Elements, Atoms and the Periodic Table
Time		Exercises 1-3 take approximately 2 hours. Until very recently, scientists believed that atoms were the smallest unit of matter which existed. It was only by studying atoms, the stuff all matter is made of, that scientists came to be able to split the atom and "look" inside. For the purpose of this lab, it will not be necessary to borrow a linear accelerator. It will be necessary to begin to develop a basic understanding of atoms. What are they? What are some of the more common atoms? Where do they exist in our bodies? In a brief set of exercises, we will explore common atoms, the periodic table, and atomic structure.
To Ponder		1.	Ashes to ashes, dust to dust. How does matter get recycled? Atoms of each element can be incorporated into many different molecules. Matter gets recycled by being broken down into small particles including atoms and molecules. For example, plants take carbon dioxide gas (CO2) from the air and use the carbon to make sugar (C6H12O6). When the sugars are eaten or burned, the carbon in them combines with oxygen and goes back into the air as carbon dioxide.
		2.	How can we eat many different kinds of animal and plants and use those materials to construct our bodies? Enzymes in our digestive tract break the animal and plant cells we eat into small molecules and atoms. We absorb these small molecules into our bloodstream and use them to construct larger molecules.
		3.	Plants don't eat as we do. How do they construct themselves? Plants obtain carbon dioxide from the air. The carbon from CO2 is converted into sugar via photosynthesis, and the sugars are then used to construct larger molecules called cellulose, which accounts for a large proportion of the dry weight of a plant.
		4.	Why doesn't all the water on earth simply disappear through evaporation? Water, like all matter, is recycled. It is held by the gravitational pull of the earth. When water evaporates into the air, it is eventually cooled and condensed into liquid form again, returning to the earth's surface as fog, rain, snow, or hail.
Supplies		Periodic Table
Objectives		Once you have completed these exercises you should be able to:
		1.	Understand what atoms are and what they are composed of.
		2.	Describe how the components of an atom may vary.
		3.	Identify the distinguishing structural features of an atom and how they affect the atom's properties.
		4.	Read the periodic table of elements.
		5.	Calculate the atomic weight of an atom, given its atomic number.
Background Information		There are 109 known elements. Oxygen, gold, and arsenic are examples of elements in the entire universe. The smallest unit into which an element may be divided while retaining all of the characteristic properties of that element is an atom. Atoms are tiny, tiny invisible particles. All substances on earth are made of different combinations of the 109 elements.
Powerful Idea		An individual atom of gold has all the attributes of the substance, gold. A gold atom, and in fact all atoms, are made up of smaller components called protons, neutrons, and electrons. However, the subunits of an atom of gold do not, by themselves, have the properties of gold. To restate a powerful idea, an atom is the smallest unit into which an element may be divided while retaining all of the characteristic properties of that element. Many non-science majors are 'turned off by' or fearful of studying chemistry. Who cares about invisible stuff? Well, it happens that a little chemistry can be amazingly empowering in helping people to answer practical questions such as: 'How can I lose weight?' 'What happens to a woman's body when she gets pregnant?' 'What happens to my body if I smoke or drink? Thus, it will be rewarding if you keep your mind open and your curiosity peaked during this and subsequent labs. If you do, you are liable to find it interesting! And relatively easy. We only need to know the basic basics in this course.
		A.	Atoms are composed of three types of particles, the neutron, the proton, and the electron.
		1.	Each type of particle has a different charge. A proton has an electrical charge of +1. An electron has an electrical charge of -1. A neutron has no electrical charge associated with it.
		2.	Two of the three particles have significant mass. A proton has a mass of 1 Atomic Mass Unit (AMU). A neutron has a mass of 1 AMU. An electron has negligible mass (zero for our purposes). Note that atomic mass may be conventionally reported as numbers alone, without the AMU designation.
		3.	Location: Two of the three particles are located in the atomic nucleus and the third orbits the nucleus. Neutrons and protons are tightly packed into the middle of the atom, a region called the atomic nucleus (not to be confused with a cell nucleus!). Electrons orbit the atomic nucleus in complex patterns or orbitals which are determined by how many electrons the particular atom has.
