Six elements that are the major constituents of living tissue. Biosphere - zone of life on earth. Hydrogen has one proton; carbon has six protons. Atomic weight - average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally occurring element. Isotope - atoms with the same number of protons but different number of neutrons. Hydrogen and helium formed within seconds of the big bang (100-300 seconds) Solar nebular disk model or solar nebular model. Stars are formed in center, planets form in disk. Heavier elements drawn to center by gravity. Star in center gets hotter and hotter. Something that"s developing into a planet that"s orbiting around a sun or star. Meteorites give access to debris left over from the formation of the solar system. Radioactive isotopes and their decay products used to date meteorites. 3 major factors that caused heating and melting in the early earth"s interior.