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Science 9 – Chemistry Unit Table of Contents Atomic Particle Review .....................................................................2 Symbols and Formulas .......................................................................3 Activity: Developing a Classification System for Elements .............4 Summary of some element family characteristics..............................6 Problems for Practice .........................................................................7 Electrons in Four Easy Steps .............................................................10 The History ............................................................................10 The Shells...............................................................................11 The Electron Behaviour .........................................................11 The Big Picture ......................................................................12 Step Diagram for the Shell Model .....................................................14 The Flame Test: Evidence for Electron Shells? ................................15 The Periodic Table of Elements .........................................................16 Ion Review .........................................................................................17 Balancing Chemical Equations ..........................................................23 Equation Balancing Questions ...........................................................25 1 Science 9 – Chemistry Unit 2 Science 9 – Chemistry Unit Symbols and Formulas Berzelius, in 1813, simplified chemical symbols. He used letters to represent the atoms of each element. Berzelius suggested that the first letter of the name of the element would make a suitable symbol. Because there were more than 26 elements known, a problem arose immediately. Carbon, cobalt, calcium, chlorine and copper could not all have the same symbol. To overcome this difficulty C is used for carbon, Co for cobalt, Ca for calcium, Cl for chlorine and Cu for copper (Latin, cuprum). Thus a chemical symbol is a capital letter, or capital letter followed by a small letter. In most cases the letters are the first or the first and second letters of the name of the element. When this is not so, the symbol has come from the name of the element in another language such as Latin. Remember that a chemical symbol represents ONE atom of the element. Thus Cl represents one atom of the element chlorine, Ag one atom of silver, and C one atom of carbon. The symbols of the elements are used to show the composition of a molecule. A chemical formula is made up of symbols placed side by side. Each symbol represents one atom of the element. A formula represents a molecule of a compound. A formula shows the elements and the number of atoms of each element in the molecule of a compound. A numeral is placed at the lower right hand corner of the symbol to show the number of atoms of the element. If no numeral is placed there, then it is understood that there is only one atom of that element in the molecule. Thus H2O shows that 2 atoms of hydrogen are joined with one atom of oxygen to form 1 molecule of water. Both compounds and elements have formulas. The formulas of compounds have two or more different symbols. The formulas of elements have only one symbol. Questions: 1. a. Define "chemical symbol". b. What does a chemical symbol represent? 2. What do the following symbols represent? Al, As, Cu, H, Pb, N, O, Sn, Zn. 3. Write the chemical symbols for the following elements: antimony, barium, carbon, fluorine, iron, magnesium, mercury, potassium, silicon, sodium, sulfur. 4. a. Define "chemical formula". b. What does a chemical formula represent? 5. What is the composition of each of the following molecules? H2 (hydrogen gas), C2H2 (acetylene gas), O3 (ozone), Na2CO3 (washing soda), (NH4)2Cr2O7 (ammonium dicromate). Activity: Developing a Classification System for the Elements When two or more elements react (combine), they produce a new substance called a compound. In every compound the atoms are bonded (joined together) in a certain way. A molecule of water (formula - H2O) is made up of two atoms joined to one atom of oxygen. There are hundreds of thousands of different compounds, each with its own particular combination of elements. Sugar is made from the elements carbon, hydrogen, and oxygen. Table salt is made from the elements sodium and chlorine. Experiments during the last one hundred and fifty years have provided evidence that elements can be organized into groups according to behaviour. Investigations like those you may have performed, the work of Rutherford, and many other experiments have been used as evidence that the numbers of protons or electrons in atoms might be the key to classifying elements into groups. According to our model of the atom - a small positive nucleus surrounded by negatively charged electrons - transfer of electric charge is a transfer of electrons from one atom to another. Some atoms tend to lose electrons, while others tend to attract and keep electrons. Your next problem will be to group elements according to their ability to gain or lose electrons. In this way you may be able to predict which of the elements should combine to form compounds. 3 Science 9 – Chemistry Unit Table 1: Properties of some elements Element Symbol Description Atomic Number Aluminum Argon Beryllium Boron Calcium Carbon Chlorine Fluorine Helium Hydrogen Lithium Magnesium Neon Nitrogen Oxygen Phosphorous Potassium Silicon Sodium Sulphur Al Ar Be B Ca C Cl F He H Li Mg Ne N O P K Si Na S Silvery metal Colourless gas Silvery metal Yellowish brown solid White powdery solid Black crystal Yellow green gas Pale yellow gas Colourless gas Colourless gas Silvery metal Silvery metal Colourless gas Colourless gas Colourless gas Red or yellow crystal Silvery metal Silvery crystal Silvery metal Yellow crystalAl 13 18 4 5 20 6 17 9 2 1 3 12 10 7 8 15 19 14 11 16 Number of electrons readily gained or lost. 3 lost 0 lost or gained 2 lost 3 lost 2 lost 4 lost or gained 1 gained 1 gained 0 lost or gained 1 lost 1 lost 2 lost 0 lost or gained 3 gained 2 gained 3 gained 1 lost 4 lost or gained 1 lost 2 gained Procedure: 1. Examine table 1 carefully. Obtain a set of paper squares of four different colour (your teacher will tell you how many of each colour). Select one colour for elements that lose electrons, another for elements that gain electrons, a third colour for elements that either gain or lose electrons, and a fourth for elements that neither gain nor lose electrons. Use a separate paper square for each element and record three pieces of information. At the top of the square, write the number of electrons the element gains or loses. In the middle of the square write the atomic symbol for the element. At the bottom of the square write its atomic number. For example, if yellow is the colour chosen for elements that lose electrons, take a yellow square for aluminum. The completed square should look like the diagram below. Make a separate square for each element in the table of elements given. number of electrons 3 gained or lost Al Atomic symbol 13 Atomic number Aluminum Element name 4 Science 9 – Chemistry Unit 2. 3. 4. Arrange the squares in a line according to atomic number (the number at the bottom of the square). Start with atomic number 1 on the left and end with atomic number 20 on the right. In your notebook, make a copy of the arrangement of squares. Study the line of squares carefully. What pattern do you see in the numbers of electrons gained and lost? Make a new arrangement of the squares according to atomic number and the number of electrons gained and lost. The new arrangement should have several rows instead of just one. Colours will also help in making the new grouping. After your arrangement has been checked by your teacher, copy it in your notebook. Sample Questions: 1. Pick an element that loses a certain number of electrons and another element that gains the same number of electrons. What compound should form as a result of the combination of these two elements? 2. Which element would be not be likely to combine with anything? 3. Compare the chart you prepared with the method of grouping that was originally used (i.e. alphabetic). Which way of arranging elements is more useful - according to alphabetic order, according to atomic number or according to appearance? Why? Summary of some element family characteristics: Name Helium Neon Argon Krypton Xenon Radon Fluorine Chlorine Bromine Iodine Oxygen Sulfur Selenium Tellurium Polonium Nitrogen Phosphorus Arsenic Antimony Bismuth Carbon Silicon Germanium Tin Lead Boron Aluminum Gallium Indium Thallium Atomic Atomic Symbol Number He Ne Ar Kr Xe Rn F Cl Br I O S Se Te Po N P As Sb Bi C Si Ge Sn Pb B Al Ga In Tl 2 10 18 36 54 86 9 17 35 53 8 16 34 52 84 7 15 33 51 83 6 14 32 50 82 5 13 31 49 81 Atomic Weight Number of electrons per group 4.0 20.2 39.9 83.8 131.3 222.0 19.0 35.5 79.9 126.9 16.0 32.1 79.0 127.6 210.0 14.0 31.0 74.9 121.8 209.0 12.0 28.1 72.6 118.7 207.2 10.8 27.0 69.7 114.8 204.4 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 5 8 8 8 8 8 7 8 8 8 6 8 8 8 8 5 8 8 8 8 4 8 8 8 8 3 8 8 8 8 8 18 8 18 18 8 18 32 18 7 18 7 18 18 8 7 6 18 6 18 18 6 18 32 18 6 5 18 5 18 18 5 18 32 18 5 4 18 4 18 18 4 18 32 18 4 3 18 3 18 18 3 18 32 18 3 