This month my molecule of the month is a simple one; hydrogen. It is the simplest possible molecule; hydrogen atoms are the smallest type of atom, composed of just one proton and one electron. When two of these atoms join together they form a molecule of hydrogen, H2.
The inspiration for writing about hydrogen came last week when I travelled to a high school with Tim Harrison, a ‘school teacher fellow’ at my own Bristol University. Being a school teacher fellow means you spend your days loading up mini busses with willing volunteers and exciting chemicals, shipping them out to schools all over the world and then blowing things up all day in front of a mass of students. Tim spent our journey to the school filling me in on his recent trip to Johannesburg, but alas, on that day we were merely on our way to the sleepy Gloucestershire town of Tetbury.
The show Tim put on was great, and amidst making disgusting ‘slime’ (which yours truly as ‘technician’ had to clear up) and other smells and bangs, we compared the result of setting fire to balloons filled with hydrogen and helium. When Tim held a flame under the balloon of hydrogen, it exploded with a deafening boom, creating an impressive fireball about two metres wide. I don’t have a video of Tim doing this unfortunately, but the effect was very similar to that shown in this video of a large american doing the same thing..
Impressive I think! But whilst hydrogen exploded violently, helium, the gas that fills may a children’s birthday balloon, refuses to do anything at all when a flame is stuck underneath it. OK, the balloon does burst, and kind of ‘pops’ (some of the less hardened kids gasped at that point) but that’s just the plastic melting and the gas expanding, nothing more exciting than that.
This difference in what chemists call the ‘reactivities’ of the two elements can be explained by the number of electrons they possess. You can easily tell how many electrons an element has by looking at its atomic number on that most useful of tools, the periodic table.
Hydrogen (H) is at the top left with one electron and helium (He) is at the top right and has two. What chemists have found is that elements have a certain quota of electrons which they can accommodate at a given energy level. For H and He this number is two, which means helium is ‘full up’, whereas hydrogen is distinctly not. Since atoms are capable of joining together and sharing electrons with other atoms or molecules, this means hydrogen will react with other things (like oxygen in the air) in order to complete in quota of electrons, but since helium is already full, it will not.
What’s perhaps worrying, is that in the future hydrogen may be powering our cars and trains. Let’s just hope we don’t end up with youtube videos of cars – rather than balloons – exploding in massive fireballs in a few years time.