what's wrong with Alkali or Alkaline? theirs nothing to react with in space
They have crap mechanical properties, crap thermal properties and they only have nothing to react with if you put them on the outside of the hull - which is actually fine by me if you just want to paint your ships with weird stuff - I'm not judging.
Other than that?
Radiators run coolant inside and it's already somewhat cheesy with sane and structurally materials that you can have kilometres of them without it limiting your manoeuvrability. Making them of something you can bend by looking at it hard enough is just adding insult to injury. Habitats have air and people moving inside. Injectors inject fluids, under pressure and often violently reactive, into infernos commonly known as rocket engines.
Sure, they may be coated, but none of those applications is suitable for something that scratches and bends easily and then catches fire (because coating broke when it bent or scratches). Yay for habitat you could put your hand through if you are not careful and then, before it vents to space it catches reactive metal fire. I'm not even going to think about trying to put, say, fluorine into stuff because this thing is mental enough in contact with normal materials, and getting a scratch in your passivated steel or something tank housing it can still give you the same kind fun alkali metals try to give you with nearly everything..
Don't get me wrong, I have nothing against first and second group elements. They are exemplary metals. Some of the best meals I ate used sodium chloride! You can make nukes with lithium, sodium is great for putting into reactors and if that's not enough for you, you can always use NaK (which isn't in game, figures). But they shouldn't even be modelled as structural materials because you just can't build sane stuff out of them. Hell, *tin* is already not a sane spaceship material, and it doesn't do any of the scary stuff alkali metals do.
What's next, metastable metallic hydrogen for structural applications? I know, let's make armour out of it - you won't get any spalling, ever.