How to know if a compound has triple bond?

Question

Which of the following substances contains a triple bond?
H2S,H2CO,HOF,C2F2,BF3.
Please explain the method used to solve.

Answer 1

You have to count up and distribute the availalble electrons. When you do not have the correct amount of electrons to give each bond a single pair, then electrons have to be pared by forming double or triple bonds. Each atom has to have the octet (there are some exceptions)

In your case, look at C2F2

The carbons bring 4 electrons each and the fluorines bring 7 electrons each. So you have a total of 22 electrons, or 11 pairs.

Putting the paired electrons around each fluorine and the C-F bond, uses 8 pairs. That leaves 3 pairs, or a triple bond, between the 2 carbons

Answer 2

If you search the web or Wikipedia for each compound, there will be a "chemical structure". If it shows one line between atoms, it is a single bond, two lines a double bond, three lines a triple bond.

Otherwise, you can make a guess about the structure based on the most common valence, or oxidation state, of each atom. I would not recommend guessing, a lot of structures are not obvious.

C2F2 is 1,2-difluoroethyne and has a triple bond

For a picture of a structure, see:

http://sci-toys.com/pages/12C22Ddifluoroethyne.html

Two carbon atoms have a total of eight electrons that will form a chemical bond. A fluorine atom will bond with one of these electrons. Two fluorine atoms will bond with one electron from each carbon atom, leaving six electrons.

You have to match the three electrons left over on one carbon atom with the three electrons left over on the other carbon atom. Each carbon carbon bond requires two electrons. So there will be three electrons from one carbon bonding with three electrons of the other carbon. This is called a triple bond.

Again, unless you have a lot of experience with Lewis dot diagrams, try to find the structure in a chemical reference.