What is the attraction between a positive ion and a negative ion?

Answer 1

Simple:

Electrostatic forces are the essential cause of an ionic bond, but these forces act within the laws of Quantum Mechanics.

Ionic bond:

The concept of an ionic bond is shorthand term for saying that two (usually neutral) atoms can combine as a bound pair with most of the equivalent of one electron's charge being increased in the vicinity of one atom and decreased in the vicinity of the other.

(We are not going to discuss the complexities of atom interactions in a solid or liquid or when there is a net charge or a net deficiency of charge on the two atoms under discussion.) As an aside, a covalent bond is typically more equal, but not exactly equal, charge on the two atoms.

Quantum Mechanics:

With that definition, the question is really asking, "What causes two atoms which are separately neutral to be attracted and become bound together strongly with a significant transfer of charge (ionic bound) from one atom to the other?"

The laws of physics governing this process are quantum mechanics. Quantum mechanics provides very restrictive conditions on the spatial relations of the electrons in atoms and molecules. (The idea of electronic orbitals is useful here.) The total energy of the two atoms and all electrons is the sum of the potential energy and the kinetic energy of these particles. (The kinetic energy of the nuclei and their quantum behavior are a small effect which is thousands of times smaller than the the quantum effect on electrons, so this answer will suppose the nuclei just sit quietly a few angstroms apart.) When the atoms get a few angstroms apart, the coulomb forces between the charges (both positive and negative) are significantly changed just from the fact that Coulomb's law says energy is inversely proportional to distance. The electronic structure of the two atoms can then change and depending on the nuclear charges and the numbers of electrons, the allowed quantum mechanics structure can change a lot. (A picture of two round balls of electronic charge sitting next to each other is incorrect.) As atoms get close together, the electrons engage in potentially complex structural rearrangements in the region of space involving both atoms. The information about that complex arrangement is contained in the many-body wave function of the electrons.

In elementary chemistry courses this complex arrangement is simplified in discussions of particular allowed orbital and combinations of orbitals that are then termed bonds. It is even sometimes said incorrectly, that in an ionic bond, an outer electron from one atom transfers from its original "high energy" orbital to a "lower energy" orbital on the receiving atom. (Recall that energy in this discussion is potential and kinetic and the idea of lower or higher energy is more complex than just knowing the electron density and Coulomb's law. Kinetic energy, which is strongly influenced by the laws of quantum mechanics can equal or dominate this energy accounting.) Such simplification allows useful pictures, provide a language for discussion and avoids the complex computational task of actually solving the Schroedinger equation in accordance with the laws of quantum mechanics.

Now, a short version of the answer can be provided. There is a continuum between the idealized ionic bond and the idealized covalent bond. When the accumulated electron density around a pair of atoms does not differ much from they state as separate neutral atoms, but electronic structural changes are rather significant with electron densities (and hence wave function) shared across the whole of the space of the two atom, then it is a strong character of a covalent bond and a weak character of an ionic bond. When the reverse occurs and a significant shift of electron charge density moved from one atom to another we say that is ionic in character with very little covalent character. In the ionic bond, the analysis of the full complex wave function of all electrons will show that much of the structure in the core regions of the atoms is not much perturbed but a sizeable change of the structure in the outer has happened. We expect the combined entity to have a strong resemblence to the original two atoms, with one atom adjusting to the loss of one electron and the other atom adjusting to the gaining of that atom.

The force pushing this process, as directed by quantum theory, included the changes in the kinetic energy configurations of electrons as well a Coulomb forces.

Answer 2

The ionic bond is the electric force of attraction between two oppositely charged ions. Two oppositely charged ions are then attracted to each other by electric force.

Answer 3

it is an ionic bond

Answer 4

An ionic bond.

Answer 5

ionic bond

Answer 6

ionic bond

Answer 7

Ionic bond.