I started commuting to work on my bicycle earlier this year and was not wearing a helmet for the first few weeks. A coworker gave me a hard time about it and I finally decided I should get one.
My chain broke on my short ride home today for lunch leaving me quickly unbalanced and on the ground in a hurry. My helmeted head definitely hit the ground and would likely have been badly injured had I not been wearing it.
I am very thankful that my coworker pressured me to wear what I thought was fairly unnecessary due to my short commute and safe riding habits.
If you ride a bicycle, please wear a helmet.
Also, get a better bicycle for commuting; chains should not break after a few weeks. This shows that the bicycle had been heavily used, or is an ancient rust bucket, and may be a "bicycle shaped object" to begin with.
I carry a Park chain linkpin press tool for opening and closing chain links. Though occasions for using that on the road are rare if the chain is well-maintained and replaced regularly, not long ago I had a fairly new chain derail off the front ring, and by some unfortunate combination of events get itself into a tight kink which jammed and caused a link to bend severely. Having the tool, I was able to splice out and discard the bent link, and be on my merry way. I also carry a short piece of chain that was left over from the original chain installation, because there is only so many links you can throw away before the chain gets too short.
When you get a new chain, the rear gear cluster should be changed at the same time, since they wear together. A new chain can slip on old sprockets even if the old chain didn't. The front rings have to be changed maybe once every three chain changes.