Science 9 – Chemistry Unit Beryllium Magnesium Calcium Strontium Barium Radium Be Mg Ca Sr Ba Ra 4 12 20 38 56 88 9.1 24.3 40.1 87.6 137.4 226.0 2 2 2 2 2 2 2 8 8 8 8 8 2 8 2 18 8 2 18 18 8 18 32 18 2 8 2 Hydrogen Lithium Sodium Potassium Rubidium Cesium Francium H Li Na K Rb Cs Fr 1 3 11 19 37 55 87 1.0 6.9 23.0 39.1 85.5 132.9 223.0 1 2 2 2 2 2 2 1 8 8 8 8 8 1 8 1 18 8 1 18 18 8 18 32 18 1 8 1 Problems for Practice Gases such as helium, neon, argon, which belong to the helium family, usually do not react (combine) with each other or with other elements. Scientists believe that the electrons of these inert gases offer an explanation for this lack of reactivity. Scientists also believe that the reactivity of other elements depends upon the ease with which their atoms gain or lose electrons and achieve electron groupings like those of inert gases. Work each of the following problems to see how different elements may combine to form compounds. 1. Examine the beryllium family on the previous page. How might the electron grouping of Calcium (Ca) be changed to resemble the electron grouping of argon (Ar)? 2. Examine the oxygen family on the previous page. How might an atom of sulfur (S) gain an electron grouping like that of argon (Ar)? 3. Calcium (Ca) and sulfur (S) combine to form calcium sulfide (CaS). Review your answers to problems 1 and 2 and explain how the combination of calcium and sulfur provides each with an electron grouping like that of an inert gas. 4. What is the electrical charge of a calcium atom before the reaction? Of a sulfur atom before the reaction? 5. What would be electrical charge of calcium after it lost two electrons? Of sulfur after it gained two electrons? Answers to Problems for Practice 1. Calcium has two electrons in its outer regions. If it lost these two electrons, its (new) outer region would have eight electrons. Calcium would then have the same electron grouping as that of argon. 2. An atom of sulfur has six electrons in its outer region (shell). If it gained two electrons, its outer region would have eight electrons. Sulfur would then have the same electron grouping as argon. 3. In this reaction, calcium loses two electrons to sulfur. Both then have an electron grouping like argon, an inert gas. 4. A calcium atom has twenty protons (positive charges) and twenty electrons (negative charges). It is electrically neutral (zero charge). Sulfur has sixteen protons and sixteen electrons and is also electrically neutral. 5. Calcium would lose two negatively charged electrons and be left with a charge of +2. Sulfur would gain two negatively charge electrons and be left with a charge of -2. Calcium (Ca) 0 has a neutral it loses (-2) two negative = is left 6 +2 a positive two Science 9 – Chemistry Unit charge electrons with charge (-2) two negative electrons = is left with -2 a negative two charge Sulfur (S) 0 has a neutral charge + it gains Formula: Ca+2 + S-2 = CaS (the charge of zero is understood) Questions: Members of the helium family (with rare exceptions) are not able to combine chemically with other elements. Chemists believe this lack of reactivity has something to do with the structure of their electron grouping. 1. How would a sodium (Na) acquire an electron grouping like that of neon (Ne)? 2. How would chlorine (Cl) acquire an electron grouping like that of neon? Like that of argon (Ar)? 3. If an atom of sodium and an atom of chlorine are brought together, how might both atoms achieve an electron grouping like that of an inert gas? 4. How would this change affect the electrical charge of each atom (use the method for calculating charge as shown above). 5. Using symbols, write formulas for the compounds most likely to be produced from a reaction between the following pairs of elements (usually the element that loses electrons is written first in a formula for a compound): a. lithium and iodine b. sodium and bromine c. potassium and chlorine d. rubidium and fluorine e. cesium and bromine 6. What would have to happen to an atom of oxygen to give it an electron grouping like that of neon? 7. How many atoms of each of the following elements would be needed to supply the electron requirements of one oxygen atom? a. sodium b. cesium c. magnesium d. barium 8. Frequently (as in Problem 7), chemical reactions require unequal numbers of atoms from the combining elements. In written formulas for such combinations, the number of atoms included from each element appears slightly below and to the right of each element's symbol. Where only one atom is included, no number is written - it is understood. For example, you probably know that the formula for water is H2O. The number 2 slightly below and to the right of H and the absence of any number after O indicates that a molecule of water is formed from two atoms of hydrogen and one atom of oxygen. Now look at a more complex example: the formula for an aluminum oxide is Al2O3; this means that the combining ratio for aluminum oxide is two atoms of aluminum (Al) to three atoms of oxygen. Now write the formula for the combination of oxygen with each element listed in Problem 7. 