Background Information: A Foundation in Chemistry
Reactivity
The first category of toxicity students will learn about is reactivity, meaning chemicals that are toxic because they are highly reactive. Understanding reactivity will require students to learn basic atomic structure because that structure determines the tendency of each element to bond or not. Students will first learn that an atom consists of protons and neutrons in the nucleus at the center of an atom, the number of protons being constant in every atom of an element and determining the atomic number of each element. We will then explore electron configuration. Students will learn that electrons orbit the nucleus in orbitals at specific energy levels, which are called electron shells. In every atom, each electron shell is capable of holding a particular number of electrons. For example, the first electron shell always holds two electrons; the second shell always holds eight. Even though each energy level has a different number of orbitals (or subshells), after the first electron shell with just two electrons, the outermost shell most often houses eight electrons, called valence electrons. If this outermost shell, also called the valence level, is not full because it is holding fewer than eight valence electrons, then the atom is more reactive because it has a tendency to fill its valence level through bonding with other atoms. This is called the octet rule; atoms seek to acquire stable octets in their outer shells by bonding with other atoms. The electron configuration of every atom can be diagrammed to predict its reactivity, or tendency to bond, by observing the number of electrons in the outermost shell. An atom that has more "space" available in its valence shell can be said to have a high electron affinity: the ability of an atom to accept electrons from other atoms. This electron affinity contributes to making an atom reactive.
Students can look at a model of any molecule to predict electron affinity and reactivity. A model of carbon, as an example of high levels of reactivity or electron affinity, indicates that it is very likely to bond because it has only four valence electrons, allowing for four more. Conversely, a model of neon would show students that, with eight valence electrons, neon is essentially un-reactive; with a full valence shell, there is no reason for neon to bond. Comprehension of atomic structure and the relationship of electron configuration to chemical bonding will lead us into a study of covalent and ionic bonds, both of which are driven by an atom's tendency to complete its octet. Covalent bonds occur when two atoms share an electron. For example, water (H 2O) is the result of one oxygen atom covalently bonded with two hydrogen atoms. Hydrogen has just one valence electron in the first electron shell, instead of a filled shell of two. Oxygen has six valence electrons in its second shell instead of a complete octet. Therefore, when hydrogen shares an electron with oxygen, its valence level is complete with two electrons: one of its own and one from sharing with oxygen. If oxygen were to share just one electron with hydrogen, it would then have seven valence electrons. Since eight is the ideal number, oxygen then bonds with a second atom of hydrogen to complete its valence level: six of its own electrons, and two from sharing with two atoms of hydrogen. Ionic bonds occur when atoms are joined by opposite electrical charges. While atoms are most often neutral because they contain the same number of positively charged protons as negatively charged electrons, they can gain or lose electrons to other atoms, creating a positive or negative charge. Charged atoms are called ions; cations are positively charged and anions are negatively charged. Atoms gain and lose electrons for the same reason as they form covalent bonds: the tendency toward a complete valence shell. One common ionic bond is sodium chloride (NaCl), or table salt. Sodium has just one valence electron; it can either gain seven or lose one to achieve a complete octet, and losing one is "easier" than gaining seven. Chlorine has seven valence electrons; it needs just one more. When sodium loses its valence electron, it gains a positive charge. Chlorine is happy to accept a free electron but, in doing so, it gains a negative charge. The positively charged sodium cation and the negatively charged chlorine anion then are electrically attracted to one another, forming an ionic bond. While water and salt have different types of bonds, both bonds occur for the same reason; they involve atoms that are reactive because they have incomplete octets in their valence energy levels. Once students understand this concept, they will understand the basis for chemical bonding as well as the nature of reactivity.
Our case study for reactive toxins will be nitrogen oxides. Nitrogen oxides are one of the six main pollutants the EPA tracks that have significant concentrations in BVHP (80 th percentile), and one cause of the area's non-attainment of state and federal air quality standards. 5 3 Before the PG&E power plant closed, it emitted approximately 321 tons of nitrogen oxides per year. 5 4 Since the plant's closure, the largest emitter of nitrogen oxides is the sewage treatment plant, releasing close to 16 tons of nitrogen oxides per year. 5 5 Nitrogen oxides are also emitted by burning fossil fuels such as at the Potrero Hill power plant, in motor vehicle exhaust, as from the two highways running through BVHP, and during industrial processes such as welding and electroplating, 5 6 both of which take place in BVHP. At low levels, nitrogen oxides can irritate the eyes, nose, throat, and lungs, causing coughing, shortness of breath, nausea, fatigue, or fluid build-up in the lungs. 5 7 High levels can cause the swelling of throat and respiratory tract tissues, reduced oxygenation of body tissues, fluid build-up in lungs, spasms, burning when in direct contact with skin, and even death. 5 8 In laboratory experiments of animals, birth defects and genetic mutations resulted from nitrogen oxide exposure. 5 9 Nitrogen oxides can also exacerbate or increase susceptibility to respiratory infections and asthma, damaging lungs, destroying lung tissue and, over time, leading to chronic lung disease such as emphysema. 6 0 Students can apply their understanding of reactivity in atoms to reactivity in molecules, creating models of nitrogen oxides to identify the chemical nature of their reactivity. Even when bonded as a molecule, nitrogen dioxide (NO 2), for example, has three available electrons for other atoms or molecules to bond with. Students can extrapolate that NO 2 can then react with molecules in the body, often to toxic effect.
Solubility
The second category of toxicity we will study is solubility, or the chemicals that are harmful because they dissolve easily. Understanding solubility will require students to learn specifically about polarity in molecules. Polar molecules are molecules that have an "uneven" electrical charge because one atom has a higher electronegativity than the others. Every atom has a different electronegativity value from 0.7 to 4.0, which is the strength an atom has to attract electrons to itself. Learning about polarity, electronegativity and the electronegativity values for different atoms is a logical progression from the study of reactivity that included atomic structure, covalent bonds, and ionic bonds. Students will already understand charge in atoms and the relationship of electron and proton charges to ionic bonds. Electronegativity can add an understanding of why some bonds are covalent and some are ionic; an atom with high electronegativity is more likely to attract an electron from one with low electronegativity, rather than share. These ionic bonds take place when the difference in electronegativity is greater than 1.5. In sodium chloride, a good example of an ionic bond, sodium has an electronegativity of 0.9, while chlorine's is 3.0. This enables chlorine to attract an electron from sodium to fill its octet, precipitating the ionic bond. In studying polarity, students can then learn that these varying values of electronegativity in atoms can also cause electrons to be shared unequally among atoms in what are called polar covalent bonds; polar molecules are the molecules in which covalently bonded atoms have an unequal sharing of electrons. Polar molecules occur when the difference in electronegativities is between 0.3 and 1.4.
The relationship between solubility and polarity is attributed to the fact that water (H 2O) is a polar molecule. Because oxygen has a higher electronegativity than hydrogen, the electrons that the oxygen atom shares with the two hydrogen atoms spend more time around the oxygen than the hydrogen. This gives the hydrogen atoms slightly positive charges and the oxygen atom a slightly negative charge. Having a positive end and a negative end make water molecules dipole molecules that attract other polar molecules. Dissolution then follows polarity; like dissolves like, so polar molecules dissolve other polar molecules. Water then becomes the solvent that dissolves polar solutes such as alcohols and even ionic compounds like salts. Students can build models of molecules and, referencing the electronegativity values of the atoms present, they can determine whether the molecules are polar or non-polar. They can then identify which molecules would have high solubility in water.
We will focus our case study of solubility on sulfur dioxide, one of the most prevalent soluble chemicals in BVHP (90 th percentile) tracked by the EPA. The PG&E power plant previously emitted 12 tons of sulfur dioxide per year, and the wastewater treatment plant continues to emit 4 tons per year. 6 1 Sulfur dioxide is a emitted by fossil fuel burning power plants such as the nearby Potrero Hill power plant, as well as by copper smelting plants and the manufacture of products such as sulfuric acid, paper, food preservatives, or fertilizers. While even short-term exposure to high levels of sulfur dioxide can be fatal or extremely harmful, long-term exposure to lower levels can result in reduced lung function and breathing ability, inflammation or infection of airways, and aggravated asthma. 6 2 Recent studies seek to determine the extent to which sulfur dioxide poses a risk to fetuses. 6 3 When students create models of sulfur dioxide that include electronegativity and an assessment of polarity, they can ascertain the relationship between solubility and toxicity. Since sulfur dioxide is a polar molecule that is highly soluble is water, it directly affects moist mucous membranes of the eyes, nose, throat, and upper respiratory tract. 6 4 Breathing air containing sulfur dioxide may cause its absorption into the body through nose and lung, where it can easily and rapidly enter the bloodstream through your lungs. 6 5
Radioactivity
Students will examine radioactivity as a third classification of toxicity by understanding the nature of radioactive elements. Unlike reactivity and solubility, which are driven by electron configuration, radioactivity lies within the "configuration" of the nucleus, which will provide students with an opportunity to review and expand their understanding of atomic structure. Radioactivity is caused when the nucleus of an atom is unstable. Generally speaking, the larger the atom, the more unstable it is because all of the positively charged protons in the nucleus are repelling each other. "Nuclear glue," which is also called nuclear force or strong force, holds most nuclei together, keeping their protons and neutrons in place. However, nuclear force can only hold so much, and all elements with 84 or more protons are unstable. Slightly smaller atoms that are "neutron rich" or have an imbalanced proton to neutron ratio are likewise unstable; the nuclear glue is not strong enough to keep the nucleus intact and the nucleus breaks apart. The process of a nucleus breaking apart with a corresponding release of energy is called radioactive decay. Atoms that decay are considered radioactive and are called radionuclides. When an atom decays, it can emit protons and/or neutrons in order to alleviate the strain of too many atomic particles in the nucleus. Elements decay, typically into smaller elements, until they become stable. There are three main forms of radioactive decay: alpha, beta, and gamma emissions. Alpha particle emissions release two protons and two neutrons, essentially a helium cation without electrons that can quickly pick up free electrons in order to become a neutral helium atom. Beta particle emissions release an electron from the nucleus when a neutron breaks down into a proton and an electron. The electron is released but the proton stays in the nucleus, increasing the atom's atomic number by one. Gamma radiation emissions occur when a nucleus emits a high-energy photon. This can take place in conjunction with alpha and beta emissions. All three types of radioactive decay give rise to forms of ionizing radiation, a release of atomic particles and energy that can cause neutral atoms to become reactive ions. In summation, radionuclides have unstable nuclei caused by a high number of protons and/or neutrons, causing them to change into more stable atoms of other elements by releasing nuclear particles and energy as ionizing radiation in the form of alpha, beta or gamma emissions.
Our case study in radioactive toxins will focus on several radioactive isotopes found in the Hunters Point Naval Shipyard. The United States Navy has disclosed 109 different radioactive chemicals contaminating the site, primarily Cesium-137, Radium-226, and Strontium-90. 6 6 We will examine these isotopes, as well as radioactive elements such as uranium and plutonium, also found in the shipyard. 6 7 Even after students learn that larger atoms with more nuclear particles are unstable, they must also learn about isotopes in order to properly understand the radionuclides affecting BVHP. While all atoms of the same element have the same number of protons in their nuclei, the number of neutrons can vary. This is why the atomic number of every element is based on the number of protons in an atom of that element. Electrons can be lost or gained, but an elements remains in its form if the number of protons remains constant. If an atom loses or gains a proton, it becomes a different element. The atomic mass of an element represents the sum of its atomic number (protons) and an average of the number of neutrons an atom of that element contains. Atoms of the same element can have different numbers of neutrons; these different versions of the same element are called isotopes. Cesium, for example, has an average atomic weight of 133. Cesium-137 has more neutrons than an average atom of cesium. This imbalance causes Cesium-137 to be a radioactive isotope.
