-Armor even if sometime you can't build armor because you are ranged ad caryy but it sould work for the rest of the team
Ranged ad carries usually rely on their teammates to defend them while they dps (which is the only reason why such squishy builds can work against intelligent players), but if their teammates cannot defend them from something they need to build defenses to deal with it.
Carries will usually only use one slot for pure defense (banshee's, ga, or Qss), but they can always take armor in a slot if they need it.
In particular, Atma's Impaler is designed to be worth taking on ranged ad carries, bruisers, and tanks. The passive is designed to make the item cost-efficient at 1125 health (will be 1500 after the nerf, you should still have this much health anyway) and it provides two damage scalars, so it is worth taking on a ranged ad carry when you need armor.