<P> The classical model identifies three main types of chemical bonds - ionic, covalent, and metallic - distinguished by the degree of charge separation between participating atoms . The characteristics of the bond formed can be predicted by the properties of constituent atoms, namely electronegativity . They differ in the magnitude of their bond enthalpies, a measure of bond strength, and thus affect the physical and chemical properties of compounds in different ways . </P> <P> An ionic bond involves a complete transfer of one or more valence electrons of atoms participating in bond formation, resulting in a positive ion and a negative ion bound together by electrostatic forces . Electrons in an ionic bond tend to be mostly found around one of the two constituent atoms due to the large electronegativity difference between the two atoms; this is often described as one atom giving electrons to the other . This type of bond is generally formed between a metal and nonmetal, such as sodium and chlorine in NaCl . Sodium would give an electron to chlorine, forming a positively charged sodium ion and a negatively charged chloride ion . </P> <P> In a true covalent bond, the electrons are shared evenly between the two atoms of the bond; there is little or no charge separation . Covalent bonds are generally formed between two nonmetals . There are several types of covalent bonds: in polar covalent bonds, electrons are more likely to be found around one of the two atoms, whereas in nonpolar covalent bonds, electrons are evenly shared . Homonuclear diatomic molecules are purely covalent . The polarity of a covalent bond is determined by the electronegativities of each atom and thus a polar covalent bond has a dipole moment pointing from the partial positive end to the partial negative end . Polar covalent bonds represent an intermediate type in which the electrons are neither completely transferred from one atom to another nor evenly shared . </P> <P> Metallic bonds generally form within a pure metal or metal alloy . Metallic electrons are generally delocalized; the result is a large number of free electrons around positive nuclei, sometimes called an electron sea . </P>

State the force which holds two or more atoms together as a stable molecule