Overview of elements, compounds and pollutants in urban areas.
· Determining which elements are pollutants in urban watershed.
Elements are composed of small particles called atoms. Atoms are the smallest units of an element that can combine with other elements. Atoms of different elements combine to form compounds. Baking soda (sodium bicarbonate), for example, is made of sodium, hydrogen, carbon, and oxygen. Hydrogen gas, composed of two hydrogen atoms is technically a compound. The chemical formula of a compound lists all the elements that make up the compound and their proportions. The smallest unit of a compound is a molecule.
A physical property of an element or compound is any characteristic that can be observed such as color, odor, density, hardness, or melting point. A chemical property refers to the chemical composition of that substance. For example, silicon (Si) and oxygen (O) make up quartz, silicon dioxide (SiO2). A physical change occurs when a compound changes state, for example, ice melting to form water. No new elements or compounds are formed. Physical changes require no modification of the internal molecular arrangement of the compound.
Minerals are either composed of one type of element (a native mineral) or two or more elements (a compound). The characteristics of minerals depend on their constituent Rock Cycle. Table salt (the mineral halite) would not have the same properties if either sodium or chlorine were replaced with another element such as silver or potassium. However, the characteristics of elements are not always expressed in minerals; for example, quartz, a hard, glassy mineral, is composed of silicon, which is a lightweight, metallic element, and oxygen, a clear gas. A compound consists of two or more different types of atoms that are chemically bonded. Halite, composed of sodium and chlorine, is an example. Electrons move around the nucleus of an element in specific and set orbitals. There are a finite number of electrons in each of these orbitals.
If an atom does not have the full number of electrons in each orbital, it seeks a partner that can "loan" one or more electrons to "fill" its molecular orbital. This is the essential cause of chemical bonding. For example, a sodium ion, which has a positive charge wants to give up an electron whereas a chlorine ion, which has a negative charge wants to accept an electron. The two elements combine to form an ionic bond (bond formed by the attraction of unlike charges) and thus form the compound, halite. There are several additional types of bonding of molecular orbitals which students will learn in high school.
The type of bonding between atoms and the characteristics of those atoms determines to a large degree how a compound will "appear" when the atoms combine. In halite, the chlorine atom is twice the size of the sodium atom. When the chlorine atoms "nestle" into a "packed" position, the sodium atoms fill in the gaps. This packed position has a cubic structure, which is reflected in the cubic nature of halite. You can demonstrate this by placing small and large plastic beads in a small, cubic, clear, plastic box. Shake the box. If one bead is twice the size of the other, they will pack in a cubic pattern. The large beads represent chlorine (Cl) atoms and the smaller beads represent sodium (Na) atoms.
Activity 1. Tell students that today they will be learning about the elements found in common pollutants in watersheds. But first, we are going to review about elements and compounds.
1. Hand out Periodic table Placemats. The periodic table lists the elements in order of increasing atomic number. Each element is described by its name, atomic weight, atomic symbol, and atomic number. Elements with similar characteristics are listed in vertical groups called families. The atomic number refers to the number of protons per atomic nucleus. Ask students to find the number and symbol of Hydrogen (1 and H). Then ask them to find the gas found in balloons that float ..Helium (2 – He). You may want them to find some elements that do not have the same symbol as they are spelt (i.e. Gold – Au; Tin – Sn; Lead, Pb)
Use the powerpoint presentation on Elements and Compounds to help
the students look at the periodic table.
go over the first 5 or 6 slides.
3.Pass out the samples of the elements and compounds. Tell the students that they need to find the atomic number and the symbol for each element and at least one or two descriptive words (they can describe color, texture, feel (heavy, light, metallic). For the compounds you need to give them the formula and then ask them the same questions (see below)
NOTE: You don't have time to
have them fill out a worksheet on this.
You can either write their answers on the board as they tell you or
just go over it verbally. The
point is to get them thinking about the variable properties of some of the
Hand out the worksheet with grid and map. Tell students to only look at the map part on the right for the moment.
1. Define a watershed (they should remember this from last time--answer: "it is the area of land where all the water drains to the same place"
Remind students that when water flows during a storm, it erodes the rock and that rocks are made from Minerals, and Minerals are made of elements. So during storms, soil and rock release elements (in an ionic state) which become part of the water. Water also cleans the roads, cement, houses, and also brings in more elements (in ionic state).
2. Using the map, have students circle: Tyson Lagoon, Mowry Slough, Mud Slough, San Francisco Bay and Lake Elizabeth. Draw arrows showing which way the water is moving in the streams.
Have students look at the grid side of the worksheet.
3. Using the periodic table,
the students should fill in the element name for the pollutants in the
grid. Cd is (cadmium), Co
(cobalt), Cu (copper), Fe (iron), Mn (Magnesium), Ni(Nickel), Pb (lead),
Next, hand out the bags of pollutants (each bag has 8 cannisters filled with the cards that have heavy metal elements found in 8 typical pollutants).
4. Students should open each
cannister (they need to share cannisters at their tables) and find out the
elements found in that pollutant and check it off the grid.
(the elements are color coded to the pollutant). These are all car
and road related pollutants. The answers are below: