Sat March 03, 2012 By:


Expert Reply
Sat March 03, 2012
1. Being a cation Mg2+ has a smaller size when compared to an anion O2-. In case of cations after removal of electrons, there happen to be more protons than electrons. This therefore, increases the attraction of electrons more toward nucleus. So, size decreases. Unlikely in anions the attraction decreses between electron and proton due to addition of more electrons to the orbitals. Also, the electron-electron repulsion increases. These factors causes increase in the size of the anion.
The size of fluorine atom is small. As a result, there are strong electron-electron repulsions in the compact 2p-orbital. Thus, the electron gain enthalpy of fluorine is less negative than that of chlorine.
3. Ionisation energy is a measure of the energy needed to pull a particular electron away from the attraction of the nucleus. A high value of ionisation energy shows a high attraction between the electron and the nucleus.

The fact is that boron's outer electron is in a 2p orbital rather than a 2s. 2p orbitals have a slightly higher energy than the 2s orbital, and the electron is, on average, to be found further from the nucleus. This has two effects.

  • The increased distance results in a reduced attraction and so a reduced ionisation energy.

  • The 2p orbital is screened not only by the 1s2 electrons but, to some extent, by the 2s2 electrons as well. That also reduces the pull from the nucleus and so lowers the ionisation energy.

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