Atomic size

Broadly speaking, the radii of atoms increase from the top to the bottom of the periodic table and decrease from left to right. Hence, the largest atoms are found at the lower left of the table, and the smallest ones are found at the upper right. The increase in radius down each group stems from the fact that in successive periods one more layer of the atomic “onion” is being formed; that is, electrons are being added to a new shell outside a closed-shell core of the atom. Thus, lithium consists of one electron outside a compact, helium-like core, sodium consists of a single electron outside a neon-like core (which itself has a helium-like core deep within its structure), and so on down the group.

The decrease in atomic radius from left to right across a period is perhaps more surprising, for a contraction in size occurs despite the presence of more electrons in each successive element. Thus, lithium has three electrons and beryllium (Be) has four, but beryllium is slightly smaller than lithium. Fluorine, with nine electrons, might be expected to be a significantly larger atom than lithium, but the opposite is true. The explanation of this seemingly counterintuitive trend is that, although successive elements have a larger number of electrons, they also have a higher nuclear charge because of the increasing number of protons. That positive charge draws in the surrounding electrons to make the atom more compact. The inner-shell, or core, electrons, which do not increase upon going across a period, effectively shield the outer-shell electrons from the positive charge of the nucleus. The outer-shell electrons that are added upon going across a period, however, do not shield other valence electrons from the increasing charge of the nucleus as well as the core electrons do. Thus, the outer-shell electrons are pulled in more closely by the greater charge of the nucleus. There is clearly competition (as is so often the case in chemistry) between the inflating effects of the presence of more electrons and the contracting effects of the stronger nuclear charge. With a few exceptions, the latter influence dominates slightly, and successive atoms are smaller on moving across a period.

Ions, both cations and anions, show a similar variation in size with the position of their parent elements in the periodic table. However, there are two gross differences. First, cations (which are formed by the loss of electrons from the valence shell of the parent atom) are invariably smaller than their parent atoms. In some cases the difference can be considerable (more than 50 percent). In effect, the outer layer of the atomic “onion” is discarded when the valence electrons are lost, so the radius of the cation is that of the compact atomic core.

Anions, which are formed by the gain of electrons by an atom—most commonly into the incomplete valence shell—are invariably larger than the parent atoms. In this case, the additional electrons repel the electrons that are already present, and the entire atom inflates.

Ionization energy

Next in order of importance for determining the number and type of chemical bonds that an atom may form is the ionization energy of the element. It is the minimum energy needed to remove an electron from an atom of the element. The energy is required because all the electrons of an atom are attracted by the positive charge of the nucleus, and work must be done to drag the electron off the atom to produce a cation. Chemical bond formation stems from the transfer or sharing of electrons, and so the energy required to remove an electron is a crucial criterion in the ability of an atom to form a bond.

In broad terms, the variation of ionization energies throughout the periodic table mirrors the variation in atomic radii, with small atoms typically having high ionization energies and large atoms usually having small ones. Thus, the elements with the lowest ionization energies (and hence from which an electron is most readily removed) are found at the lower left of the periodic table, near cesium and francium, and elements with the highest ionization energies are found at the upper right of the table, close to fluorine and helium. The variation in ionization energy correlates with the variation in atomic radius because a valence electron in a bulky atom is on average far from the nucleus and therefore experiences only a weak attraction to it. On the other hand, a valence electron in a small atom is close to its parent nucleus and is subject to a strong attractive force.

At this point the relative inertness of the noble gases can in part be explained. They lie on the right in the periodic table, and the members of the family that are closest to helium (namely, neon and argon) have ionization energies that are among the highest of all the elements. Thus, their electrons are not readily available for bond formation. Only lower in the group, at krypton and xenon, do the ionization energies become comparable to those of other elements, and these elements can be coaxed into compound formation by sufficiently aggressive reagents (most notably by fluorine).

An important feature of the ionization energy is that the energy required to remove a second electron from an atom is always higher than the energy needed to remove the first electron. Once an electron has been removed, there are fewer electrons to repel one another in the cation, so more work must be done to drag the next electron away from the nucleus. The same is true of the third electron, which is even less available than the second electron. However, an important point is that, if an electron needs to be removed from the core of the atom (as is the case for a second electron removed from sodium), then the ionization energy may be exceedingly high and not attainable in the course of a typical chemical reaction (as will be justified below). The reason for the high ionization energies of core electrons is largely that these electrons lie much closer to the nucleus than do the valence electrons, and thus they are gripped by it much more strongly.

It is a general rule that for elements on the left in the periodic table, which have one, two, or three electrons in their valence shells, sufficient energy is attainable in chemical reactions for their removal, but not enough energy is available for removing any electrons from inner shells. Hence, sodium can form Na⁺ ions, magnesium can form Mg²⁺ ions, and aluminum can form Al³⁺ ions.

One reason for the importance of noble gas configurations in chemical bond formation now becomes apparent. Once a noble gas, closed-shell configuration is obtained, the ready removal of electrons to form cations ceases (as does the opportunity for the partial removal of electrons for the sharing required in the formation of covalent bonds, as discussed below). A large energy barrier is encountered when going beyond the removal of the valence electrons of an atom.

Ionization energies do not correlate with atomic radii exactly, because there are other influences beyond the distance of the electron from the nucleus that determine the energy needed to remove an electron. These influences include the details of the occupation of the orbitals in the valence shell. Once again, the origin of a further possibility for competition becomes apparent, in this case between effects that stem from size alone and those that are determined by the energy requirements for ionization.