Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Chemical Foundations: Elements, Atoms, and Ions
Natural States of the Elements

Objective

· To learn the natures of the common elements.

As we have noted, the matter around us consists mainly of mixtures. Most often these mixtures contain compounds, in which atoms from different elements are bound together. Most elements are quite reactive: their atoms tend to combine with those of other elements to form compounds. Thus we do not often find elements in nature in pure form—uncombined with other elements. However, there are notable exceptions. The gold nuggets found at Sutter’s Mill in California that launched the Gold Rush in 1849 are virtually pure elemental gold. And platinum and silver are often found in nearly pure form.

Gold, silver, and platinum are members of a class of metals called noble metals because they are relatively unreactive. (The term noble implies a class set apart.)

Other elements that appear in nature in the uncombined state are the elements in Group 8: helium, neon, argon, krypton, xenon, and radon. Because the atoms of these elements do not combine readily with those of other elements, we call them the noble gases. For example, helium gas is found in uncombined form in underground deposits with natural gas.

When we take a sample of air (the mixture of gases that constitute the earth’s atmosphere) and separate it into its components, we find several pure elements present. One of these is argon. Argon gas consists of a collection of separate argon atoms, as shown in Fig. 4.11.

Figure 4.11.An illustration shows a cube that contains argon atoms moving rapidly in random directions.

Argon gas consists of a collection of separate argon atoms.

Air also contains nitrogen gas and oxygen gas. When we examine these two gases, however, we find that they do not contain single atoms, as argon does, but instead contain diatomic molecules : molecules made up of two atoms, as represented in Fig. 4.12. In fact, any sample of elemental oxygen gas at normal temperatures contains molecules. Likewise, nitrogen gas contains molecules.

Figure 4.12.An illustration shows two cubes. The first cube contains molecules consisting of two nitrogen atoms each, moving rapidly in random directions. The second cube contains molecules consisting of two oxygen atoms each, moving rapidly in random directions.

Gaseous nitrogen and oxygen contain diatomic (two-atom) molecules.

Hydrogen is another element that forms diatomic molecules. Although virtually all of the hydrogen found on earth is present in compounds with other elements (such as with oxygen in water), when hydrogen is prepared as a free element, it contains diatomic molecules. For example, an electric current can be used to decompose water (Fig. 4.13 and Fig. 3.3) into elemental hydrogen and oxygen containing and molecules, respectively.

Figure 4.13.An illustration uses space-filling models to show the decomposition reaction of two water molecules into two molecules of hydrogen and one molecule of oxygen when electric current is passed through them. Each water molecule is composed of an atom of oxygen and two atoms of hydrogen on either side of the oxygen at angles. The products of the reaction are a diatomic molecule of oxygen, consisting of two oxygen atoms, and two diatomic molecules of hydrogen, each consisting of two hydrogen atoms.

The decomposition of two water molecules to form two hydrogen molecules and one oxygen molecule . Note that only the grouping of the atoms changes in this process; no atoms are created or destroyed. There must be the same number of atoms and atoms before and after the process. Thus the decomposition of two molecules (containing four atoms and two atoms) yields one molecule (containing two atoms) and two molecules (containing a total of four atoms).

Several other elements, in addition to hydrogen, nitrogen, and oxygen, exist as diatomic molecules. For example, when sodium chloride is melted and subjected to an electric current, chlorine gas is produced (along with sodium metal). This chemical change is represented in Fig. 4.14. Chlorine gas is a pale green gas that contains molecules.

Figure 4.14.Two photos with accompanying illustrations show the compound sodium chloride and the elements sodium and chloride. The first photo shows a small pile of salt on a watch glass. A molecular zoom shows a three-dimensional lattice structure of alternating larger Cl minus and smaller n a plus ions. The second photo shows a sodium metal, a light-gray, amorphous solid, on a watchglass, and chlorine gas inside a sealed beaker. A molecular zoom to the sodium metal shows a tightly packed lattice of sodium ions, and a molecular zoom to the chlorine gas shows randomly arranged and separated molecules that each consist of two chlorine atoms.

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Chlorine is a member of Group 7, the halogen family. All the elemental forms of the Group 7 elements contain diatomic molecules. Fluorine is a pale yellow gas containing molecules. Bromine is a brown liquid made up of molecules. Iodine is a lustrous, purple solid that contains molecules.

Table 4.5 lists the elements that contain diatomic molecules in their pure, elemental forms.

Table 4.5. Elements That Exist as Diatomic Molecules in Their Elemental Forms

Element Present

Elemental State at

Molecule

hydrogen

colorless gas

nitrogen

colorless gas

oxygen

pale blue gas

fluorine

pale yellow gas

chlorine

pale green gas

bromine

reddish-brown liquid

iodine

lustrous, dark purple solid

So far we have seen that several elements are gaseous in their elemental forms at normal temperatures . The noble gases (the Group 8 elements) contain individual atoms, whereas several other gaseous elements contain diatomic molecules ( , , , , and ).

Only two elements are liquids in their elemental forms at : the nonmetal bromine (containing molecules) and the metal mercury. The metals gallium and cesium almost qualify in this category; they are solids at , but both melt at .

Elements That Exist as Diatomic Molecules in Their Elemental Forms

Harry Taylor/Dorling Kindersley/Getty Images

Platinum is a noble metal used in jewelry and in many industrial processes.

The other elements are solids in their elemental forms at . For metals these solids contain large numbers of atoms packed together much like marbles in a jar (Fig. 4.15).

Elements That Exist as Diatomic Molecules in Their Elemental Forms

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Liquid bromine in a flask with bromine vapor.

Figure 4.15.An illustration shows a three-dimensional structure of atoms tightly packed together.

In solid metals, the spherical atoms are packed closely together.

The structures of solid nonmetallic elements are more varied than those of metals. In fact, different forms of the same element often occur. For example, solid carbon occurs in three forms. Different forms of a given element are called allotropes. The three allotropes of carbon are the familiar diamond and graphite forms plus a form that has only recently been discovered called buckminsterfullerene. These elemental forms have very different properties because of their different structures (Fig. 4.16). Diamond is the hardest natural substance known and is often used for industrial cutting tools. Diamonds are also valued as gemstones. Graphite, by contrast, is a rather soft material useful for writing (pencil “lead” is really graphite) and (in the form of a powder) for lubricating locks. The rather odd name given to buckminsterfullerene comes from the structure of the molecules that form this allotrope. The soccer-ball-like structure contains five- and six-member rings reminiscent of the structure of geodesic domes suggested by the late industrial designer Buckminster Fuller. Other “fullerenes” containing molecules with more than carbon atoms have also been discovered, leading to a new area of chemistry.

Figure 4.16.An illustration shows the bonding structures of diamond, graphite, and buckminsterfullerene. The molecular structure of diamond shows a network of carbon atoms, each of which is bonded to four other carbon atoms. The structure of graphite shows carbon atoms single bonded together in a repeating hexagonal pattern, forming flat sheets. Three such sheets are aligned together vertically with dashed lines connecting carbon atoms between adjacent sheets.The molecular structure of buckminsterfullerene shows carbon atoms single bonded together to form a spherical cage-like structure of repeating pentagons and hexagons.

Frank Cox

The three solid elemental (allotropes) forms of carbon. The representations of diamond and graphite are just fragments of much larger structures that extend in all directions from the parts shown here. Buckminsterfullerene contains molecules, one of which is shown.