4.) What are valence electrons?

Valence electrons are electrons that tend to stay in the highest energy levels of an atom and are involved in chemical bonding.

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.