UT Knoxville BIOL 140 - Chapter 2 (54 pages)

Unformatted text preview:

CHAPTER 2 2011 Pearson Education Inc Basic Atomic Structure Atoms are composed of Protons positively charged particles Neutrons neutral particles Electrons negatively charged particles Protons and neutrons are located in the nucleus Electrons are found in orbitals surrounding the nucleus 2011 Pearson Education Inc Elements The Building Blocks of Chemical Evolution Every different atom has a characteristic number of protons in the nucleus called the atomic number Atoms with the same atomic number have the same chemical properties and belong to the same element Forms of an element with different numbers of neutrons are isotopes The mass number is the number of protons neutrons of the most common isotope 2011 Pearson Education Inc 2011 Pearson Education Inc Electron Arrangement around the Nucleus Electrons move around atomic nuclei in specific regions called orbitals Each orbital can hold up to two electrons Orbitals are grouped into levels called electron shells Electron shells are numbered with smaller numbers closer to the nucleus The electrons in the outermost shell are called valence electrons Elements commonly found in organisms have at least one unpaired valence electron The number of unpaired electrons in an atom is its valence 2011 Pearson Education Inc Chemical Bonding Unfilled electron orbitals allow formation of chemical bonds and atoms are most stable when each electron orbital is filled Covalent bond Each atom s unpaired valence electrons are shared by both nuclei to fill their orbitals Substances held together by covalent bonds are called molecules Ionic bond Electrons are transferred from one atom to another 2011 Pearson Education Inc Covalent Bonds Electrons are not always shared equally An atom in a molecule with a high electronegativity will hold the electrons more tightly and have a partial negative charge whereas the other atom will have a partial positive charge Differences in electronegativity dictate how electrons are distributed in covalent