The size of an ionAn atom or covalently bonded set of atoms that carries an overall net charge. is governed not only by its electronic structure but also by its charge. This relationship is evident in the following figure comparing ionic radii. Ions in the first row of this figure, H–, Li+, and Be2+, all have the same 1s2 electronic structure as the helium atomThe 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..
Species which have the same electronic structure but different charges are said to be isoelectronicHaving the same number of electrons. Sometimes used to describe species that have the same number of valence electrons.. For any electronic series, such as H–, He, Li+, Be2+, in which the nuclear charge increases by 1 each time, we find a progressive decrease in size due to the increasingly strong attraction of the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. for the electron cloud. Each row in the figure corresponds to an isoelectronic series involving a different noble-gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. 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.. As we move from the more negative to the more positive ions in each row, there is a steady decrease in size. If we move down any of the columns, ionic sizes increase due to the increasing principal quantum numberOne of a set of numbers that specifies the state of an electron in an atom; the set of quantum numbers summarize results from quantum mechanics. of the outermost electrons. The sizes of singly charged cations, for example, increase in the following order: Li+ < Na+ < K+ < Rb+ < Cs+.
A further point of interest is the size of an ion relative to the atom from which it was formed. Recall the electron dot densityThe ratio of the mass of a sample of a material to its volume. diagram showing the formation of LiH. when the Li atom lost its electron and became a Li+ ion, its size decreased dramatically. Comparing atomic radii to the ionic radii above reveals that this is also true for the other alkali metals. For example, the van der Waals radius of K is 280 pm, while the ionic radiusAn estimate of the size of an ion in an ionic compound; found from the internuclear distance between ions in a crystal lattice. of K+ is only 133 pm.
The large reductionThat part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a reactant must occur. in size is in sharp contrast to what occurs when an atom accepts one or more electrons to attain a noble-gas structure. Since the added electron goes into a subshell which already has occupants, rather than starting on a new subshell, there is very little change in size. This is clearly seen in the electron dot density diagram mentioned above, where formation of an H– ion from an H atom produces no perceptible increase in size. Comparing the figure of atomic radii to ionic radii above, we also find that the van der Waals radii of nonmetals are only slightly smaller than the radii of their anions.
The sizes of the ions involved have considerable influence on both the chemical and physical properties of ionic compounds. There is a strong correlation, for example, between ionic size and the melting pointThe temperature at which a solid becomes a liquid. Also called freezing point. of an ionic compound. Among the halides of sodium the meltingThe process of a liquid forming from a solid. point decreases in the order of NaF (995°C) > NaCl (808°C) > NaBr (750°C) > NaI (662°C). The larger the anion, the farther it is from the sodium ion, and the weaker the coulombic force of attraction between them. Hence the lower the melting point. When a very small cation combines with a very large anion, the resulting compound is less likely to exhibit the characteristic macroscopic properties of an ionic substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula..