Question

Explain each of the following observations. You should refer to any general expected trends, and then explain the reasons for any observed exceptions.

a) The difference between the third and fourth ionization energies of Sc is much larger than the difference between the third and fourth ionization energies of Ti.

b) The electron affinity of lithium is negative, but the electron affinity of its neighbor beryllium is positive.

c) The Cl atom is smaller than the K atom, but the Cl- ion is larger than the K+ ion.

d) Although the first ionization energy of Rb is less than the first ionization energy of Sr, the second ionization energy of Rb is greater than the second ionization energy of Sr.

Answer 1

(a) Outer electronic configuration of Scandium is 4s², 3d¹

Outer electronic configuration of Titanium is 4s², 3d²

Inorder to remove the 4 ^th electron from scandium , we have to take an electron from a completely filled shell, so that the configuration will be highly unstable and it takes more energy to remove 4 ^th electron. But for titanium if the 4 ^th electron is removed it will end up in a stable configuration. So the difference between the third and fourth ionization energies of Sc is much larger than the difference between the third and fourth ionization energies of Ti.

(b) electron affinity is the energy  relesed when one electron is added to one mole of atoms in the gaseous state. If energy is released when an extra electron is added to the atom, the electron affinity will be negative.Similarly, if energy is required to add an electron to an atom, then electron affinity will be positive.   

electronic configuration of Li is [He]2s¹ if an extra electron is added it will end up in a stable electronic configuration and energy will be released. So electron affinity will be negative.

electronic configuration of Be is [He]2s². It is a stable electronic configuration, so inorder to add an extra electron energy has to be given to the atom. So electron affinity will be positive.

(c) Atomic number of Cl is 17(17 electrons), when it gains one more electron then becomes Cl- having 18 electrons. Because of the presence of excessive electron, electron-electron repulsion causes Cl- size becomes larger.

Atomic number of K is 19 (19 electrons). When it loses one electron it becomes K+ having 18 electrons . Here electron-proton attraction becomes dominant due to excess number of proton than electron. Due to this attraction K+ size becomes small.

(d) electronic configuration of Rb is [Kr]5s¹ and Sr is [Kr]5s²

When one electron is removed from Rb it will end up in a stable configuration but for Sr if one electron is removed it will end up in an unstable configuration.So first ionisation energy of Rb is less than that of Sr. But when a second electron is removing , an electron should be removed from a 4p subshell in the case of Rb. 4p subshell has a lower energy than the 5s subshell.Therfore the second ionization energy of Rb will be greater than the second ionization energy of Sr.