AP Chemistry - Unit 1 - Principles & Concepts

• Avogadro's number represents the amount of formula units, atoms, or molecules present in one mole of a substance.
• Mass number is a combination of protons and neutrons of the element or isotope.
• An element's atomic number gives us its number of protons and electrons.
• Atomic mass of element or isotope (rounded to nearest whole number) - atomic number = neutrons
• Effective nuclear charge is the net positive charge experienced by valence electrons.
• Electronegativity...

  • increases from L to R
  • due to an increase in valence shell stability
  • decreases from T to B
  • due to an increase in atomic radius
  • does not include noble gases.
• Coulomb's Law states that...

  • the electrostatic attraction between two charged particles is
  • proportional to the magnitude of the charge
  • inversely proportional to the distance between them
• Electronegativity is the measure of the ability of an atom (or a group of atoms) to attract shared electrons.
• Ionization energy is the energy required to remove an electron from a neutral atom in its gaseous phase.
• The lower the ionization energy, the more readily an atom becomes a cation.
• The higher the ionization energy, the more likely it is to become an anion.
• Elements to the right have higher ionization energies because their valence shells are nearly filled.
• Elements to the left have lower ionization energies because they are more willing to lose electrons and become cations.
• Ionization energy

  • increases from L to R
  • due to valence shell stability
  • decreases from T to B
  • due to electron shielding
• Helium has the highest ionization energy of all the elements.
• Electron affinity is the ability of an atom to accept an electron. It is the energy change that occurs when an electron is added to a neutral gas atom.
• Electron affinity...

  • increases from L to R
  • due to the decrease in atomic radius
  • decreases from T to B
  • due to the increase in atomic radius
• Atomic radius

  • decreases from L to R
  • due to the increase in the number of electrons and protons
  • increases from T to B
  • due to increase in the energy level
• Lattice energy is the energy required to completely separate one mole of a solid ionic compound into its gaseous phase.
• The formation of ionic compounds is an exothermic process.
• Exothermic reaction


  A chemical reaction that releases energy in the form of heat or light, resulting from atom-to-atom interactions.
• Endothermic reaction


  A chemical reaction that absorbs energy in the form of heat or light, resulting from atom-to-atom interactions.
• The magnitude of the lattice energy of an ionic solid depends on the charges of their ions, their sizes, and their arrangement in the solid.
• The lattice energy increases as the ionic charges increase and as ionic radii decrease.
• Adding electrons to nonmetals is either exothermic or only slightly exothermic.
• Removing electrons from metals is endothermic.
• Ionic bonding occurs when one metal loses its valence electron(s) to another element (nonmetal).
• Adding electrons to nonmetals is either exothermic or slightly endothermic as long as the electrons are added to the valence shell.
• Transition metals lose the valence shell-s electrons first, then as many electrons required to reach the charge of the ion.
• A covalent bond is a chemical bond formed by sharing a pair of electrons.
• Most polyatomic ions are negatively charged. ***Aside from NH4
• Ionic solids do not conduct electricity due to the solidarity of their ionic compounds. Ionic solids are non-conductors.