Large polyelectrolytes, such as nucleic acids, and polyampholytes, such as proteins, are classified together as macroions. The electrostatic forces of attraction or repulsion between such charged particles play a major role in determining their behavior in solution.

Each macroion collects about it (in solution) a counterion "atmosphere" (Figure 2.22) enriched in oppositely charged small ions. This cloud of ions tends to screen the molecules from one another.