ATOMIC STRUCTURE

• Atom


  The smallest unit of matter that can take part in a chemical reaction
• Element

  Atoms of a single type constitute an element
• Subatomic particles

  • Protons
  • Neutrons
  • Electrons
• Nucleus


  The dense central core of an atom
• A proton weighs nearly the same as a neutron
• A proton and a neutron each weigh 1,836 times as much as an electron
• Protons (p+)


  Closely packed particles in the atom's nucleus that have a positive charge
• Each element has a distinct number of protons
• Molecule

  A combination of two or more atoms
• Element vs. Compound

  An element is a single atom; a compound is formed from different elements
• Molecule of an element

  Molecules made of the same atoms
• Compound


  A molecule formed from different elements
• An element's number of protons determines its atomic number and positive charge
• All carbon atoms—and only carbon atoms—have six protons
• The atomic number of carbon is 6 (6p+)
• Neutrons (n)


  Uncharged, or neutral, particles in the atom's nucleus
• Atomic mass number


  The sum of the number of protons and neutrons in the nucleus of an atom
• Helium, with two protons and two neutrons, has an atomic mass number of 4
• Isotope


  A form of an atom that has a different number of neutrons
• Atomic weight


  The average of the relative weights (atomic mass numbers) of all the element's isotopes
• Electrons (e-)

  Negatively charged particles that orbit the nucleus in electron shells
• The number of electrons in an atom equals the number of protons in its nucleus
• Electrons' negative charges cancel out the protons' positive charges, making atoms electrically neutral
• Valence


  An atom's ability to combine with other atoms, equal to the number of unpaired electrons in its outer shell
• Sodium (Na+) has a plus-one valence because its outer shell contains an unpaired electron
• Chemical bond


  A force of attraction that binds a molecule's atoms together
• Formation of a chemical bond usually requires energy
• Breakup of a chemical bond usually releases energy
• Types of chemical bonds

  • Hydrogen bond
  • Ionic bond
  • Covalent bond
• Hydrogen bond


  Occurs when two atoms associate with a hydrogen atom
• Ionic bond

  Occurs when valence electrons transfer from one atom to another
• Covalent bond


  Forms when atoms share pairs of valence electrons
• Synthesis reaction


  Combines two or more substances to form a new, more complex substance
• Decomposition reaction


  A substance decomposes into two or more simpler substances
• Exchange reaction

  A combination of a decomposition and a synthesis reaction
• Reversible reaction


  The product reverts to its original reactants, and vice versa