Welcome to Molecular Mondays, a relatively new series here on Planet Pailly. Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction. Today, we turn our attention to:
Ever since I started this Molecular Mondays series, I’ve wanted to identify chemicals that could serve as alternatives to oxygen in alien biochemistries. My research eventually led me to the concept of reduction potentials, which has now led me to the more fundamental concept of electronegativity.
When atoms join together as molecules, they share each other’s electrons, but they rarely share equally. Certain atoms (notably oxygen) tend to hog electrons. Electronegativity measures how much electron hogging an atom is wont to do.
When considering how electronegative an atom is, we should ask two key questions:
- How many protons are in the nucleus?
- How many layers of electrons surround that nucleus?
Since protons have a positive charge, an atom with more protons can exert a stronger attractive force on nearby negatively charged electrons.
But that attractive force is mitigated by the atom’s own electron shells. More shells mean less electronegativity.
Oxygen’s exceptionally high electronegativity means it’s very eager to participate in any chemical reaction that will give it more electrons, including the biochemical reactions that make multicellular life possible on Earth.
So what do aliens breathe if they don’t breathe oxygen? A quick look at the periodic table shows that several elements have electronegativities in a similar range to oxygen. So in terms of astrobiology, or at least in terms of writing science fiction, I believe two elements deserve special attention: fluorine and chlorine.