Ionic bonds are a type of chemical bond based on electrostatic forces between two oppositely-charged ions. In ionic bond formation, a metal donates an electron, due to a low electronegativity to form a positive ion or cation. In ordinary table salt, the bonds between the sodium and chlorine ions are ionic bonds. Often ionic bonds form between metals and non-metals. The non-metal atom has an electron configuration just short of a noble gas structure. They are electronegative, and so readily gain electrons to form negative ions or anions. The two or more ions are then attracted to each other by electrostatic forces. Such bonds are stronger than hydrogen bonds, but similar in strength to covalent bonds.
Ionic bonding occurs only if the overall energy change for the reaction is favourable when the bonded atoms have a lower energy than the free ones. The larger the resulting energy change the stronger the bond.
Pure ionic bonding is not known to exist. All ionic bonds have a degree of covalent bonding or metallic bonding. The larger the difference in electronegativity between two atoms the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water.
Electron configurations of lithium and fluorine. Lithium has one electron in its outer shell, held rather loosely because the ionisation energy is low. Fluorine carries 7 electrons in its outer shell. When one electron moves from lithium to fluorine, each ion acquires the noble gas configuration. The bonding energy from the electrostatic attraction of the two oppositely-charged ions has a large enough negative value that the overall bonded state energy is lower than the unbonded state
Ions in crystal lattices of purely ionic compounds are spherical, but, if the positive ion is small and/or highly charged, it will distort the electron cloud of the negative ion. This polarization of the negative ion leads to a build-up of extra charge density between the two nuclei, i.e., to partial covalency. Larger negative ions are more easily polarized, but the effect is usually only important when positive ions with charges of 3+ (e.g., Al3+) are involved (e.g., pure AlCl3 is a covalent molecule). However, 2+ ions (Be2+) or even 1+ (Li+) show some polarizing power because their sizes are so small (e.g., LiI is ionic but has some covalent character). The Polarizing Power depends on the ratio of charge and size of the ion, often called the charge density.
Ionic compounds in the solid state form a continuous ionic lattice structure in an ionic crystal. When all the ions are approximately the same size, they can form a structure that is face-centered cubic, but, when the ions are different sizes, the structure is often body-centered cubic. In ionic lattices the coordination number refers to the number of ions that each is connected, too.
In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond, atoms are bound by sharing electrons. In covalent bonding, the molecular geometry around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules. Thus, a compound can be classified as ionic or covalent based on the geometry of the atoms.
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