Procesos irreversibles

When scientists at NAsA first began sending spacecraft into orbit, they discovered that the spacecraft were being corroded by the environment of the upper atmosphere.the culprit was the atomic oxygen formed by the interaction of ultraviolet radiation with oxygen gas, or O2. researchers using atomic oxygen to test the durability of satellite coat- ings found that atomic oxygen could remove carbon-based materials from the surface of objects without damaging them. Furthermore, NAsA engineers discovered that atomic oxygen would not react with and damage glass composed of silicon dioxide because this silicon had already reacted with oxygen and would not react further. Armed with this knowledge, they began to coat the space station and shuttle parts with a thin layer of silicon dioxide to protect them from the corrosive effects of atomic oxygen.

Back in the lab, NAsA researchers looked for new ways to utilize the power of atomic oxygen. they discov- ered several useful applications.

suRgiCal imPlanTs When implant- ing foreign objects in a patient, a surgeon must be sure the parts are clean and sterile. It

is easy to sterilize these parts and kill all bacteria, but oc- casionally there is organic debris left behind from the dead bacteria. this organic material can cause postoperative inflammation. treatment of the sterile implants with atomic oxygen dissolves away the organic matter, leading to more successful and less painful recovery times.

Cleaning oF DamageD aRT Artwork that has been damaged by soot can be cleaned using atomic oxygen. the oxygen atoms react with the soot and cause it to vaporize away. the chief conservator at the Cleveland museum of Art

tried the atomic oxygen treatment on two paint- ings damaged in a church fire. Although the

paintings were not extremely valuable (and so were good subjects for an experiment),

all other attempts to restore them had failed. Atomic oxygen proved to work

wonders, and the soot and char were removed to reveal the original image. Because the treatment is a gas, the underlying layers were not harmed. the treatment doesn’t work on ev- erything and won’t replace other tech- niques altogether, but the conservator was impressed enough to continue to work with NAsA on the process.

>ChemisTRy in action

learning ObjeCtive

P R a C T i C e 3 . 3

Identify the physical state of each of the following elements at room temperature (20°C): H, Na, Ca, N, S, Fe, Cl, Br, Ne, Hg

Hint: You may need to use a resource (such as the Internet or a chemical handbook) to assist you.

P R a C T i C e 3 . 4

Identify each of the following elements as a nonmetal, metal, or metalloid: Na, F, Cr, Mo, Kr, Si, Cu, Sb, I, S

NASA

3.3 • Compounds and Formulas 53

Figure 3.5

Compounds can be classified as molecular and ionic. Ionic compounds are held together by attractive forces between their positive and negative charges. molecular compounds are held together by covalent bonds.

A molecule is the smallest uncharged individual unit of a com- pound formed by the union of two or more atoms. Water is a typi- cal molecular compound. If we divide a drop of water into smaller and smaller particles, we finally obtain a single molecule of water consisting of two hydrogen atoms bonded to one oxygen atom, as shown in Figure 3.6. This molecule is the ultimate particle of water; it cannot be further subdivided without destroying the water mol- ecule and forming hydrogen and oxygen.

An ion is a positively or negatively charged atom or group of atoms. An ionic compound is held together by attractive forces that exist between positively and negatively charged ions. A positively charged ion is called a cation (pronounced cat-eye-on); a negatively charged ion is called an anion (pronounced an-eye-on).

Sodium chloride is a typical ionic compound. The ultimate par- ticles of sodium chloride are positively charged sodium ions and negatively charged chloride ions, shown in Figure 3.6. Sodium chlo- ride is held together in a crystalline structure by the attractive forc- es existing between these oppositely charged ions. Although ionic compounds consist of large aggregates of cations and anions, their formulas are normally represented by the simplest possible ratio of the atoms in the compound. For example, in sodium chloride the ratio is one sodium ion to one chloride ion, so the formula is NaCl.

Example: NaCl Example: H2O Ionic Molecular Compounds Figure 3.6

Representation of molecular and ionic (nonmolecular) compounds. O

H

H Na+ Cl

–

A positively charged sodium ion and a negatively charged chloride ion form the compound

sodium chloride (NaCl). Two hydrogen atoms combined

with an oxygen atom to form a molecule of water (H2O).

There are more than 50 million known registered compounds, with no end in sight as to the number that will be prepared in the future. Each compound is unique and has characteristic properties. Let’s consider two compounds, water and sodium chloride, in some detail. Water is a colorless, odorless, tasteless liquid that can be changed to a solid (ice) at 0°C and to a gas (steam) at 100°C. Composed of two atoms of hydrogen and one atom of oxygen per molecule, water is 11.2% hydrogen and 88.8% oxygen by mass. Water reacts chemically with sodium to produce hydrogen gas and sodium hydroxide, with lime to produce calcium hydroxide, and with sulfur trioxide to produce sulfuric acid. When water is decomposed, it forms hydrogen and oxygen molecules (see Figure 3.7). No other compound has all these exact physical and chemical properties; they are characteristic of water alone.

Sodium chloride is a colorless crystalline substance with a ratio of one atom of sodium to one atom of chlorine. Its composition by mass is 39.3% sodium and 60.7% chlorine. It does not conduct electricity in its solid state; it dissolves in water to produce a solution that con- ducts electricity. When a current is passed through molten sodium chloride, solid sodium and

+

¡ Hydrogen molecules

Water molecules Oxygen molecule

+

¡

Figure 3.7

a representation of the decomposition of water into oxygen and hydrogen molecules.

54 chapter 3 • Elements and Compounds

gaseous chlorine are produced (see Figure 3.8). These specific properties belong to sodium chloride and to no other substance. Thus, a compound may be identified and distinguished from all other compounds by its characteristic properties. We consider these chemical properties further in Chapter 4.