		B.	Most, but not all, atoms have equal numbers of protons, electrons and neutrons.
		1.	The protons in the atomic nucleus is the primary determining factor for the general characteristics of the atom. Each different element has a different number of protons in its atomic nucleus. Atoms with the same number of protons in the atomic nucleus are the same element, regardless of whether they have the same numbers of neutrons or electrons.
		2.	For example, all atoms with 1 proton are hydrogen, all atoms with 6 protons are carbon, all atoms with 7 protons are nitrogen, all atoms with 8 protons are oxygen, regardless of the numbers of neutrons and electrons they may have associated with them.
		3.	Hydrogen is different from all other atoms in that the hydrogen atom normally does not contain a neutron. That is, the hydrogen atom is composed of one proton and one electron but no neutron.
		4.	Isotopes are atoms of an element which have differing numbers of neutrons in the atomic nucleus. For example, even though oxygen always has eight protons, it may have from seven to ten neutrons, each representing a different isotope of oxygen. The most prevalent and stable form of oxygen contains eight neutrons as well as eight protons and eight electrons, and has an atomic weight of 16. Isotopes in which the number of neutrons is different from the number of protons are often unstable and radioactive.
		5.	Ions result from changes in the number of electrons orbiting the atomic nucleus. In most atoms, the number of electrons is the same as the number of protons. When the number of electrons does not equal the number of protons in the nucleus, that atom will have a positive or negative electrical charge. For example, a hydrogen atom easily gives up its single electron to another atom or molecule and then becomes a positively charged hydrogen ion (H+). In the hydrogen atom, a single negatively charged electron and a single positively charged proton give a total or net charge of zero. The charge of the hydrogen ion comes from the fact that it has lost an electron but still has a positively charged proton. Atoms with an electrical charge are called ions.
Exercise 1		Atoms, Isotopes and Ions
To Do		1.	Complete the table below, considering 'normal' atoms only (not ions or isotopes). Except for hydrogen, which as we noted earlier is exceptional in having no neutron, the elements should have equal numbers of electrons, protons, and neutrons. Table 1. Characteristics of Some Elements Element # Electrons # Protons # Neutrons hydrogen 1 1 0 helium 2 2 2 sodium 11 11 11 chlorine 17 17 17 argon 18 18 18 carbon 6 6 6 nitrogen 7 7 7 oxygen 8 8 8
Question		2.	What is the atomic weight of the normal oxygen atom? Of the normal argon atom? The atomic weight of the oxygen atom is 16 Atomic Mass Units. This comes from 8 neutrons and 8 protons each contributing 1 AMU. Oxygen exists in the air as a molecule of two oxygen atoms (O2). An O2 molecule has a mass of 32 atomic mass units.
		3.	What is the atomic weight of a hydrogen atom? Of a hydrogen ion? The hydrogen atom (H) weighs 1 AMU, since it contains one proton and no neutron. The hydrogen ion (H ) weighs 1 AMU as well, since it has lost a negatively charged electron but retains the proton with mass 1. Like oxygen gas, hydrogen gas is made of molecules containing two hydrogen atoms (H2).
		4.	When table salt (sodium chloride or NaCl) is dissolved in water, its two components separate as they go into solution. Each sodium atom in salt gives one of its electrons to a chlorine atom. As a consequence, the sodium atoms are positively charged and the chlorine atoms are negatively charged. Are they ions or isotopes? Sodium chloride is a salt that exists in an ionized state. With the transfer of an electron from sodium to chlorine, Na+ and Cl- ions. are formed. Ions are charged particles (whereas isotopes have a different mass.)
		5.	What is the charge on each one? The sodium ion has a charge of plus 1 (because it lost a negatively charged electron) and the chlorine atom has a charge of minus 1 (because it gained the negatively charged electron).
		6.	Deuterium or 'heavy hydrogen' is frequently used in scientific research. It contains a neutron as well as a proton in its nucleus. This makes it an _____________ of hydrogen. Deuterium is an isotope of hydrogen. It has a different mass than normal hydrogen due to the gain of a neutron in its nucleus.
		7.	Does deuterium have an electrical charge? Describe your reasoning. Deuterium has no charge. It arises from a hydrogen atom (neutral) which gains a neutron. A neutron has no charge.
		8.	What is the approximate mass of deuterium? A deuterium atom has a mass of approximately 2, whereas the hydrogen atom has a mass of about 1.
