Use Figure 7.6 to predict the lengths of the C-S, C-H, and S-H bonds in this molecule.Īnalyze and Plan We are given three bonds and told to use Figure 7.6 for bonding atomic radii. One such substance is methyl mercaptan, CH 3SH. Because natural gas leaks pose the danger of explosion or suffocation, various smelly substances are added to the gas to allow detection of a leak. Natural gas used in home heating and cooking is odorless. SAMPLE EXERCISE 7.1 Bond Lengths in a Molecule Which part of the periodic table (top/bottom, left/right) has the elements with the largest atoms?įIGURE 7.6 Trends in bonding atomic radii for periods 1 through 5. In CCl 4 the measured length of the C-Cl bond is 1.77 Å, very close to the sum (0.77 + 0.99 Å) of the bonding atomic radii of C and Cl.įIGURE 7.5 Distinction between nonbonding and bonding atomic radii within a molecule. For example, the Cl-Cl bond length in Cl 2 is 1.99 Å, so a bonding atomic radius of 0.99 Å is assigned to Cl. Knowing atomic radii allows us to estimate bond lengths in molecules. (For helium and neon, the bonding atomic radii must be estimated because there are no known compounds of these elements.) The bonding atomic radii of other elements can be similarly defined ( FIGURE 7.6). ![]() For example, in the I 2 molecule, the distance separating the nuclei is observed to be 2.66 Å, which means the bonding atomic radius of an iodine atom is (2.66 Å)/2 = 1.33 Å.* Similarly, the distance separating adjacent carbon nuclei in diamond (a three-dimensional solid network of carbon atoms) is 1.54 Å thus, the bonding atomic radius of carbon is 0.77 Å. From observations of these distances in many molecules, each element can be assigned a bonding atomic radius. Scientists have developed a variety of techniques for measuring the distances separating nuclei in molecules. Unless otherwise noted, we mean the bonding atomic radius when we speak of the “size” of an atom. Note from Figure 7.5 that the bonding atomic radius (also known as the covalent radius) is shorter than the nonbonding atomic radius. The bonding atomic radius for any atom in a molecule is equal to half of the nucleus-to-nucleus distance d. We can define an atomic radius based on the distance separating the nuclei when two atoms are bonded to each other, shown as distance d in Figure 7.5. For now, we only need to realize that this attractive interaction brings the two atoms closer together than they would be in a nonbonding collision where the atoms ricochet apart. In molecules, an attractive interaction exists between any two adjacent atoms in the molecule, leading to a chemical bond between the atoms. We call this radius the nonbonding atomic radius or the van der Waals radius ( FIGURE 7.5). The shortest distance separating the two nuclei during such collisions is twice the radii of the atoms. This ricocheting happens because the electron clouds of the colliding atoms cannot penetrate each other to any significant extent. When two of these atoms collide with each other, they ricochet apart like colliding billiard balls. ![]() Imagine a collection of argon atoms in the gas phase. (Section 6.5) Nevertheless, we can define atomic size in several ways, based on the distances between atoms in various situations. According to the quantum mechanical model, however, atoms do not have sharply defined boundaries at which the electron distribution becomes zero. ![]() We often think of atoms as hard, spherical objects. CHEMISTRY THE CENTRAL SCIENCE 7 PERIODIC PROPERTIES OF THE ELEMENTS 7.3 SIZES OF ATOMS AND IONS
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