When to use double and when to use triple bonds?

[Kabigon]

I seemed to be confuse when to use double and triple bonds. For example, take CH2O and HCN. How do you know if you can use a triple or a double bond on the carbon and oxygen for CH2O? I know the correct answer is a double bond, with two lone pairs on the oxygen. And for HCN, it is a triple bond on the carbon and nitrogen with only one lone pair on the nitrogen.

But how do you know when to use a triple or double? I am very confused.. is there a rule to know when you can use a double bond, and leave two lone pairs hanging out, or a triple bond with only one lone pair? Thanks.

 

[Kurdt]

Well consider the CH2O molecule. The carbon has four free electrons in its outer shell and 2 of those are used with the 2 Hydrogen atoms so it only has 2 bonds left and so forms a double bond with the oxygen. I hope this simple explanation has clarrified things.

 

[Kabigon]

Oh. I'm so stupid. I almost forgot about that part. Thanks!