The last few posts were completely dedicated to studying the periodic table – A table of elements, which is very fundamental to many sciences like chemistry, physics and material science. This table is at the core of almost everything that is tangible in the universe.It took great effort to develop the entire table, to study the impeccable organization that already existed in nature and to develop means to quantitatively put forth parameters , that will help describe the elements to the fullest. 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 H+ ion, just like alkali metals.
However, still placing it in Group in debatable as –
- Hydrogen is a non-metal. Then why is it placed above all the alkali metals?
- Hydrogen also behaves like halogens in some reactions. Hydrogen can gain one electron and form hydride ion(H– ) , just like halogens.
With electropositive elements, hydrogen forms hydrides like Li+H– , Na+H– etc. exactly like halogens form LiCl and NaCl !
The ionisation energy (IE) of hydrogen = 1312kJ/mol.
IE for alkali metals ≈ 375 – 520kJ/mol
IE for halogens ≈ 1008 -1681 kJ/mol
Thus, we can observe that the ionisation energy of hydrogen is comparable to that of halogens than the alkali metals.The IE for alkali metals are low.
- Hydrogen is a bad conductor of heat and electricity and exists as H2 just as halogens exist as Cl2,Br2 .
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 electron has entered the 5f or 6s shell.
e.g.– 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
Group 3 has f-block elements in its 6th and 7th periods.The f–block elements or inner transition elements are elements which have valence electrons in the f-orbital.Lanthanum(La),Actinum(Ac) are placed in the f-block,but both these elements do not have electrons in the outer most f-orbital !
La ⇒[Xe] 5d1 6s2
Ac⇒[Rn] 6d1 7s2
There is controversy with Lu and Lr elements too !
Lu ⇒[Xe] 4f14 5d1 6s2
Lr ⇒ [Rn] 5f14 6d17s2
In these elements the last electron enters the 5d and 6d shells. So, they technically should be under the d- block !!Recent studies have shown that the valence electron in Lr goes to 7p orbital rather than the 6d! So this element behaves differently altogether !!
So, we can conclude that the periodic table is not yet completely unerring. Some adjustments have been made to get the table as we know it today. Nonetheless , this does not lessen its immense importance in giving us an overview of all the elements in nature in such a systematized and beautiful 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 –