Learn Portuguese (Portugal): A picada da aranha era radioativa.

The sentence describes a characteristic the bite already had in the past, so we use era (the imperfect of ser). If you wanted to say the bite actually became radioactive, you would indeed use a change of state verb like ficou or tornou-se.

Why is the verb era in the imperfect tense rather than the past tense foi?

We use the imperfect era for descriptions and background facts in the past. The radioactivity was an ongoing property of the bite, rather than a single, completed event.

Could I use estava instead of era here?

No, because being radioactive is considered an inherent characteristic of this specific bite. We use ser (here as era) for permanent traits, whereas estar (estava) is for temporary states.

Why is it picada instead of mordida for a bite?

In Portuguese, picada is used for punctures, stings, or bites from spiders, insects, and snakes. For a bite from an animal with larger teeth, like a dog, you would use mordida.