In what manner is the long from of periodic table better than Mendeleef's periodic table? Explain with examples.
The long form of the periodic table is better than Mendeleef's periodic table because it classifies the elements on the basis of electronic configurations of their atoms. The characteristics of this table are
(i) The table consists of 9 vertical columns, called the groups and 7 horizontal rows, called the periods.
(ii) The groups are marked 0 to VIII out of which group I to VII are subdivided into subgroups $A$ and $B$.
(iii) The group IA elements ( $\mathrm{Li}, \mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs}$ and Fr ) are known as alkali metals and the group IIA elements ( $\mathrm{Be}, \mathrm{Mg}, \mathrm{Ca}, \mathrm{Sr}, \mathrm{Ba}$ and Ra ) are known as alkaline earth metals. Elements in group VIII A ( $\mathrm{F}, \mathrm{Cl}, \mathrm{Br}, \mathrm{I}$ and At ) are called halogens and elements in group VIII ( $\mathrm{He}, \mathrm{Ne}, \mathrm{Ar}, \mathrm{Kr}$, Xe and Rn ) are called noble gases or rare gases.
(iv) The group VIII has there similar elements placed together in one place. These are called transition triads, e.g., $\mathrm{Fe}, \mathrm{Co}$ and $\mathrm{Ni}, \mathrm{Ru}, \mathrm{Rh}$ and $\mathrm{Pd} ; \mathrm{Os}, \mathrm{Ir}$ and Pt etc.
(v) In the 6th and 7th period, 14 elements present called as lanthanides and actinides respectively.
(vi) Based on their electronic configuration, elements have been grouped into $s-, p-, d-$ and $f$-blocks. This has helped us to understand their properties more easily.
(vii) There is gradual change in properties seen from one end to the other.
Discuss and compare the trend in ionisation enthalpy of the elements of group 1 with those of group 17 elements.
The ionisation enthalpies decreases regularly as we move down a group from one element to the other. This is evident from the values of the first ionisation enthalpies of the elements of group 1 (alkali metals) and group 17 elements as given in table and figure.
| Group 1 | First ionisation enthalpies (kJ mol$$^{-1}$$) |
Group 17 | First ionisation enthalpies (kJ mol$$^{-1}$$) |
|---|---|---|---|
| H | 1312 | F | 1681 |
| Li | 520 | Cl | 1255 |
| Na | 496 | Br | 1142 |
| K | 419 | I | 1009 |
| Rb | 403 | At | 1009 |
| Cs | 374 |
Given trend can be easily explained on the basis of increasing atomic size and screening effect as follows
(i) On moving down the group, the atomic size increases gradually due to the addition of one new principal energy shell at each succeeding element. Hence, the distance of the valence electrons from the nucleus increases.
Consequently, the force of attraction by the nucleus for the valence electrons decreases and hence the ionisation enthalpy should decrease.
(ii) With the addition of new shells, the shielding or the screening effect increases. As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence the ionisation enthalpy should decrease.
(iii) Nuclear charge increases with increase in atomic number. As a result, the force of attraction by the nucleus for the valence electrons should increase and accordingly the ionisation enthalpy should increase.
The combined effect of the increase in the atomic size and the screening effect more than compensates the effect of the increased nuclear charges. Consequently, the valence electrons become less and less firmly held by the nucleus and hence the ionisation enthalpies gradually decrease as move down the group.