Ionic Bond
An ionic bond forms when one atom transfers electrons to another, creating oppositely charged ions that attract each other, as in sodium chloride.
Definition
An ionic bond happens when one atom gives an electron to another atom, so one becomes positive, the other negative, and opposites attract. It is the electrostatic attraction between a positive cation and a negative anion formed when a metal transfers one or more valence electrons to a nonmetal; ionic compounds form crystal lattices rather than separate molecules, have high melting points, are often soluble in water, conduct electricity when melted or dissolved, and are written as the smallest whole-number ratio of ions that balances the charges. Ionic bonding is favored when the electronegativity difference is large enough (typically above about $1.7$ on the Pauling scale) that electron transfer produces ions that assemble into a lattice maximizing attraction and minimizing repulsion. The lattice energy, the enthalpy released when gaseous ions form the solid, follows Coulomb's law, increasing with ionic charge and decreasing with ionic radius, and the Born-Haber cycle accounts for the ionization energy, electron affinity, and lattice energy that together make ionic compound formation exothermic.
Example
Sodium hands one electron to chlorine, and the positive sodium and negative chlorine snap together to make salt. Magnesium loses $2$ electrons to become $Mg^{2+}$ and each chlorine gains $1$ to become $Cl^-$, so it takes two chloride ions to balance one magnesium, $MgCl_2$, and sodium chloride melts at $801^\circ\text{C}$ because every ion is held by several neighbors. Forming $NaCl$ from the elements costs $496\text{ kJ/mol}$ to ionize sodium and returns only $349\text{ kJ/mol}$ from chlorine's electron affinity, an uphill step, but the lattice energy of $787\text{ kJ/mol}$ pays for it, giving an overall enthalpy of formation of $-411\text{ kJ/mol}$; $MgO$, with doubly charged ions, has a lattice energy near $3800\text{ kJ/mol}$ and melts at $2852^\circ\text{C}$.
Key Insight
Ionic bonds usually form between a metal and a nonmetal: the metal gives, the nonmetal takes. Solid salt does not conduct electricity even though it is made of charged ions, because the ions are locked in place; melt it or dissolve it and the ions can move, and then it conducts. Electron transfer alone is not favorable, the lattice makes the bond, so "sodium wants to give up an electron" is a shortcut rather than the physics, and sodium gives up an electron only because the crystal that results is so stable.