Classification

The group of elements in which the differentiating electrons enters in the antipenultimate (inner to the penultimate shell, ie, ) shell are called block elements or inner-transition elements.

lonisation potential is the amount of energy required to remove an electron from an isolated gaseous atom. Since, on moving down the group, the size of atom increases, thus the outer electron gets farther and farther away from the nucleus and hence, the less amount of energy is required to remove it. Thus, ionisation potential decreases and hence, has lowest ionisation potential.

and are isoelectronic species (containing equal number of electrons, i.e., ). Their radii would be different because of their different nuclear charge. Anion with the greater negative charge will have the larger radius hence has larger size than . For isoelectronic species, radii Further and are also isoelectronic species but they have electrons. Hence, their sizes are smaller than ion.

Lanthanides and actinides constitute transition series within transition series and their last electron occupies the antipenultimate shell, so they are named as inner transition elements also.

Generally, electron gain enthalpy increases as we move from left to right in a period and decreases as we go down in a group. But electron affinities of second period elements are less negative as compared to corresponding third period element. This is because of the small atomic size of second period elements. Hence, the correct order electron affinities is Least negative Most negative

The fast lonisation Energy increases in a. period.
Thus, the first IE of the elements of second period should be as follows:

But in practice, the elements do not follow the above order.
The first IE of these element is

The lower IE of B than that of Be is because in , electron is to be
removed from which is easy while in , electron is to be removed from which is difficult.
The low IE of is lower than that of because of the half- filled orbitals in .

The elements of group have relatively high negative electron gain enthalpy values which decrease down the group from to Po. Oxygen has lowest negative electron gain enthalpy due to its small size which makes electron density high and repels the incoming electron

Arsenic is a metalloid while rubidium, titanium and cadmium are are metals