9. The atomic number of the element hydrogen is 1, and its average atomic weight is 1.008 Atomic mass units. a. An atom of hydrogen has how many protons? How many neutrons? How many 7 Science 9 – Chemistry Unit b. c. electrons? From its electron arrangement, to which family/ies might hydrogen belong? What would be the formula for a combination of hydrogen and chlorine? of lithium and hydrogen? of calcium and hydrogen? 8 Science 9 – Chemistry Unit ELECTRONS IN FOUR EASY STEPS Figure 1: History of the Atomic Model 1. The History: The concept of electrons in energy shells is not necessarily an easy one to grasp. A short history of the development of a model for the atom shows that there were many theories along the way. The Raison Bun Theory pictured the atom as a bun whose raisons represented electrons. The electrons were just mixed in with the general "atomic" dough. This theory, however, did not do anything to explain why atoms interacted. The next theory to come along was the Two-Dimensional Planetary Theory. This model had the nucleus in the center of a series of concentric (common centered) circular orbits. It was discredited because it only had matter existing in two dimensions. Everything would be flat!! The Three-Dimensional Planetary Theory was then introduced. It explained matter's 3-D nature but did not adequately explain why matter combined chemically. It also implied that the exact location and speeds of the electrons could be determined. This was found, through experimentation, to be incorrect. In fact, Heisenberg went so far as to state that "the exact location and speed of an electron could never be known". Finally, the Electron Cloud or Electron Shell Model was introduced. What it said was that electrons could be found in shells around the nucleus. Their exact positions and speeds could never be determined but their approximate locations could be found based upon the energy which they were thought to possess. 2. The Shells The shells of which we speak are not physical, tangible shells. Instead they are zones where the electrons are thought to possess certain amounts of energy. Like gravitational potential energy, this amount of energy increases as the electron gets further from the nucleus. Therefore it "costs" more energy to keep an electron in a distant shell than it does to keep it in an inside shell. When an atom's electrons begin to fill shells, they start with those shells closest to the nucleus and work their way outward. The following graphic shows the order in which electrons fill the shells. The lowest steps of the graphic fill first and then successively higher steps fill. Figure 2: Energy Steps for Shell Model 9 Science 9 – Chemistry Unit The height of the steps indicates the relative amount of energy that it costs to stay in that position. Notice that the L shell is further subdivided into subshells. Though the L shell has a capacity of 8 electrons, it is divided into two subshells - one of which has a capacity of 2 electrons and the other of which has a capacity of 6 electrons. However, we will not concern ourselves with these details for the moment. 3. The Electron Behavior The electrons of elements can be stable or unstable. Stability depends upon whether the outer shell of the atom is full or not. Which is the outer shell? The outer shell is any outermost shell which contains electrons. If one outer shell loses all of its electrons then the next closest shell to the nucleus becomes the outer shell. Atoms will gain or lose electrons in order to fill their outer shells. When an atom gains electrons it becomes a negative ion. If it loses electrons, it becomes a positive ion. Some atoms already have full outer shells. These are: Helium, Neon, Argon, Krypton, Xenon and Radon. These elements are said to belong to the Inert Gas family. Because they do not easily gain or lose electrons, they do not react chemically with other elements. When other elements gain or lose electrons they are attempting to mimic the electron configuration of a member of the inert gas family. 