Exercise 2		Reading the Periodic Table Look at the periodic table of the elements on the next page. Notice the following points:
Background		1.	Each element is identified by an abbreviation (H = hydrogen, Li = lithium, Na = sodium, K = potassium and so on - see the page following the table for a complete list). Find these elements in the table and identify the group to which each belongs. Groups are designated by a Roman numeral at the top of each column. Table 2. Group Identification Atom   Group   hydrogen     lithium     sodium     potassium     All four elements are in Group IA. This means they each have a single electron in their outer orbit which is readily given up to create an ion with a positive charge of one. They are quite reactive.
		2.	Elements in each group have common structural and functional properties. For example, the elements in Group IA each have one electron in the outer shell of their atoms and are thus highly reactive. They tend to lose this electron in chemical reactions and to become ions with a single positive charge (e.g., H+, Na+, K+). Atoms in Group IIA each have two electrons in their outer orbit which are also easily given up to form ions. What is the charge on these ions? _____________ How would you write the abbreviations for calcium and magnesium ions? Elements in Group IIA readily lose the two electrons in their outer orbits, creating ions with a charge of +2, as in Ca++ and Mg++.
		3.	In the periodic table that follows, the number of protons (and in most cases, also of electrons and neutrons) is designated by the number in the lower left corner of each box in the periodic table (e.g., H has 1, Li has 3, and Na has 11 protons). This is the atomic number of the element. What is the atomic number of: phosphorus (P) 15 sulfur (S) 16
		4	As noted previously, each neutron and proton has a mass of one, while the weight of each electron is infinitesimal (close to zero). Thus, helium (He ) has an atomic number of 2, meaning it generally contains 2 protons, 2 electrons, and 2 neutrons. Helium has an atomic weight of 4.003, as shown at the top of the box. What is the atomic weight of:
		carbon (C) 12.01 nitrogen (N) 14.01 oxygen (O) 16.00 The atomic weight represents the average weight of the normal atom plus all of its isotopes, in the proportions in which they exist in the natural world. That is why the atomic weight is not exactly double the atomic number.
		5.	Elements are arranged in the periodic table from left to right and top to bottom in order of increasing mass. The table starts with hydrogen (with an atomic number of one) and goes to unnilennium (with an atomic number of 109). There is an element for each atomic number in between, except for 108 which has not yet been observed. What element has an atomic number of 26? Fe or iron Is this element important in living things? Yes Explain. Iron is essential for human beings because there are four iron atoms within every hemoglobin molecule. Iron plays a critical role in the binding of oxygen to hemoglobin, the transport of oxygen throughout the body, and the release of oxygen to the cells.
		6.	Only the lighter elements occur in living things. In particular, living organisms are composed of about 25 of the elements in the top four rows of the periodic table. For example, what element has an atomic weight of 40.08 and where does this occur in living things? Calcium has an atomic weight of 40,08. Calcium is an important component in bones and plays a role in several areas of metabolism.
To Do		7.	Approximately 25 elements occur in living things. Six of these 25 elements are of critical importance in that they comprise the four major groups of molecules found in living organisms: carbohydrates, lipids, proteins and nucleic acids. These elements are called CHNOPS for short.
			Find the six elements in the periodic table and complete the chart below. Learn all you can about these six atoms.
		Table 3. Some Characteristics of CHNOPS Abreviation Atom Atomic Number Atomic Weight Group C carbon 6 12.01 IVA H hydrogen 1 1.008 IA N nitrogen 7 14.01 VA O oxygen 8 16.00 VIA P phosphorous 15 30.97 VA S sulfur 16 32.06 VIA
Question		8.	Which of the other five elements in CHNOPS is most like oxygen? Sulfur is most like oxygen because it is in the same Group, VIA. Both sulfur and oxygen have six electrons in their outer orbit. They readily accept two additional electrons to complete their orbits, forming two covalent bonds. The ability of oxygen to accept electrons is critical in respiration. Sulfur serves the same purpose in respiration by deep sea organisms living around hot vents.
		9.	Which of the other five elements in CHNOPS is most like nitrogen? Phosphorus is most like nitrogen since both are in Group VA and both have five electrons in their outer orbit. That means that both nitrogen and phosphorus can accept three electrons and form three covalent bonds. Because of its size and shifts that occur in its orbitals, phosphorus can also form five bonds.
