What is Catenation and Tetravalency?

Answer 1

Catenation is when the same atom bonds to itself in a linear chain. There are very few atoms that do this, carbon being the most common by far. In fact, catenation is the basis of organic chemistry and biochemistry (all biological macromolecules have a carbon backbone). Silicon and sulfur also catenate (form chains) but silicon's aren't stable in oxygen and sulfur's can't bond to anything else, so they are very simple.

Tetravalency means an atom typically forms four bonds. This also mostly applies to carbon.