2013 Destroyer GC #43

[Moka11]

An unknown compound needed to be identified. Studies revealed that it reacted with Oxygen and Fluorine to yield Y2O3 and YF3. Which element listed below could be element Y?

a) As
b) C
c) Mg
d) Ar
e) Hg

Answer is a. Can someone please explain to me why it cant be b?

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[Meat Gyver]

This question just deals with the charges of the ions.

Long way: Given Y2O3 you know that 2Y + 3O = 0 (neutral compound) Oxygen (as an ion) carries a -2 charge (that is, it will accept 2 e- to get a filled octet). If you plug -2 for oxygen in the equation, the net charge for all 3 oxygens is -6. Seeing that this is a neutral compound you need 2Y to have a +6 charge. So 1 Y ion should carry a +3 charge. If you look on a periodic table, which element has 3 valence electrons it is willing to share to get a filled octet? It can't be Carbon because it has 4 valence electrons, nor can it be Magnesium or Mercury because they only have 2. Ar is a noble gas, which has 8 valence electrons.

Fast way: F- has a -1 charge, and if 3 F- (-3 charge in total) = 1 Y then Y is +3. So which of these can share 3 electrons and get a filled octet in the process? As

 

[chlrbwls]

Because when it reacts with Fluoride it would be C F4 not CF3

and for these situations you would want an element with different oxidation states that suffice the conditions.

 

[Moka11]

Thanks so much both you guys! Got it!