		10.	Which element in CHNOPS has the greatest mass? The elements in the acronym CHNOPS are not ordered by size, but it happens that the last two (phosphorus and sulfur) are the largest. Sulfur has the greatest mass with an atomic weight of 32.06.
		11.	Which element in CHNOPS has the least mass? Hydrogen has the least mass with 1 AMU.
Describe		12.	Describe how your body obtains oxygen. About 20% of air is molecular oxygen, O2. When people breathe air into their lungs, oxygen diffuses across several membranes (lung alveoli, capillary, red blood cell) and into a red blood cell where it binds to a hemoglobin molecule. The hemoglobin binds to oxygen at high concentrations of oxygen as in the lung; when the concentration of surrounding oxygen is low, as in the tissues, the hemoglobin releases the oxygen. The O2 then diffuses into the cells of the tissue where it is used in cellular respiration.
		13.	What is the primary means by which your body obtains phosphorus, carbos and fats? These elements are obtained primarily by eating food which consists of organic materials derived from other living things.
		14.	How does your body obtain the hydrogen it needs? Water is a major source of hydrogen for the body's reactions. Hydrogen gas is explosive and is fortunately not a normal part of air.
		15.	Can your body obtain what it needs by eating carbohydrates or fats alone? Which of the CHNOPS elements are completely absent from carbos and fats? Carbos and fats contain CHO. Anyone who lives on an 'empty' diet of carbos and fats will be deficient in the elements needed for making the all-important proteins and nucleic acids: nitrogen, phosphorus, sulfur.
Exercise 3		Electron Orbitals and Reactivity
Review		1.	Electrons revolve around atomic nuclei in specific patterns called orbitals. The first orbital holds up to two electrons. The second and third orbitals hold up to eight electrons each. (Actually it is more complicated than this but we will use the simple model.) The first orbital fills up before any electrons go into the second orbital, the second orbital fills up before any electrons go into the third orbital, and so on. An orbital diagram for phosphorus is shown below. Phosphorus has an atomic number of 15. Thus phosphorus has 15 protons and 15 neutrons in its nucleus with 15 electrons orbiting around the nucleus. Of these 15 electrons, 2 are in the first orbit, 8 are in the second orbit, and 5 are in the third orbit. There are three 'empty' places for electrons in the third orbit. Figure 1. Phosphorus
To Do		2.	Draw an orbital diagram for each of the other five CHNOPS elements and label each one with the name of the element, its atomic number and atomic weight.
Drawing 1. Orbitals of Other CHNOPS Atoms The larger dot in the center of each drawing represents the atomic nucleus where the neutrons and the protons are located. The rings around the atomic nucleus are stylized representations of orbitals. The smaller dots on the rings represent electrons (they are drawn much larger than scale). The first orbital holds up to 2 electrons and the second and third orbitals hold up to 8 electrons each. The number of electrons in the outer orbital determines (in part) the bonding properties of the element.
To Do		3.	Use your orbital drawings to fill in the columns in the table below, showing how many electrons each atom has in its outer orbital and how many 'empty slots' remain in the outer orbital.
		Table 4. Bonding Characteristics of CHNOPS Abreviation Atom # Electrons in Outer Shell # Empty Slots # Bonds Formed C carbon 4 4 4 H hydrogen 1 1 1 N nitrogen 5 3 3 O oxygen 6 2 2 P phosphorous 5 3 3 or 5 S sulfur 6 2 2, 4, 6 Notice the correspondence between the empty slots in the outer orbit and the number of bonds formed. Except for phosphorus and sulfur the correspondence is exact. For the smaller atoms, the number of slots available indicates the number of bonds that will be formed. Things get more complicated in the larger atoms.
Explain		4.	The last column in the table above shows how many covalent bonds are formed by each atom when they join with other atoms to form molecules. Each covalent bond arises as the result of two atoms sharing a pair of electrons, one donated by each atom. Explain the bonding behavior of each atom in terms of the number of electrons in its outer shell.
			4a. Carbon Carbon with four electrons in the outer orbit and four empty slots can form four covalent bonds, including single, double and triple bonds. It is an exceptionally versatile 'connector.' 4b. Hydrogen Hydrogen has only one electron and one empty slot. It can form a single covalent bond. 4c. Nitrogen Nitrogen has five electrons in the outer orbit and 3 empty slots. It can form three bonds. d. Oxygen Oxygen has six electrons in the outer orbit and 2 empty slots. It can form two covalent bonds, two single or one double. The oxygens in oxygen gas are joined by a double bond. e. Phosphorus Phosphorus has five electrons and 3 empty slots in its outer orbit. Like nitrogen, it can form three covalent bonds. Because of its large size and certain rearrangements that occur as a result, however, it can also form five covalent bonds. f. Sulfur Like oxygen, sulfur has six electrons in the outer orbit and 2 empty slots. It can form two covalent bonds. Sulfur and oxygen are electron receptors and 'electron hungry.'
To Do		5.	Group 0 in the right column of the periodic table contains inert, unreactive or noble gases, including helium, neon, and argon. The atomic numbers of these gases are 2, 10, and 18. Use orbital diagramming to explain why these gases are inert. These gases are inert because their outer orbitals are completely filled, as in the case of argon below. They can neither receive nor give electrons nor are they able to share pairs of electrons. Thus they are unreactive. Drawing 2 - Argon (Atomic Number 18) an inert gas with outer shell filled
Question		6.	What atom is drawn below? Figure 2. Hydrogen This is the hydrogen atom with one electron, one proton and no neutron.
		7.	Two elements in Group IA in addition to hydrogen play a critical role in living tissue. What are they, what are their abbreviations, and what do they do? sodium    Na+ potassium    K+ Sodium and potassium ions are important in producing electrical potentials across cell membranes, as in nerve cells. Neurons (nerve cells) selectively pump K+ into the cell and Na+ out of the cell.
		8.	Carbon in Group IVA forms the backbone of all organic molecules and thus of all intelligent life. The element below it in Group IVA forms the backbone of intelligent machines. What intelligent machines do we mean, and what is the element used to create them? Computers are silicon-based and silicon has characteristics similar to carbon. Some biologists have speculated that life on some other planets may also be silicon-based. Both silicon and carbon have the great versatility in bonding that allows for formation of many different kinds of molecules. All the major molecules in living things on earth, including proteins, carbohydrates, lipids, and nucleic acids, have carbon backbones.
		9.	Anaerobic organisms often use sulfur for respiration in place of oxygen. Can you give one reason why such a substitution may be possible? Both oxygen and sulfur are effective electron receptors (with two empty slots in their outer orbits) and this is the important role they play in respiration.
Supplementary Resources		Atkin, Peter. (1987). Molecules.The abstract concept of molecules is related to everyday experiences. The illustrations clarify the explanations. Salem, Lionel. (1987). Marvels of the Molecule.The author describes the formation and behavior of molecules. Although the concepts are complex, the illustrations and explanations make the book understandable and interesting. Chemistry Teaching Resources, Umea University, Sweden URL: http://www.anachem.umu.se/eks/pointers.htm
Related AAAS Benchmarks		Chapter 4: THE PHYSICAL SETTING Section D: Structure of Matter Grade 6-8 (Benchmark 1 of 7) All matter is made up of atoms, which are far too small to see directly through a microscope. The atoms of any element are alike but are different from atoms of other elements. Atoms may stick together in well-defined molecules or may be packed together in large arrays. Different arrangements of atoms into groups compose all substances. Grade 6-8 (Benchmark 3 of 7) Atoms and molecules are perpetually in motion. Increased temperature means greater average energy of motion, so most substances expand when heated. In solids, the atoms are closely locked in position and can only vibrate. In liquids, the atoms or molecules have higher energy, are more loosely connected, and can slide past one another; some molecules may get enough energy to escape into a gas. In gases, the atoms or molecules have still more energy and are free of one another except during occasional collisions. Grade 6-8 (Benchmark 6 of 7) There are groups of elements that have similar properties, including highly reactive metals, less-reactive metals, highly reactive nonmetals (such as chlorine, fluorine, and oxygen), and some almost completely nonreactive gases (such as helium and neon). An especially important kind of reaction between substances involves combination of oxygen with something else as in burning or rusting. Some elements don't fit into any of the categories; among them are carbon and hydrogen, essential elements of living matter.