Quote:You're comparing oxygen and chlorine, both gases only produces in quantity by life - but that's the wrong side of the equation. It's much more illuminating to look at the other side - the chlorine anion compared with water. That's where chlorine falls down.
Admittedly, my background is in electrical and aero engineering, not chemistry, so there are probably holes in this.
To start with, water is far, far, more common, even in the absence of life.
Free oxygen is rare, absent life, but oxygen itself is as common as dirt, literally - most minerals are full of the stuff. Oxygen is the third most abundant element, hydrogen the first, making water probably the commonest compound in the universe.
When doing photosynthesis, you need a reducing agent. Terran life uses water, mostly, but alternatives include ammonia, Cl-, Fe2+,H2S, and many other small molecules.
Water is far more common than any of them. Ammonia comes closest, but using that produces nitrogen gas, a terrible oxidising agent. Inhaling N2 and exhaling NH3 is a non-starter.
It's not the easiest to work with, terran life experimented with less energetic alternatives first, but when the others run short, water is there. Photosynthesisers using water can outcompete everything, simply because there's so much more of it, by more than enough to make up for the extra problems of handling it - and it beats chlorine on both counts.
Once the photosynthisers make the inevitable switch to using water, the atmosphere will fill up with oxygen, an excellent oxidising agent. Animals will use it, and the cycle will start turning.
To plausibly get chlorine based life, you pretty much have to handwave away the oxygen and water from the solar system - you don't want a comet landing in your ecosystem - a triviality for an elder race.