Size, of atoms and ions

We often think of atoms as hard, spherical objects. According to the quantum mechanical model, however, atoms do not have sharply defined boundaries at which the electron distribution becomes zero. — (Section 6.5) Nevertheless, we can define atomic size in several ways, based on the distances between atoms in various situations. 

Which part of the periodic table (top/bottom, left/right) has the elements with the largest atoms  

M FIGURE 7.6 Trends in bonding atomic radii for periods 1 through 5. 

C — S bond length = bonding atomic radius of C -t- bonding atomic radius of S 

Comment Notice that our estimated bond lengths are close but not exact matches to the measured bond lengths. Bonding atomic radii must be used with some caution in estimating bond lengths. 

In earlier chapters, we discovered the importance of atomic masses in matters relating to stoichiometry. To understand certain physical and chemical properties, we need to know something about atomic sizes. In this section we describe atomic radius, the first of a group of atomic properties that we will examine in this chapter. 

Unfortunately, atomic radius is hard to define. We have seen that atomic orbitals extend, in principle, to infinity. Although the probability of finding an electron decreases with increasing distance from the nucleus, there is always nonzero probability of finding an electron at very large distances from the nucleus. Thus, an atom has no precise outer boundary. We might describe an effective atomic radius as, say, the distance from the nucleus within which 95% of the electron charge density is found, but this distance cannot be measured experimentally. 

From an experimental standpoint, we cannot make a measurement of the radius of a single, isolated atom. We can, however, obtain a measure of the size (radius) of an atom when it is combined with other atoms. For this reason, we define atomic radius in terms of internuclear distance. 

We will emphasize an atomic radius based on the distance between the nuclei of two atoms joined by a chemical bond. The covalent radius is one-half the distance between the nuclei of two identical atoms joined by a single covalent bond. The ionic radius is based on the distance between the nuclei of ions joined by an ionic bond. Because the ions are not identical in size, this distance must be properly apportioned between the cation and anion. One way to apportion the electron density between the ions is to define the radius of one ion and then infer the radius of the other ion. The convention we have chosen to use is to assign 0 an ionic radius of 140 pm. An alternative apportioning scheme is to use F as the reference ionic radius. When using ionic radii data, carefully note which convention is used and do not mix radii from the different conventions. Starting with a radius of 140 pm for 0 , the radius of Mg can be obtained from the internuclear distance in MgO, the radius of CU from the internuclear distance in MgCl2, and the radius of Na from the internuclear 

A FIGURE 9-3 Covalent, metallic, and ionic radii compared 

Imagine a collection of argon atoms in the gas phase. When two of these atoms collide, they ricochet apart like colliding billiard balls. This ricocheting happens because 

Nuclei cannot get any closer to each other because of electron-electron repulsion 

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Another property that is closely related to electronegativity and position in the periodic table is polarizability. Polarizability is related to the size of atoms and ions and the... 

The electron configuration or orbital diagram of an atom of an element can be deduced from its position in the periodic table. Beyond that, position in the table can be used to predict (Section 6.8) the relative sizes of atoms and ions (atomic radius, ionic radius) and the relative tendencies of atoms to give up or acquire electrons (ionization energy, electronegativity). 

Before we discuss the sizes of atoms and ions, let ns review briefly some basic scientific principles ... 

Figure 13.2 Periodic Table Showing the Relative Sizes of Atoms and Ions (Atomic and Ionic Radii in Angstrom units 1 A = 10 ° m = 0.1 nm...

Down a column of the Periodic Table, the size of atoms and ions with comparable electronic structure increases with atomic number the effect of the electrons added to outer orbitals outweighs the overall shrinkage due to increasing nuclear charge. 

The sizes of atoms and ions influence how they interact in chemical compounds. Although atomic radius is not a precisely defined concept, these sizes can be estimated in several ways. If the electron density is known from theory or experiment, a contour surface of fixed electron density can be drawn, as demonstrated in Section 5.1 for one-electron atoms. Alternatively, if the atoms or ions in a crystal are assumed to be in contact with one another, a size can be defined from the measured distances between their centers (this approach is explored in greater detail in Chapter 21). These and other measures of size are reasonably consistent with each other and allow for the tabulation of sets of atomic and ionic radii, many of which are listed in Appendix F. 

Discnss the factors that lead to systematic variation of sizes of atoms and ions throngh the periodic table (Section 5.5, Problems 31-34). 

Utilize the periodic table and its predictive power to estimate the relative sizes of atoms and ions, as well as relative magnitudes of ionization energy and electron affinity. 

More detailed periodic tables may also include such information as the electron arrangement, relative sizes of atoms and ions, and most probable ion charges. 

Understanding the wealth of information found in the organization of the periodic table is a central skill for general chemistry. You will always have a periodic table available for ACS exams, and likely for most classroom tests as well. Therefore, knowing the trends within the periodic table will allow prediction of properties, even for unfamiliar elements. Relative sizes of atoms and ions, trends in ionization energy, and trends in electronegativity are all important to understanding the behavior of elements. The differences between metals and nonmetals and their reactions are also based on periodic trends. Trends within families and trends within periods can both reveal much about the physical properties and chemical reactions expected for the elements. 

SIZES OF ATOMS AND IONS (SECTION 7.3) The size of an atom can be gauged by its bonding atomic radius, which is based on measurements of the distances separating atoms in their chemical compounds. In general, atomic radii increase as we go down a colrnnn in the periodic table and decrease as we proceed left to right across a row. 

Figure 1.3. The size of atoms and ions of the elements. Neutral atoms have covalent sizes marked with circles, cations with triangles, and anions with squares.

Next, we examine the periodic trends in physical properties such as the size of atoms and ions in terms of effective nuclear charge. (8.3)... 

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