Trends in Atomic Size

In Chapter 7, we noted that an electron in an atom can lie relatively far from the nucleus, so we commonly represent atoms as spheres in which the electrons spend 90% of their time. However, we often define atomic size in terms of how closely one atom lies next to another. In practice, we measure the distance between identical, adjacent atomic nuclei in a sample of an element and divide that distance in half. (The technique is discussed in Chapter 12.) Because atoms do not have hard surfaces, the size of an atom in a compound depends somewhat on the atoms near it. In other words, atomic size varies slightly from substance to substance. 

In a covalent compound, the bond length and known covalent radii are used to determine other radii. Here the C—Cl bond length (177 pm) and the covalent radius of Cl (100 pm) are used to find a value for the covalent radius of C (177 pm - 100 pm = 77 pm). 

Changes in n. As the principal quantum number (n) increases, the probability that the outer electrons will spend more time farther from the nucleus increases as well thus, the atoms are larger. 

Changes in Z ff. As the effective nuclear charge —the positive charge felt by an electron—increases, outer electrons are pulled closer to the nucleus thus, the atoms are smaller. 

The net effect of these influences depends on shielding of the increasing nuclear charge by inner electrons  

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On the basis of the trend in atomic size, what trend is expected in the ionization energy E of the halogen atoms Compare your prediction with the actual trend in Eu given in Table 19-1. 

Sketch an outline of the periodic table and use it to compare the trends in atomic size, first ionization energy, and electron affinity. 

The group 4A elements exemplify the increase in metallic character down a group in the periodic table Carbon is a nonmetal silicon and germanium are semimetals and tin and lead are metals. The usual periodic trends in atomic size, ionization energy, and electronegativity are evident in the data of Table 19.4. 

In addition to the information below, the periodic table shows trends in atomic size, ionization energy, electronegativity, valence electrons, and melting points. 

Figure 13.1 General Periodic Trends in Atomic Size...

Thinking Critically Lithium behaves more like magnesium than sodium. Use what you learned in Chapter 6 about trends in atomic sizes to explain this behavior. 

No. If all you know is that the atomic number of one element is 20 greater than that of the other, then you will be unable to determine the specific groups and periods that the elements are in. Without this information, you cannot apply the periodic trends in atomic size to determine which element has the larger radius. 

Trends in Atomic Size Trends in Ionization Energy Trends in Electron Affinity... 

Figure 8.11 Periodicity of first ionization energy (IE,). A plot of IEi vs. atomic number for the elements in Periods 1 through 6 shows a periodic pattern the lowest values occur for the alkali metals (brown) and the highest for the noble gases (purple). This is the inverse of the trend in atomic size (see Figure 8.10).

Explain the relationship between the trends in atomic size and in ionization energy within the main groups. 

Summarize the trend in metallic character as a function of position in the periodic table. Is it the same as the trend in atomic size Ionization energy ... 

Solution Recall that the general periodic trends in atomic size are ... 

Our two periodic trends in atomic size do not supply enough information to allow us to determine, however, whether B or Si (represented by the two question marks) has the larger radius. Going from B to Si in the periodic table, we move down (radius tends to increase) and to the right (radius tends to decrease). It is only because Figure 7.7 provides numerical values for each atomic radius that we know that the radius of Si is greater than that of B. If you examine the figure carefully, you will discover that for the s- andp-block elements the increase... 

How does the effective nuclear charge help explain the trends in atomic size across a period of the periodic table ... 

Use the electronic configuration of the alkali metals and your knowledge of orbitals and quantum numbers to explain the trend in atomic size of alkali metal atoms. 

Remember the trends in atomic size—across within a row and verticaiiy within a coiumn. 

As indicated by their high positive ionization energies, the halogen atoms do not lose electrons readily. Ionization potentials show a decrease (greater ease of electron loss) with an increase in atomic number, as would be expected for the trend in atomic sizes (weaker binding of the valence electrons resulting from increased distance from the nucleus). 

Notice that the trends in ionization energy are consistent with the trends in atomic size. Smaller atoms are more difficult to ionize because their electrons are held more tightly. Therefore, as you go across a period, atomic size decreases and ionization energy increases. Similarly, as you go down a column, atomic size increases and ionization energy decreases since electrons are farther from the nucleus and therefore less tightly held. 

Electronegativity increases across a period and decreases down a group, the reverse of the trends in atomic size. 

Although the overal trend in atomic size for transition elenienis is also to decrease from left to light and inaease from top to bottom, the observed radi do not vary in as regular a way as do the main group elements. 

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