Why can cyclic ethers exist, but not cyclic alcohols whose alcohol functional group is part of (embedded in) the ring?

Valency of H can not be equal to 2.

Explanation:

Let's consider a cyclic ether e.g. tetrahydropyran.

In this cyclic structure, valency of all constituent atoms e.g. C, H and O are satisfied. So, it is a potentially stable structure. Hence, cyclic ether exist.

If we replace O in tertrahydropyran by a -OH group and tries to embed it into ring then it requires valency of H equal to 2. This is potentially not possible because H has only one valence electron and no energetically available vacant orbital.

So, cyclic alcohol structure with -OH group embedded in the ring would give an unstable structure. Hence, it does not exist.

answer:

technetium is the most reactive. this is followed by silver. the least reactive out of these three elements is strontium.