4. The Big Picture When atoms of different elements react chemically they undergo a process of electron swapping. The "most famous" example is the Sodium/Chlorine union. In this reaction, Na (sodium) loses an electron to have its outer shell full and Cl (Chlorine ) gains an electron in order to have its outer shell full. When Na loses its electron it becomes a +1 ion and when chlorine gains its electron it becomes a -1 ion. Since Na is positive and Cl is negative they attract one another and react chemically. Figure 3: Electron Configuration of Sodium 10 Science 9 – Chemistry Unit Figure 4: Electron Configuration of Chlorine Figure 5: The Bonding of Sodium and Chlorine There are many ways to represent the electron configuration of elements and in the next two pages three such ways are shown. The first diagram is intended to show how many electron positions are available in each shell and that each shell is made up of even small subdivisions call subshells. The next representation is the step model which is intended again to show the numbers of electrons in each shell and the relative amounts of energy necessary for the electrons to exist in those shells. The higher the shell is on the steps, the more energy is necessary to keep the electrons in that shell. When occupying space around a nucleus, electrons will always fill the shells closest to the nucleus first and then move outward until all of the electrons are used up. The final diagram is, of course, a part of the Periodic Table of elements. It should be familiar to you since you developed this same portion in the original activity in this chemistry section. 11 Science 9 – Chemistry Unit The Flame Test: Evidence for Electron Shells? An electric current passing through a light bulb heats the tungsten filament. The heat causes the filament to glow and it appears white hot. When an electric current is passed through a glass tube containing neon, a red glow is produced. A wide variety of coloured neon lights is produced by adding small amounts of other elements to the neon and by tinting the glass tube. Is the colour produced a characteristic property of the gas? Do other elements produce a characteristic colour? Does the colour of the light change when the element combines with other elements? In this demonstration we will heat elements and compounds in a Bunsen Flame until they glow or give off some light. The light produced colours the Bunsen flame. This procedure is called a flame test. In the table provided, record the results of the experiment beside the name of the element or compound used. 12 Science 9 – Chemistry Unit Questions: 1. What evidence suggests that the flame colours are produced by the first or metallic element of a compound? 2. What is a "positive" flame test for each of the following: copper, calcium, potassium, strontium, sodium, and barium? 3. What is the definition of "characteristic property"? What evidence suggests that flame colour is a characteristic property? 13 Science 9 – Chemistry Unit Ion Review: This review should help to find what type of ion is formed by each of the first 20 elements in the periodic table. Refer to the periodic table above for the necessary information. Remember that Atomic Number is the same as the number of protons and the number of electrons in a neutral atom. Also remember that the atomic weight minus the atomic number gives you the number of neutrons. When filling in the charts start placing the atoms electrons in the bottom step and work your way up until all of the available electrons are used up. Then to decide what type of ion is formed, find out what would be the easiest way for the atom to have a full outer shell - by losing some electrons or by gaining some electrons. The chart that you made should help you visualize this need. 14 Science 9 – Chemistry Unit 15 Science 9 – Chemistry Unit 16 Science 9 – Chemistry Unit 17 Science 9 – Chemistry Unit 18 Science 9 – Chemistry Unit 19 Science 9 – Chemistry Unit Directions: With the following pairs of elements, put the number of atoms of each necessary to create a balanced compound. # 1 2 3 4 5 6 7 8 9 10 Atoms of Element 1 Be Na C Li Ca P Al B H Si Atoms of Element 2 F S N O Si N O F C S Balancing Chemical Equations: By now you should know what elements and compounds are; you should know what the difference is between chemical symbols and chemical formulas. Molecular weight is the combined weight of all of the atoms in a molecule of a compound. The next step to be taken is the balancing of chemical equations. This is a skill that may at first seem very imposing but when one realizes that all that is necessary is an understanding of the law of Conservation of Mass and lowest common denominators, it all becomes relatively straight-forward. Conservation of mass states that "in a chemical reaction, the mass of the reactants before the reaction is equal to the mass of the products after the reaction". This