All of these radionuclides are toxic because the ionizing radiation they release as they decay can produce reactive ions in materials including our flesh. Reactive ions can damage cells, including DNA, hinder production of hemoglobin, cause severe anemia, mutations, birth defects, cancer, and death. 6 8 While alpha particles have a low penetrating power, they can enter the body through ingestion and cause major damage to exposed tissue. Beta particles are more penetrating than alpha particles, and can penetrate skin and membranes. Gamma radiation in the form of high-energy wavelengths can penetrate our bodies most easily. By examining models of radioactive atoms, students will understand the process of radioactive decay and the cause of radiation.
Volatility
Volatility is the final category of toxicity students will focus on, studying volatile organic compounds (VOCs) that evaporate easily. In order to understand volatility, students must understand molecular mass. VOCs typically have low molecular mass, meaning the atomic mass of all of the atoms in the compounds is relatively low. Many VOCs are short hydrocarbon chains, wherein the atomic mass of hydrogen (1) and the atomic mass of carbon (12) form compounds that are still able to evaporate. Other VOCs are organohalides. Calculating atomic mass will offer students a means of reviewing atomic structure. Protons and neutrons in the nucleus determine the atomic mass of each element in a compound, and electron configurations will illustrate why hydrogen and carbon bond so well into hydrocarbon chains, as well the reasons halogens are common in compounds. The lower the molecular mass of a compound, the higher its volatility, and the more likely it is to be emitted out of liquids or solids as a gas. VOCs can evaporate from water or soil and disperse into the air; they vaporize easily into a gaseous state that enters the atmosphere. By identifying these characteristics of VOCs, students will be able to understand the nature of their toxicity. While humans may be exposed to VOCs through contact with solid or liquid forms, they are most likely to be encountered in gaseous form as they affect the quality of the air we breathe. Their volatile behavior also creates many challenges in managing VOC emissions.
There are many sources of volatile organic compounds (VOCs) in BVHP. Like nitrogen oxides and sulfur dioxide, VOCs are tracked by the EPA 6 9 and BVHP ranks in the 80 th percentile for VOC prevalence. 7 0 Darling International, a rendering facility in the neighborhood, emits more than 47 tons of VOCs per year; Pan-Glo Services, Inc., an industrial baking pan coating factory, emits 29 tons of VOCs per year, and the sewage treatment plant emits around 8 tons of VOCs annually. 7 1 Prior to its closure, the PG&E power plant emitted 13 tons of VOCs each year. 7 2 VOC emissions from motor vehicles travelling on the two freeways in BVHP are also a concern. While the data sets for VOCs in BVHP are not disaggregated into specific compounds, there are several common VOCs we can examine by calculating their molecular mass and/or carbon chain structure: benzene, formaldehyde, methylene chloride, and perchloroethylene. Illustrating the volatility of VOCs will also enable students to conceptualize the reasons they present health issues; we absorb VOCs by breathing the air into which they have evaporated. Inhalation of VOCs, as well as absorption through skin, can have many impacts on health: eye, nose, and throat irritation or discomfort, nosebleeds, difficulty breathing, headaches, fatigue, dizziness, loss of coordination, impaired memory, allergic skin reactions, nausea or vomiting, damage to liver, kidney, and central nervous system, and cancer. 7 3 Some hydrocarbon solvents can dissolve lipid coverings around nerve fibers when inhaled, which results in a condition called peripheral neuropathy. 7 4
Comments: