Why 4th is not occur
but triple bond occurs

N has 5 valence electrons, meaning it can triple bond, have one lone pair, and still obey the octet rule. 

F can't triple bond - if it does, it would either disobey the octet rule (can't happen, it's in period 2 with no d orbitals) or have a really high positive formal charge, which isn't likely since F is really electronegative and tends to hold on to its electrons. 

Pb has 4 valence electrons. If it triple bond, it would leave one floating, unpaired electron. Unlikely. 

Finally, S has 6 valence electrons. If it triple bonds, it would have one lone pair, and an extra electron to bond with something else. It's logistically possible, since elements beyond period 2 can go disobey the octet rule, but it's not as likely to occur. 

N just fits.

The answer to that would be magnesium

The gas would begin to expand, and eventually the can would burst or explode.