Use information from Section 6.7 to estimate which form of electromagnetic radiation is the lowest energy ionizing radiation.

Data from section 6.7

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When we first introduced the concept of the periodic table, we noted that various observations about the behavior of atoms led to its development. The reactivity of halogens and the inertness of rare gases are examples of the periodic law. Using the understanding of orbitals and atomic structure we have just gained, it is possible to explain some of these periodic variations in greater detail. These atomic properties will then be used to help understand reactivity in later chapters. Atomic Size One important characteristic of atoms is their size. The measurement of atomic size is sometimes complicated, because atoms do not have well-defined boundaries. But com- putational techniques allow us to determine the sizes of different atoms in a consistent way, and the results of these calculations are summarized in Figure 6.20. There are two ways to explore trends in the periodic table, within a group and within a period. As we go down a group, we observe an increase in atomic size. As we go across a period, we observe a decrease. An easy way to remember this trend is to draw what look like x, y coordinate axes with the origin at the bottom left of the periodic table. An increase in either the x or y direction on these axes corresponds to a decrease in atomic size. How do we understand this trend? The size of the atom is determined largely by its valence electrons because they occupy the outermost orbitals. Two factors are important: (1) the shell in which the valence electrons are found and (2) the strength of the interaction between the nucleus