Kaplan MCAT General Chemistry Review

Metallic solids consist of metal atoms packed together as closely as possible. Metallic solids have high melting and boiling points as a result of their strong covalent attractions. Pure metallic masses (consisting of a single metal element) are usually described as layers of spheres of roughly similar radii, stacked layer upon layer in a “staggered” arrangement such that one sphere in one layer fits into the indented space between the spheres that sit above and below it. These staggered arrangements (body-centered cubic and face-centered cubic, see below) are more common than layers of spheres stacked to form perfectly aligned columns of spheres (see simple cubic, below), because the staggered arrangement minimizes the separation between the atoms.

The repeating units of crystals (both ionic and metallic) are represented by unit cells. There are many types of unit cells. Chapter 4, Compounds and Stoichiometry, referred to the geometric arrangement of Na⁺ and Cl^- ions in table salt as 6:6 coordinated, meaning that each sodium ion is surrounded by (coordinated by) six chloride ions, and each chloride ion is surrounded by (coordinated by) six sodium ions. This particular arrangement is also known as face-centered cubic. There are three cubic unit cells, and you should recognize these for the MCAT: simple cubic, body-centered cubic, and face-centered cubic. Figure 8.2 illustrates these unit cells as ball-and-stick models.

Figure 8.2

In the ball-and-stick models illustrated, the anions are represented as small spheres separated by a lot of space, but more accurately, the spheres are packed quite closely. Figure 8.3 illustrates this. In the ionic unit cell, the spheres represent the anions; the spaces between the anions are occupied by the smaller cations. In most ionic compounds, the anion, which has gained one or more electrons, is much larger than the cation, which has lost one or more electrons. (The cations are not shown in Figures 8.2 and 8.3.)

Figure 8.3

Liquids

The liquid phase, along with the solid phase, is considered a condensed phase because the spacing between the particles is reduced in comparison to that between gas particles. Furthermore, we assume that liquids, like solids, are incompressible, meaning that their volumes do not change in any significant way as a result of moderate pressure changes. But don’t let all this talk of liquid as a “condensed phase” mislead you: There is still a lot of space separating liquid particles. You can “observe” this space for yourself through a very simple experiment. Fill a glass with very hot tap water all the way to the top of the glass, making sure that the water surface is actually “bulging” up above the level of the rim of the glass. Carefully add powdered sugar, by the teaspoon, to the water. Do not allow the spoon to touch the surface of the water, and do not stir the solution. The hot water will dissolve the sugar on its own. Repeat the process. You will be able to add several teaspoons of sugar to the water before it spills out of the cup. The sugar dissolved into the empty spaces that separate the water molecules!

Liquids are categorized as fluids (along with gases) because they do not resist shearing forces and flow when subjected to them. They also conform to the shapes of their containers. These behaviors are a result of the high degree of freedom of movement liquids possess. Like gas molecules, liquid molecules can move about in random motion and are disordered in their arrangement. Both liquids and gases are able to diffuse. Liquid molecules near the surface of the liquid can gain enough kinetic energy to escape into the gas phase; this is called evaporation.