Periodic trends atomic radii

Atomic radii typically decrease from left to right across a period and increase down a group (Fig. 14.2 see also Fig. 1.46). As the nuclear charge experienced by the valence electrons increases across a period, the electrons are pulled closer to the nucleus, so decreasing the atomic radius. Down a group the valence electrons are farther and farther from the nucleus, which increases the atomic radius. Ionic radii follow similar periodic trends (see Fig. 1.48). 

II. The general trend is for ionization energy to increase as one moves from left to right across the periodic table and to decrease as one moves down this is the inverse of the trend one finds in examining the atomic radius. 

Figure 3.23 shows the periodic trends associated with the atomic radius. You can see that atomic radii generally decrease across a period. Furthermore, atomic radii generally increase down a group. Two factors affect differences in atomic radii. 

Zeff governs the trend of decreasing atomic radius across a period. 

Ionization energy generally increases across a period. Again, this trend is linked to the atomic radius. Across a period, the atomic radius decreases because Zeff increases. The force of attraction between the nucleus and valence electrons is subsequently increased. Therefore, more energy is needed to remove one such electron. 

The periodic trend for electronegativity is the inverse of the trend for atomic radius. 

FIGURE 14.1 The general tendency of atomic radius is to decrease across a period and increase down a group. This diagram is a highly schematic representation of those trends. 

Use the Interactive Periodic Table in eChapter 5.15 to determine the trend in atomic radius as you move across a period and as you move down a group. Explain the factors that account for these trends. 

Use the Interactive Periodic Table (eChapter 5.1) to compare the atomic radius and the ionic radius of the elements in group 2A. How does the ionic radius compare to the atomic radius Explain this trend. 

A horizontal row is called a period. As one travels from left to right across a period certain trends occur. In general, atomic radius decreases from left to right (disregarding the noble gases). Metallic properties also decrease from left to right. 

Within a given group of the periodic table, the first ionization energy decreases with increasing atomic number. This is related to the increase in atomic radius and the decreasing attraction of the nucleus for the increasingly distant outermost electron. It should be mentioned that this trend is not uniformly noted for the transition metals. 

Because of the arrangement of elements on the periodic table, there are several patterns that can be seen between the elements. These patterns, or periodic trends, can be observed for atomic radius, ionic radii, ionization energies, electron affinities, and electronegativities. You should be familiar with the periodic and group trends for each of these. 

Your understanding of periodic trends such as atomic radius and ionization energy will help you identify some unknown elements in the Chemistry Course Challenge at the end of this book. 

Know the trends for electronegativity, ionization energy, and atomic radius across the periodic table. 

---