[leena]

X,Y and Z are 3 consecutive non-transition elements in the Periodic Table.The 1st and 3rd ionisation enthalpies of the elements are
                                                                                                                                                                                                                              X                Y             Z
1st ionisation enthalpy/kJmol^-1                      1012            999         1251
3RD ionisation enthalpy/kJmol^-1                      2912           3361        3822

The element X belongs to the following group in the periodic table:
(1) 1       (2) 2   (3) 3      (4) 4       (5)   5  (ans: 5)

I tried to figure it out by writing the electronic configurations,
(n-1)p⁶ns¹
(n-1)p⁶ns²
ns²np¹
ns²np²
ns²np³
ns²np⁴
ns²np⁵
ns²np⁶

but it's still very difficult for me to solve this.

[Astrokel]

Observe any irregularity in the data compared to the general trend expected. Assume X is in group 1 or so and why it is illogical for it to be in that group.

[leena]

Yes.I tried thinking of it like that,but I still can't figure it out.

X can't be in Group 1,cause then Y would have to be group 2 and the 1st IE of Group 2 should be higher than that of group1 or group 3(in Z),but it's not the case here.

Then according to the 1st IE ,X can be group 2 ,Y-GROUP 3 and Z-group 4,but not acording to the 3rd IE

X can't be group 3 cause the 1st IE gradually increases from group 3 to 5 (but not in X to Z,1st IE of Y<Z)

X can't be group 4 cuz Y would then be group5 and Y should theoretically have a higher 1st IE ,but here it's less.

X can't be group 5 cause although,according to me it agrees with the 1st IE values given,but not the 3rd IE as Y would then be group 6 and Z would be Group 7,and theoretically shouldn't the 3rd IE of group 7 elements be more than that of group 6,but here the IE of Y has tripled ,and even according to the electronic configuration of Group 6,the 3rd IE would be less than that given(compared to the 3rd IE of group 5),right?

So where am I going wrong?

THANK YOU

[Astrokel]

You are right. At a first look at 1st IE, 1-3 is definitely out. If you know, group 5&6 has a very noticeably 1st IE due to electronic repulsion. IE cannot be compared across, IE of Y tripled does not mean anything. Why would the 3rd IE of group 6 less than that of group 5?

[leena]

Yes,my mistake.i looked too much into the actual values given,and now I think the IEs of group 6 and 5,should be very close,i guess.