The last few posts were dedicated entirely to studying the periodic table of elements. This table is at the core of almost everything that is tangible in the universe. It is the culmination of the efforts of innumerable scientists, all over the world, to organize all the elements in the best possible way. We know that it took great effort to develop this table, to study the impeccable organization of elements, and to quantitatively put forth parameters describing each element. But is this table completely foolproof? We try to find the answer to this question in this concluding post on the periodic table.
The placement of hydrogen.
Hydrogen is placed at the extreme left of the periodic table just above the alkali metals. Hydrogen has 1 electron in its valence shell, just like alkali metals. In some reactions, it donates this electron to form an H+ ion, just like alkali metals.
However, placing it in Group 1 is debatable. Why? It is because of the following reasons –
- Hydrogen is a non-metal. So, placing it above alkali metals is technically not the right thing to do.
- Hydrogen behaves like halogens in some reactions. Hydrogen can gain one electron and form a hydride ion(H- ), just like halogens.
Hydrogen forms hydrides with the electropositive elements- Li+H-, Na+H– etc. This behavior is exactly like halogens, when they form compounds like LiCl and NaCl! - The ionization energy (IE) of hydrogen = 1312kJ/mol.
IE for alkali metals ≈ 375 – 520kJ/mol (The IE for alkali metals is low).
IE for halogens ≈ 1008 -1681 kJ/mol
Thus, we can observe that the ionization energy of hydrogen is comparable to that of the halogens, than to the alkali metals. - Hydrogen is a bad conductor of heat and electricity and exists as H2, just as halogens exist as Cl2, and Br2.
Thus the position of hydrogen in the periodic table is questionable.
Actinide elements in f-block?
There isn’t much difference in the energies of 5f and 6s orbitals. So, it is difficult to predict whether the valence electron has entered the 5f or 6s shell.
e.g.– The electronic configuration of Thorium is [Rn] 6d2 7s2
So, there is no electron in the outer f-orbital in thorium and yet is placed in the f-block of the periodic table! This is an anomaly too !!
Group 3 Anomaly.
The third group in the periodic table has an anomaly too. Group 3 has f-block elements in its 6th and 7th periods. The f-block or inner transition elements, as the name suggests, should have their valence electron/s in the f-shell. However, Lanthanum(La), Actinium (Ac), which are placed in the f-block, do not have electrons in the f-orbital! The electronic configuration of these elements is as follows-
La ⇒[Xe] 5d1 6s2
Ac⇒[Rn] 6d1 7s2
Moreover, there is controversy with Lu and Lr elements too!
Lu ⇒[Xe] 4f14 5d1 6s2
Lr ⇒ [Rn] 5f14 6d17s2
The last electron enters the 5d and 6d shells in Lutetium (Lu) and Lawrencium(Lr) respectively. Thus, technically they should be under the d- block !! Recent studies have shown that the valence electron in Lr goes to the 7p orbital rather than the 6d! So this element behaves differently altogether !! In spite of these differences, these elements are placed with the other f-block elements in the periodic table.
After looking at the anomalies above, we can conclude that the periodic table is not completely unerring. Nonetheless, this does not lessen the importance of the periodic table. This table undoubtedly gives us an overview of all the elements in a systematized way!
We end our discussion on the periodic table with this post. We shall start talking about a very important branch of chemistry in our upcoming posts. Till then,
Be a perpetual student of life and keep learning…
Good day!
References and further reading –
Madoverchemistry 