Origin of Earth  What determines which element an atom is?  The Number of Protons  What is an isotope? A radioactive isotope? What is fusion? Fission?  An Isotope is a chemical element which has the same number of Protons but a different Number  of Neutrons.  Radioactive isotope is radioactive elements (parents) which decay into nonradioactive elements(daughters).  A radioactive isotope is an  Fusion- nucleus of two or more elements combine  Fission- nucleus of an element breaks apart.  Describe the protoplanet hypothesis of the formation of the solar system. Why is it implausible that the planets were captured by the sun?  A large gas cloud begins to condense, then most mass goes to the center, turbulence in out parts. Turbulent eddies collect matter meters across; small chunks grow and collide, eventually becoming large aggregates of gas and solid chunks. Protoplanets, much bigger than present planets, eventually contracted due to their own gravity.  Be able to order the eras of the geologic time scale? Which ofthese were dominated by dinosaurs? Mammals?  Era- Paleozoic(old life), Mesozoic (dinosaurs), Cenozoic (mammals).  Period-Quaternary  Epoch-Pleistocene, Holocene  Why do the planets rotate about their axis and the sun in the same direction?  Axis Parallelism.  Why are the inner planets small and rocky and the outer planets big and gaseous?