IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: moon on November 02, 2010, 01:03:17 pm

Title: Chemistry - Urgent - help Question in ppr 2
Post by: moon on November 02, 2010, 01:03:17 pm
plz help me to solve this question. thanx in advance.

element       Atomic radius/pm          Ionic radius/pm

lithium                   152                    ??????
sodium                  186                       95
potassium              227                      138
Title: Re: Chemistry - Urgent - help Question in ppr 2
Post by: elemis on November 02, 2010, 01:32:50 pm
Is the answer 59 ?
Title: Re: Chemistry - Urgent - help Question in ppr 2
Post by: moon on November 02, 2010, 01:46:24 pm
Quote from: Ari Ben Canaan on November 02, 2010, 01:32:50 pm
Is the answer 59 ?

No Ari the answer is 78.
Title: Re: Chemistry - Urgent - help Question in ppr 2
Post by: iluvme on November 02, 2010, 02:06:50 pm
(95/186)*152
Title: Re: Chemistry - Urgent - help Question in ppr 2
Post by: Deadly_king on November 02, 2010, 04:52:40 pm
Quote from: moon on November 02, 2010, 01:03:17 pm
plz help me to solve this question. thanx in advance.

element       Atomic radius/pm          Ionic radius/pm

lithium                   152                    ??????
sodium                  186                       95
potassium              227                      138

Hmm..............are you sure we need to find the ionic radius exactly?

Or is it just an estimate?

Koz i've never come across such a question. :-[
Title: Re: Chemistry - Urgent - help Question in ppr 2
Post by: moon on November 02, 2010, 05:09:33 pm
it just estimate.
Title: Re: Chemistry - Urgent - help Question in ppr 2
Post by: yasser37 on November 02, 2010, 05:18:53 pm
I've never seen this but

(95/186) * 152
Title: Re: Chemistry - Urgent - help Question in ppr 2
Post by: Deadly_king on November 02, 2010, 05:23:12 pm
Quote from: moon on November 02, 2010, 05:09:33 pm
it just estimate.

Hehe.............now it's simpler. ;)

First you should note that all of them are in Group 1. Hence their outermost electrons lie in an s-orbital.

However as we move down the group from lithium to potassium, the electric force between the positively charged nucleus and the outermost electron decreases since there is more shielding effect by the other preceding shells.

Moreover lithium has electronic configuration of 1s22s1. Losing one electron to form its corresponding Li+ ion will cause the electric force between the nucleus and the remaining two electrons from the 1s orbital to increase.

So ionic radius decreases. However when compared to sodium and potassium, this decrease is low since the electric force involves less protons(Li has only 3 protons while sodium has 11).

Decrease for sodium was (186 - 95) = 91.

For lithium, this decrease would be much lower, lets say about 70.
Hence ionic radius comes out to be around 80. ;)