Ionic compounds are the source of electrolytes when dissolved in a solvent like water. An electrolyte is defined as any substance that creates an electrically conducting solution when dissolved or melted. This conductivity depends entirely upon the presence of mobile, charged particles.
An ionic bond forms when valence electrons are completely transferred from one atom, typically a metal, to another, usually a nonmetal. This transfer results in the formation of positively charged cations and negatively charged anions. The strong electrostatic attraction between these oppositely charged ions holds the compound together.
The Structure of Ionic Compounds
The formation of an ionic compound begins with electron transfer between two atoms with a large difference in electronegativity. The atom that loses electrons becomes a cation, while the atom gaining electrons becomes an anion. For example, in table salt, a neutral sodium atom transfers its valence electron to a neutral chlorine atom, creating a positive sodium ion (\(\text{Na}^{+}\)) and a negative chloride ion (\(\text{Cl}^{-}\)).
These oppositely charged ions are drawn together by electrostatic forces. This attraction results not in discrete molecules, but in a massive, repeating, three-dimensional arrangement known as a crystal lattice. In the solid state, the ions are fixed in their positions within the lattice structure, maximizing the attraction between opposite charges and minimizing the repulsion between like charges. Although the compound contains charged particles, its solid form does not conduct electricity because the ions lack the mobility to carry a current.
How Ionic Compounds Become Electrolytes in Water
The process that transforms a solid ionic compound into an electrically conductive solution is called dissociation. When an ionic compound is introduced to a polar solvent like water, the polar water molecules interact strongly with the charged ions. Water molecules have a slight negative charge on the oxygen atom and a slight positive charge on the hydrogen atoms, allowing them to surround and attract the ions in the crystal lattice.
The negative end of the water molecules pulls on the cations, while the positive end pulls on the anions, breaking the ionic bonds within the crystal. This separation process, known as solvation, releases the ions, which become surrounded by a shell of water molecules. Once dissolved, the ions are free to move throughout the solution, making the liquid electrolytic.
Electrical conductivity requires the presence of these freely mobile, charged ions. When a voltage is applied, positive ions migrate toward the negative electrode, and negative ions move toward the positive electrode, establishing an electrical current. This contrasts with non-electrolytes, such as sugar, which dissolve but remain as neutral molecules and cannot conduct electricity. Since most soluble ionic compounds dissociate almost completely, they are categorized as strong electrolytes.
The Essential Role of Electrolytes in Biology
Electrolytes derived from ionic compounds are fundamental to nearly all biological processes. Common biological electrolytes include sodium (\(\text{Na}^{+}\)), potassium (\(\text{K}^{+}\)), calcium (\(\text{Ca}^{2+}\)), and chloride (\(\text{Cl}^{-}\)). These ions are responsible for generating and conducting electrical signals necessary for the nervous system and muscle function.
Sodium ions are primarily located outside of cells, while potassium ions are concentrated inside. This maintained concentration difference, governed by the sodium-potassium pump, creates an electrical potential across the cell membrane. The rapid flow of these ions generates action potentials, which are the electrical impulses that allow nerve cells to communicate and trigger muscle contractions.
Calcium ions play a role in muscle contraction, blood clotting, and the transmission of nerve signals. Chloride ions, the most abundant extracellular anion, help regulate fluid volume and osmotic pressure. Maintaining the correct concentration of these ions is important, and the body regulates them through hormones and organs like the kidneys. Fluid loss can deplete these essential ions, which is why oral rehydration solutions replenish the specific balance of electrolytes.