Ionic bonding involves transfer of an electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. from one atom (which becomes a positively charged cationA positively charged ion, attracted toward the cathode in an electrolytic cell.) to another (which becomes a negatively charged anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell.). The two ions attract strongly to form a crystal latticeAn orderly, repeating arrangement of points in 3-D space in which each p;oint has surroundings identical to every other point. A crystal's constituent atoms, molecules, and ions are arranged about each lattice point..
Since ionic bonding requires that the atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. involved have unequal attraction for their valence electrons, an ionic compoundA compound containing oppositely charged ions held together by electrostatic attraction. Usually the ions are in a crystal lattice with positive ions surrounded by negative ions and negative ions surrounded by positive ions. must involve atoms of two quite different elements. Attraction for electrons depends on the distance of the electrons from the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. (which in turn depends on the amount of shieldingThe effect of (negatively charged) electrons close to a (positively charged) nucleus in diminishing the attractive force between the nucleus and other electrons in the atom. Also called screening. by inner electrons). Ionic compounds generally form between metals toward the left and bottom of the periodic tableA chart showing the symbols of the elements arranged in order by atomic number and having chemically related elements appearing in columns., and nonmetals toward the right and top of the periodic table. The size of atoms increases as main shells are added going down groups, and decreases as electrons are added to the same shell across periods, because the increasing number of protons draws the outer shell closer. Metal atoms are large compared to nonmetal atoms with the same valence shell (in the same period)
The simplest example of a binary ionic compound is provided by the combination of elements number 1 (H) and number 3 (Li) in lithium hydride, LiH. On a microscopic level the formula LiH contains four electrons. In separate Li and H atoms these electrons are arranged as shown in part a of the following figure. The H atom has the electron configurationA representation of the number of electrons of an atom or ion and the orbitals in which they lie. For example, the electron configuration of oxygen is 1s22s22p4. 1s1, and Li is 1s22s1. When the two atoms are brought close enough together, however, the striking rearrangement of the electron clouds shown part b takes place. Here the color coding shows clearly that the electron densityThe ratio of the mass of a sample of a material to its volume. which was associated with the 2s orbitalA mathematically defined region of electron density around one or more atoms; a wave function that defines the properties of a particular electron in an atom or molecule. in the individual Li atom has been transferred to a 1s orbital surrounding the H atom. As a result, two new microscopic species are formed. The extra electron transforms the H atom into a negative ion or anion, written H– and called the hydride ion. The two electrons left on the Li atom are not enough to balance the charge of +3 on the Li nucleus, and so removal of an electron produces a positive ion or cation, written Li+ and called the lithium ion. The electron-transfer process can be summarized in of Lewis diagrams as follows:
The opposite charges of Li+ and H–attract each other strongly, and the ions form an ion pair in which the two nuclei are separated by a distance of 160 pm (1.60 Å).