In the followings ,decreasing order of 1st ionisation energy will be ,i)Xe,ii)Al,iii)Be,iv)As. ?

1 Answer
Apr 6, 2017


Got a Periodic Table handy?


.......of course you have one nearby because you are doing your chemistry homework.

Now ionization energies are defined by the following rxn:

#"Atom(g)" + Delta rarr "Cation(g)" + e^(-)#

Now we know the ionization energies INCREASE across a Period from left to right as we face the Table, but DECREASE down a Group, a column of the Periodic Table.

And simple ideas of electrostatics can rationalize this trend. Incomplete electronic shells shield the nuclear charge very imperfectly. And across the Period, ionization energies should increase. This also manifests in the well known decrease in atomic radii from left to right as we face the Table. As a valence shell is completed, shielding of the nuclear charge becomes more effective, and the electrons occupy a new valence shell to begin the process again.

So we have (i) 5th Period xenon, an inert gas; (ii) 3rd Period aluminum; (iii) 2nd Period beryllium; and 4th Period arsenic. Because we compare (i) different valence shells, and (ii) different electronic configurations, it is BY NO MEANS straightforward to predict the relative order of ionization energies.

At a push we might predict the following order of DECREASING ionization energies:

#"(i) xenon; (ii) Be; (iii) As; and (iv) Al"#. Am I right? I don't know. But I am required to interpret data NOT REMEMBER IT (and of course so are you!). And so now this is your job. Find the relevant ionization energies, and INFORM YOUR ARGUMENT WITH DATA...........This question is quite unsuitable, even at the undergraduate level, WITHOUT reference to the relevant ionization energies. No-one expects you to remember these.