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Chapter 2: The Chemistry of Life
2.1 What are the chemical elements that make up living organisms?
x All matter is composed of atoms t protons, neutrons, and electrons
x The mass of a proton serves as a standard unit of measure called the atomic mass unit or Dalton
x An element is a pure substance that contains only one kind of atom
x 98% of the mass of a living organism is composed of six elements: carbon, hydrogen, oxygen,
nitrogen, phosphorus, and sulphur.
x Atomic number t number of protons in an atom
x Mass number t the total composition of protons and neutrons in the nucleus
x Isotopes of an element have the same number of protons but different number of neutrons
x Atomic weight t average of mass numbers of isotopes in their normally occurring proportions
x Radioisotopes are unstable isotopes, and thus spontaneously give off energy in the form of
alpha, beta, or gamma radiation
x The radioactive decay transforms the atom, including the atomic number
x The radioactivity can be used as a marker (such as tumor marking)
x The number of electrons determine how the atoms will interact
x The region of space ÁZZo}v](}µvoõì9}(Z]u]ooZo}v[
x Orbitals occur in series called electron shells or energy levels
x The s orbitals fill with electrons first and then subsequent shells are filled according to their
x Reactive atoms have unpaired electrons in their outermost (valence) shell
x Atoms can share electrons, lose, or gain electrons t resulting in bonding
x The octet rule t the tendency of atoms in a stable molecule to have eight electrons in their