Ionic Bonding and Structures

From atoms to oppositely charged ions

Ionic compounds usually form when electrons are transferred from atoms of one element to atoms of another. A metal atom tends to lose one or more outer-shell electrons, becoming a positive ion. A non-metal atom gains the transferred electron or electrons, becoming a negative ion. Only electrons are transferred: the nuclei of the atoms do not change.

For example, a sodium atom can lose one electron to form a $\mathrm{Na^+}$ ion, while a chlorine atom can gain that electron to form a $\mathrm{Cl^-}$ ion. The charges show that the ions now contain unequal numbers of protons and electrons.

Electron transfer explains how the ions are formed, but it is not itself the ionic bond.

The nature of an ionic bond

Positive and negative charges attract. Once oppositely charged ions have formed, a strong electrostatic force of attraction acts between them. This attraction is the ionic bond.

The force is not a material object joining two particles, like glue or a tiny stick. It is an attraction between charges. It also acts in all directions, so a positive ion can attract all the nearby negative ions around it, rather than becoming exclusively paired with just one negative ion.

This distinction matters. In sodium chloride, it is misleading to imagine one $\mathrm{Na^+}$ ion joined only to one $\mathrm{Cl^-}$ ion as a separate molecule. The formula $\mathrm{NaCl}$ gives the simplest ratio of ions—one sodium ion for every chloride ion—not the existence of individual sodium chloride molecules.

How the ions are arranged

Because every ion attracts nearby ions of the opposite charge, the ions assemble into a giant ionic lattice. This is a regular, repeating arrangement that extends through the solid in three dimensions. Positive and negative ions alternate so that each ion has oppositely charged ions as its nearest neighbours.

The word giant means that the structure contains very many ions joined by ionic bonding; it does not mean that an individual ion is unusually large. The word lattice describes the ordered, repeating three-dimensional arrangement.

A drawing normally shows only a small part of the lattice. The same arrangement continues beyond the edges of the picture and behind and in front of the page. A flat diagram must therefore be interpreted as a representation of a three-dimensional structure.

Nature and arrangement together

The structure follows directly from the bonding:

• Nature of the bond: electrostatic attraction between positive and negative ions.
• Arrangement: a giant, regular, repeating three-dimensional lattice of alternating charges.
• Extent of bonding: each ion is attracted to several nearby oppositely charged ions, with the attraction acting in all directions.

A ball-and-stick model can help reveal the repeating arrangement in three dimensions, but its sticks can create a false impression that ionic bonds are physical rods or that each ion has only a few isolated links. The real bonding is the electrostatic attraction between the charged ions throughout the lattice.

Ion formation

• Metals lose electrons and form positive ions.
• Non-metals gain electrons and form negative ions.
• The nuclei do not change.
• Electron transfer forms the ions; it is not the ionic bond.

Ionic bonding

An ionic bond is the electrostatic force of attraction between oppositely charged ions. It acts in all directions, so each ion attracts several nearby ions of the opposite charge.

Ionic structure

Ionic compounds form giant three-dimensional lattices:

• ions have a regular, repeating arrangement;
• positive and negative ions alternate;
• opposite charges are nearest neighbours;
• there are no separate molecules or exclusive ion pairs.

A formula such as $\mathrm{NaCl}$ gives the simplest whole-number ratio of ions, not a single molecule.

Models

A small or two-dimensional lattice diagram represents part of a structure that continues in all three dimensions. Sticks in a model represent attractions; ionic bonds are not physical rods.