A: According to BBC, an endoskeleton is a bone structure located in the interior of an animal, and an exoskeleton is a bone structure located on the exterior of an animal. Each type of skeleton has unique advantages and disadvantages in helping the animal survive.
Minerals made of only one element are called NATIVE minerals (i.e., silver, gold, mercury, copper). Minerals composed of compounds are simply referred to as minerals (i.e., quartz, which is made up on silicon and oxygen).
An element is made up of only one type of atom. Compounds are made up of different types of atoms combined together. A lump of copper is made up of only copper atoms. Water particles are made up of hydrogen atoms and oxygen atoms combined together. Scientists call this type of particle a molecule. There is, however, an almost uncountable number of ways of combining atoms of different elements together. That is why there is an almost uncountable number of different compounds. Think about the number of different words we can make from just the 26 letters of the English alphabet.