Explain how ionic bonds form through electron transfer between metals and nonmetals, predict the resulting formula, and describe the properties of ionic compounds.
An ionic bond forms when a metal atom, which loses electrons easily (low ionization energy), transfers one or more valence electrons to a nonmetal atom, which attracts electrons strongly (high electronegativity). The metal becomes a positively charged cation, the nonmetal becomes a negatively charged anion, and the two oppositely charged ions attract each other electrostatically -- that attraction between opposite charges IS the ionic bond.
Sodium (1 valence electron) transfers that single electron to chlorine (7 valence electrons, needing just 1 more to fill its outer level). The result is Na+ and Cl-, which combine in a 1-to-1 ratio to form NaCl. Magnesium (2 valence electrons) needs two chlorine atoms to accept its two electrons, one each, forming Mg2+ and 2 Cl-, which combine as MgCl2. The subscripts in an ionic formula are exactly whatever ratio makes the total positive and negative charge cancel out to zero.
Ionic compounds share a recognizable set of properties: they form hard, brittle crystal lattices (a repeating 3D grid of alternating positive and negative ions, not individual molecules); they have high melting and boiling points, since breaking apart that whole lattice of strong electrostatic attractions takes a great deal of energy; and they conduct electricity when dissolved in water or melted, because the ions are then free to move and carry charge, but NOT as a solid, where the ions are locked in place.
Mastery looks like: They correctly predict the ions formed and the resulting balanced formula for a metal-nonmetal pair, and explain the properties of ionic compounds in terms of the crystal lattice.
Common stumbles: Forgetting to balance total charge to zero when writing a formula; assuming ionic compounds conduct electricity even as a solid.