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Author Topic: Criss Cross method  (Read 4341 times)

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ericman5

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Criss Cross method
« on: January 28, 2010, 02:46:55 PM »

Hello anyone who knows chemistry im new here.

Anyways im trying to study for my mid term toomrow in chemistry, but i don't know how to do the criss cross method. You do it to find the sub script number when naming compunds.

I know you change the two numbers, but I dont know how to find that number.

Example: Al +3 and Cl -1 The 3 becomes the subscript for the chloride ion and the 1 becomes understood for aluminum, to form aluminum chloride: AlCl3

But how do you find the initial + 3 and -1???

after i have those + 3 and such I know how to do it.
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Wreath

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Re: Criss Cross method
« Reply #1 on: January 28, 2010, 09:47:16 PM »

look at electron configuration. these are called oxidation numbers. Cl, when is acting like anion, can only have -1, because if it gain this electron, it will have the electron configuration of Ar, which is the most stable electron configuration for Cl (in general, in each period the gas in VIII.A represents the most stable electron configuration). so, you should know halogen anions always has -1, I.A elements +1, II.A elements +2...so when you have AlCl3, you know this: Cl always -1, and there are three of them - so that's -3, molecule has to be neutral - so only one Al must have +3.
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