# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Problem of the Week Archive => Topic started by: Borek on March 19, 2012, 11:29:34 AM

Title: Problem of the week - 19/03/2012
Post by: Borek on March 19, 2012, 11:29:34 AM
This one is completely different. I remember answering this question in the past, but I don't remember where I got it from.

Add 4 more substances to the list:

H2, Li, He, Be, N2, B, O2, C, F2, Na, Ne, Mg...
Title: Re: Problem of the week - 19/03/2012
Post by: UG on March 20, 2012, 07:23:25 AM
P Al S Si  :-X
Title: Re: Problem of the week - 19/03/2012
Post by: XGen on March 20, 2012, 09:23:40 PM
P4, Al, S8, Si?

Basically UG but in standard states?
Title: Re: Problem of the week - 19/03/2012
Post by: Borek on March 22, 2012, 06:46:58 AM
Neither is what I expected. Can you explain the logic behind your answer?
Title: Re: Problem of the week - 19/03/2012
Post by: XGen on March 22, 2012, 06:59:57 AM
This is borderline guess, but the first, third, fifth, etc. elements progress along the periodic table and so do the second, forth, sixth, etc elements.
Title: Re: Problem of the week - 19/03/2012
Post by: Borek on March 22, 2012, 07:10:24 AM
I see.

OK, it is not the right answer  :P
Title: Re: Problem of the week - 19/03/2012
Post by: Ann1234 on March 23, 2012, 02:38:20 AM
H2, Li, He, Be, N2, B, O2, C, F2, Na, Ne, Mg...

I took the "math sequence" path-I may be way out but - I'll give this a try:

first I put the elements next to its atomic number- Then I found what do I need to add or subtract (in atomic number units) to get the next element:for example H is 1, plus 2 gives Li; Li is 3, minus 1 gives He, and so on:

H2--1  (+2)
Li---3  (-1)
He--2  (+2)

Be--4  (+3)
N2--7  (-2)
B---5  (+3)

02--8  (-2)
C---6  (+3)
F2--9  (+2)

Na--11 (-1)
Ne--10 (+2)
Mg--12  (?)

Then I studied the sequence of signs:
(+ - +) ( + - +)   (- + +) (-  +  ?)   ( ? ? ?)  ( ? ? ?)  and I continued the sequence as follows:
(+ - +) ( + - +)   (- + +) ( - +  +)   (+ - +)  (+ - +).

Lastly the number sequence:
(2 1 2) (3 2 3) (2 3 2)  (1 2 ?) ( ? ? ?) (? ? ?) I continued the sequence as:
(2 1 2) (3 2 3) (2 3 2 ) (1 2 1) (2 1 2) (3 2 3)

so putting all together: this will the completed sequence:
( +2 -1 +2) (+ 3 -2 +3) ( -2 +3 +2) (- 1+ 2+ 1) (+2 -1 +2) ( +3 -2 +3)

so completing the sequence on the top:

H2--1  (+2)
Li---3  (-1)
He--2  (+2)

Be--4  (+3)
N2--7  (-2)
B---5  (+3)

02--8  (-2)
C---6  (+3)
F2--9  (+2)

Na--11 (-1)
Ne--10 (+2)
Mg--12 (+1)

Al ---13 (+2)
P4----15 (-1)
Si---14 (+2)

S8----16

and I wrote them in standard states as all are on the list:so Al, P4, Si, S8 is my answer   ;D
Title: Re: Problem of the week - 19/03/2012
Post by: Fluorine on March 23, 2012, 03:09:02 AM
I think I got it after trying out many strange approaches for 20min...Cl, K, Ar, Ca?

Highlight text for explanation (possible spoiler): They are following 'gas, solid, gas, etc.' pattern on the periodic table going 1st gas, 1st solid, 2nd gas, 2nd solid so on. Very cool!
Title: Re: Problem of the week - 19/03/2012
Post by: Ann1234 on March 23, 2012, 03:23:05 AM
you' re right =) I think you got it- real cool!
Title: Re: Problem of the week - 19/03/2012
Post by: UG on March 23, 2012, 04:54:51 AM
This question is definitely a different one, keen to see the answer.
And that's a lot of typing Ann1234 :o
Title: Re: Problem of the week - 19/03/2012
Post by: Borek on March 23, 2012, 05:37:34 AM
I think I got it after trying out many strange approaches for 20min...Cl, K, Ar, Ca?

You are on the right track - but still not there.
Title: Re: Problem of the week - 19/03/2012
Post by: DrCMS on March 23, 2012, 07:18:53 AM
Cl2, Al, Ar, Si?
Title: Re: Problem of the week - 19/03/2012
Post by: Borek on March 23, 2012, 08:43:19 AM
Cl2, Al, Ar, Si?

Yep  :D
Title: Re: Problem of the week - 19/03/2012
Post by: Fluorine on March 23, 2012, 04:42:26 PM
Oops, I see where I made the silly mistakes.