Magnesium chloride (MgCl₂) is an ionic compound. It forms when magnesium, a metal, transfers electrons to chlorine, a nonmetal, creating oppositely charged ions that attract each other. This isn’t a borderline case. Every major indicator, from electronegativity difference to physical properties to electrical behavior, points clearly to ionic bonding.
Why the Bond Is Ionic
The simplest way to classify a bond is by looking at how unevenly the two atoms share electrons. On the Pauling electronegativity scale, magnesium scores 1.31 and chlorine scores 3.16. That’s a difference of 1.85. As a general rule, a difference greater than 1.7 indicates an ionic bond, while anything below about 0.4 is considered purely covalent, with a gray zone of polar covalent bonds in between. At 1.85, magnesium chloride falls squarely in ionic territory.
What happens at the atomic level is straightforward. Magnesium has two electrons in its outermost shell, and it readily gives both away. Each chlorine atom needs one electron to complete its outer shell. So one magnesium atom donates one electron to each of two chlorine atoms, producing a magnesium ion with a +2 charge (Mg²⁺) and two chloride ions, each with a −1 charge (Cl⁻). The resulting compound balances out electrically: one Mg²⁺ paired with two Cl⁻ ions.
Physical Properties That Confirm It
Ionic compounds share a set of recognizable physical traits, and magnesium chloride checks every box. Its melting point is 712 °C and its boiling point is 1,412 °C. Those extremely high temperatures reflect how much energy it takes to pull the oppositely charged ions apart from one another. Covalent compounds, by contrast, tend to have much lower melting and boiling points because the forces holding their molecules together are weaker.
In its solid form, magnesium chloride arranges itself into a repeating crystal lattice, with each magnesium ion surrounded by chloride ions and vice versa. This rigid, orderly structure is a hallmark of ionic solids. It also makes the solid brittle rather than flexible: if force shifts one layer of ions, like charges suddenly line up and repel each other, causing the crystal to crack.
How It Behaves in Water and When Melted
Another reliable test for ionic character is electrical conductivity. Solid magnesium chloride does not conduct electricity because the ions are locked in place within the crystal. But dissolve it in water or melt it, and the ions become free to move. Molten magnesium chloride conducts electricity well enough that industrial magnesium production relies on running an electric current through the molten salt to extract pure magnesium metal.
Magnesium chloride is also highly soluble in water, dissolving at roughly 166 grams per 100 mL at room temperature. When it dissolves, the crystal breaks apart into individual Mg²⁺ and Cl⁻ ions. This complete dissociation into charged particles is characteristic of ionic compounds. Covalent compounds that dissolve in water typically stay as intact molecules rather than splitting into ions.
How Strong the Ionic Bond Is
Not all ionic bonds are equally strong. The lattice energy of a compound, which measures how much energy it takes to completely separate all the ions in a crystal, gives a useful comparison. Magnesium chloride has a lattice energy of about 2,526 kJ/mol. For perspective, sodium chloride (table salt) has a lattice energy of 787 kJ/mol, roughly a third as much.
The reason for that large difference is charge. Sodium carries only a +1 charge, while magnesium carries +2. The electrostatic attraction between a +2 ion and two −1 ions is considerably stronger than between +1 and −1 ions. This is why magnesium chloride has a higher melting point than table salt (712 °C vs. 801 °C for NaCl) despite NaCl being the more famous example of an ionic solid. The stronger ionic attractions in MgCl₂ also explain why it releases so much more energy when its crystal lattice forms.
Common Forms You’ll Encounter
Pure anhydrous magnesium chloride is what chemists refer to when discussing its bond type and crystal structure. But in everyday life, you’re more likely to encounter the hexahydrate form, which has six water molecules attached to each unit of MgCl₂. This version appears as white, odorless crystals that readily absorb moisture from the air (a property called deliquescence). It’s used in road de-icing, dust control, and as a dietary magnesium supplement. Regardless of the form, the fundamental bond between magnesium and chlorine remains ionic.