2.11: Ionic Bonds

When atoms gain or lose electrons to achieve a more stable electron configuration they form ions. Ionic bonds are electrostatic attractions between ions with opposite charges. Ionic compounds are rigid and brittle when solid and may dissociate into their constituent ions in water. Covalent compounds, by contrast, remain intact unless a chemical reaction breaks them.

Opposing Charges Hold Ions Together in Ionic Compounds

Ionic bonds are reversible electrostatic interactions between ions with opposing charges. Elements that are the most reactive (i.e., have a higher tendency to undergo chemical reactions) include those that only have one valence electron, (e.g., potassium) and those that need one more valence electron (e.g., chlorine).

Ions that lose electrons have a positive charge and are referred to as cations. Ions that gain electrons have a negative charge and are called anions. Cations and anions combine in ratios that result in a net charge of 0 for the compound they form. For example, the compound potassium chloride (KCl) contains one chloride ion for each potassium ion, because the charge of potassium is +1 and the charge of chloride is -1. The compound magnesium chloride (MgCl₂) contains two chloride ions for each magnesium ion because magnesium’s charge is +2.

Ions Can Dissociate from One Another in Water

The electrostatic forces holding ionic compounds together are strong when the compounds are in solid form. Since the melting points of ionic compounds tend to be very high, they are generally found as solids on the Earth’s surface. However ionic bonds are not as strong as covalent bonds because the ions can be pulled apart or dissolved in a chemical reaction liquids like water. Ions dissolved in water are said to be in a chemical reaction, and many ions are capable of conducting electric currents when in this form.

Electrolytes Are Important for Biological Systems

Electrolytes are ions that can conduct electricity when dissolved in water. In biological systems, electrolytes are essential for osmotic regulation—the balance of water across cellular membranes. Electrolytes also contribute to critical biological processes that rely upon electrical charges across the cell membrane, such as muscle contractions and nerve impulses. Common biological electrolytes include the calcium ion (Ca²⁺), the sodium ion (Na⁺), the magnesium ion (Mg²⁺), the potassium ion (K⁺), the phosphate ion (PO₄^3-) and the chloride ion (Cl^-).

Electrolyte imbalances can cause severe physical symptoms and even death. One of the most commonly-encountered electrolyte imbalances is hyponatremia, insufficient sodium levels in the blood. Hyponatremia can be a symptom of another medical condition or caused by ingesting too much water without adequately replacing sodium. Treatments for this serious condition aim to restore the balance of sodium in the body so that the brain, heart, and other organs can function properly.

An ionic bond is a noncovalent attraction that holds two oppositely charged ions together, like the sodium cation and chloride anion in sodium chloride.

In this case, a valence electron of sodium is transferred to the chlorine atom to complete the octet of electrons in the combining atoms.

The strength of an ionic bond depends upon the environment of the ionic compound.

Ionic compounds are highly stable in air but dissociate to their constituent ions in an aqueous solution in the presence of polar water molecules.