Table salt holds together because sodium gives away an electron
Drop sodium and chlorine together and each sodium atom hands one electron to a chlorine atom. The result is two ions of opposite charge, pulled together so strongly that they stack into the hard crystals sprinkled on dinner. That electrical grip, called ionic bonding, is among the strongest in chemistry.
An atom that gains electrons becomes a negative ion, an anion; one that loses them becomes a positive cation. Typically a metal gives up electrons and a nonmetal takes them, each ending up with a full outer shell. Alkali metals, holding just a few loosely bound outer electrons, give them up readily, while halogens, a slot or two short of a full shell, grab them. Ions need not be single atoms: ammonium and sulfate are charged groups that bond the same way.
The energy accounting is subtle. Pulling an electron off the metal actually costs energy. What pays the bill is the attraction of the new ions for each other, released as lattice energy, and the bond forms only when the overall balance is favourable. Charge matters enormously: by Coulomb's law, a pair of doubly charged ions is held roughly four times as tightly as a pair with single charges. Typical ionic bond strengths run from about 170 to 1500 kilojoules per mole.
That strength shapes how salts behave. Ionic compounds usually form crystal lattices with no separate molecules, just alternating ions, and they tend to melt at high temperatures, higher still when the charges are larger. As solids they barely conduct electricity, but melt them or dissolve them in water and the freed ions carry current well. Many dissolve easily in water, though stronger cohesion lowers solubility.
Chemists stress that a purely ionic bond cannot exist. Ions sit close enough to share some electron density, so every ionic compound has a little covalent character. A small, highly charged cation distorts the electron cloud of its partner, an effect described by Fajans' rules; even lithium iodide, clearly ionic, shows some sharing. The label ionic simply means the charge-transfer side outweighs the sharing side.
Source: Ionic bonding
