electron from oxygen is less as compared to the energy required to remove one of the three 2p-electrons from nitrogen. Hence, oxygen has lower than nitrogen.
Fluorine contains one electron and one proton more than oxygen. As the electron is being added to the same shell, the increase in nuclear attraction (due to the addition of a proton) is more the increase in electronic repulsion (due to the addition of an electron). Therefore, the valence electrons in fluorine atom experience a more effective nuclear charge than that experienced by the electrons present in oxygen. As a result, more energy is required to remove an electron from fluorine atom than that required to remove an electron from oxygen atom. Hence, oxygen has lower than fluorine.
Q: 17. How would you explain the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium?
The first ionization enthalpy of sodium is lower than that of magnesium. This is primarily because of two reasons:
The atomic size of sodium is greater than that of magnesium.
2. The effective nuclear charge of magnesium is higher than that of sodium
For these reasons, the energy required to remove an electron from magnesium is more than the energy required in sodium. Hence, the first ionization enthalpy of sodium is lower than that of magnesium.
However, the second ionization enthalpy of sodium is higher than that of magnesium. This is because after losing an electron, sodium attains the stable noble gas configuration. On the other hand, magnesium, after losing an electron still has one electron in the 3s-orbital. In order to attain the stable noble gas configuration, it still has to lose one more electron. Thus, the energy required to remove the second electron in case of sodium is much higher than that required in case of magnesium. Hence, the second ionization enthalpy of sodium is higher than that of magnesium.
Q: 18. What are the various factors due to which the ionization enthalpy of the main group elements tends to decrease down a group?
The factors responsible for the ionization enthalpy of the main group elements to decrease down a group are listed below:
(i) Increase in the atomic size of elements: As we move down a group, the number of shells increases. As a result, the atomic size also increases gradually on moving down a group. As the distance of the valence electrons from the nucleus increases, the electrons are not held very strongly. Thus, they can be removed easily. Hence, on moving down a group, ionization energy decreases.
(ii) Increase in the shielding effect: The number of inner shells of electrons increases on moving down a group. Therefore, the shielding of the valence electrons from the nucleus by the inner core electrons increases down a group. As a result, the valence electrons are not held very tightly by the nucleus. Hence, the energy required to remove a valence electron decreases down a group.
Q: 19. The first ionization enthalpy values of group 13 elements are:
How would you explain this deviation from the general trend?
On moving down a group, ionization enthalpy generally decreases due to an increase in the atomic size and shielding. Thus, on moving down group 13, ionization enthalpy decreases from to. But, has higher ionization enthalpy than. follows immediately after s-block elective. These electrons do not shield the valence electrons very effectively. As a result, the valence electrons of experience a greater effective nuclear charge than those of. Further, moving from to, the ionization enthalpy decreases. In the periodic table, follows after 4f and 5d electrons. The shielding provided by the electrons in both these orbitals is not very effective. Therefore, the valence electron is held quite strongly by the nucleus. Hence, the ionization energy of is on the higher side.
Q: 20. Which of the following pairs of elements would have a more negative electron gain enthalpy?
(i) and are present in the same period of the periodic table. An atom has one proton and one electron more than and as an electron is being added to the same shell; the atomic size of is smaller than that of. As F contains one proton more than, its nucleus can attract the incoming electron more strongly in comparison to the nucleus of atom. Also, needs only one more electron to attain the stable noble gas configuration. Hence, the electron gain enthalpy of is more negative than that of.
(ii) and belong to the same group of the periodic table. The electron gain enthalpy usually becomes less negative on moving down a group. However, in this case, the value of the electron gain enthalpy of is more negative than that of F. This is because the atomic size of is smaller than that of . In F, the electron will be added to quantum level , but in , the electron is added to quantum level . Therefore, there are less electron-electron repulsions in and an additional electron can be accommodated easily. Hence, the electron gain enthalpy of is more negative than that of .
Q: 21. Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.
When an electron is added to O atom to from ion, energy is released. Thus, the first electron gain enthalpy of O is negative.
On the other hand, when an electron is added to ion to from ion, energy has to be given out in order to overcome the strong electronic repulsions. Thus, the second electron gain enthalpy of O is positive.