comes from the fact that during a chemical reaction new matter is not created and matter is not destroyed - atoms simply rearrange themselves and attach to other atoms. The mathematical concept of lowest common denominators comes into play because numbers of atoms of each element must be balanced on each side of a chemical equation. A simple example of a balanced equation is given below along with an explanation. ____ H + ____ O2 ----> ____ H2O In this example, hydrogen (monatomic - exists as a one atom element) reacts with oxygen (diatomic - exists as a two atom molecule) to give us water. On the left side there is one atom of hydrogen and there are two atoms of oxygen. On the right side there are two atoms of hydrogen and one atom of oxygen. Immediately an imbalance should be seen which violates the law of conservation of mass. One can count the atoms of each element and make a table to keep track of numbers. This table is filled with preliminary numbers of the equation the way you find it originally. Later the values will be changed as you get closer to the balanced equation. Before Reaction 1 H 2 O || || || || || || || After Reaction 2 H 1 O 20 Science 9 – Chemistry Unit If this equation was balanced the numbers for each element would be the same on both sides of the table. Since they aren't, we must chose one of the elements to start with. We can start with oxygen here (the reason for this choice will be given later). The question we must ask ourselves is, "what is the lowest common denominator between the left side and the right side"? This is a rather trivial example because the lowest common denominator is obvious - 2. The right side must therefore be multiplied by two to achieve this common denominator. Before || After Reaction || Reaction || 1 H || 2 H || 2 O || 2 x 1 = 2 O || In the chemical equation, the only way that we can accomplish this is to multiply the complete water molecule by 2. We cannot just multiply the oxygen by 2 because this destroys the unique structure of water - H2O. We can, however, say that there are two molecules of water. But when we do this we must also adjust our table to reflect this multiplication. ____ H + ____ O2 ----> 2 H2O Before Reaction || After || Reaction || 1 H || 2 x 2 = 4 H || 2 O || 2 x 1 = 2 O || Now we must balance hydrogen and this can only be accomplished by multiplying the left side by 4. Before || After Reaction || Reaction || 1 x 4 = 4 H || 2 x 2 = 4 H || 2 O || 2 x 1 = 2 O || The equation is now balanced if we put the numbers from the able into the equation. 4 H + 1 O2 ----> 2 H2O As mentioned earlier the reason for the choice of oxygen as a starting point would be given. The oxygen was solved first because on the right side it is in the more simpler form - there is only one atom in the molecule. Hydrogen, on the other hand, is slightly more complicated since there are two atoms of it in the molecule. As a rule, then, do the simpler elements first and then try to work your way to the more difficult elements. You will be given plenty of questions in the next exercise to practice this procedure. Questions: 1. The formula for table sugar is C12H22O11. 21 Science 9 – Chemistry Unit 2. 3. a. How many atoms of hydrogen are found in the sugar molecule? b. What other atoms are present and how many of each? c. Translate the English phrase "three sugar molecules" into chemical symbols. d. What is the molecular weight of a molecule of table sugar? The formula for the insecticide DDT is C14H9Cl5. a. What atoms combine to make DDT, and how many of each are present? b. How would a chemist symbolize "four DDT molecules"? c. What is the molecular weight of a molecule of DDT? Balance the chemical equations that follow. Remember, you cannot change the chemical formula of a molecule, you can only write numbers in front of the chemical formula. a. A smoldering fire: coal (carbon) + oxygen ------> carbon monoxide C + O ------> CO b. Scratching an aluminum can will reveal a shiny surface. This surface dulls quickly according to the reaction: aluminum + oxygen ------> aluminum oxide Al + O ------> Al2O3 c. Burning Magnesium: Mg + d. e. f. g. O2 ------> gas barbecue flame: C3H8 + O2 a soft drink fizzing: H2CO3 ------> H2O + photosynthesis: CO2 + H2O ------> Acid rain: SO3 + H2O ------> ------> MgO CO2 + H2O CO2 C6H12O6 + O2 H2SO4 Bibliography Aikenhead, Glen S., Logical Reasoning in Science & Technology, Saskatoon: University of Saskatchewan, 1988. Andrews, William A, et al, Physical Science - An Introductory Course, Scarborough: Prentice-Hall of Canada, Ltd., 1978. Interaction Science Curriculum Committee, Interaction of Matter and Energy (second edition), Chicago: Rand McNally & Company,1973. 22