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Physical Science: Ch. 15 & 17 Atomic Structure and the Periodic Table Atom - the smallest particle of an element which has all the properties of that element Ex.: each gold atom has the exact same properties as a gold bar (color, density, melting point, etc.) (although this is hard to tell, since a single atom is so small) • Element - a substance made of only 1 kind of atom • All known elements are illustrated on the Periodic Table Compounds • A compound is a combination of atoms from 2 or more different elements, in a definite ratio. Example: water (H2O) and hydrogen peroxide (H2O2) The properties of a compound may be completely different from those of the elements which make it up. • Sodium, for example, is a very reactive metal. Chlorine is a poisonous gas. But combine them together in a 1:1 ratio and you get ……. • Sodium Chloride………salt. Chemical and Physical Properties • Physical properties are properties which can be easily observed without changing the chemical make-up of the material. • Ex.: color, density, melting point, odor • A physical change is one where the identity of the material remains the same, but physical properties such as size, shape, or state of matter may change. • Ex.: boiling water, breaking a pencil, melting and reshaping a metal • Chemical properties tell how a substance will react to form new substances. • Ex.: flammability, reactivity, corrosiveness • A chemical change occurs when a new substance(s) is created from other substances • Ex.: rusting, photosynthesis, burning • Sunny went to the beach and laid out in the sun for 1 hour, after which time she had a dark tan. Was this a chemical change, a physical change, both, or neither? Explain. • Could you chemically combine substances containing sodium, hydrogen, oxygen, and chlorine, and end up with one of your resulting substances containing nitrogen? • No! Even though you can create new substances, you can only create them from what you already have. You cannot end up with the element nitrogen if you had none to begin with. Doh!! • Alchemists tried to produce gold and other valuable materials. But with gold being a pure element, it wasn’t possible. Law of Conservation of Mass • In a chemical reaction, mass can neither be created or destroyed, it can only change form. Therefore, the mass of the reactants before the reaction occurs must be exactly equal to the mass of the products following the reaction. • The same thing applies to physical changes. mass before = mass after = • For example, if a log were placed in a sealed metal box and set on fire. The products (smoke, ash, gases, etc.) would have the exact same mass as the log before it were burnt. • If the box were sitting on a scale as the log burned, the weight would not change at all. • Cletus and Brandine decided to see what would happen when they mixed baking soda and vinegar in a bottle and capped the top with a balloon. As the illustration shows, the balloon inflated. Wouldn’t this be a violation of the Law of Conservation of Mass, since matter which was not originally there was produced? Why or why not? The Periodic Table • The Periodic Table is an arrangement of all the different elements based on their chemical and physical properties. • Elements are arranged by increasing atomic #'s on the Periodic Table (from left to right) • Elements in the same group (vertical column) have similar properties. • Chemical symbols are used to identify elements on the Periodic Table. The symbols consist of 1 or 2 letters, the first of which is always capitalized. There are 2 main areas inside an atom: -The nucleus is the tiny central core of the atom. It contains protons and neutrons and has a positive charge. -The electron cloud is the very large area surrounding the nucleus. It contains electrons and has a negative charge. Atomic Charge • There are usually the same number of positively charged protons (p+) in the nucleus as there are negatively charged electrons (e-) in the electron cloud. • These charges cancel out and make the atom neutral. Protons & neutrons are located in the nucleus of the atom Protons have a positive charge Neutrons have no charge (neutral) Both protons and neutrons have a mass of 1 amu (atomic mass unit) Almost all of the mass of the atom is located in the nucleus, even though it is the smallest part of the atom. Electrons • Electrons are negatively charged particles which are located in the electron cloud (the area surrounding the nucleus) • Electrons orbit the nucleus and have essentially 0 mass. Atomic number and atomic mass • An element is identified by it's atomic # and atomic mass • Atomic number is simply the # of protons in the nucleus of an atom • This number is usually the same as the # of electrons in the atom Atomic Mass • The atomic mass of an element is the mass of a single atom of the element. This is determined by adding the # of protons and neutrons in the atom. Mass number = # protons + # neutrons • So...if mass number = # protons + # neutrons, and the # of protons is just the atomic #, what would the formula be for finding the # of neutrons? • Correct! Mass number - # protons = # neutrons Find the # of neutrons in an atom of Mercury. • Round 200.59 to 201, subtract the # of protons (80), the # of neutrons is 121. • Now, provide the requested info. for the following elements: Cadmium Iron - atomic # ________________________________________________________________________________________________________________ - atomic mass _________________________________________________________________________________________________________________ - # protons _________________________________________________________________________________________________________________ - # electrons _________________________________________________________________________________________________________________ - # neutrons - atomic # - atomic mass - # protons - # electrons - # neutrons Cadmium Iron 48 112.411 48 48 64 26 55.845 26 26 30 Isotopes • An isotope is an atom of an element with a different number of neutrons than normal. • This has a direct effect on the atomic mass of the atom. • Ex.: C-12 and C-14 are isotopes of carbon • Polydactyle cats have a 6th digit on each paw. They’re still cats, just like an isotope of carbon is still carbon. They just happen to have a different number of digits, like C-14 happens to have 8 neutrons. • If fingers were neutrons, this guy would be an isotope. Quarks • A quark is a fundamental particle which makes up protons and neutrons. • Quarks are detected and identified by accelerating protons to very high speeds, colliding them together, and then looking at their component parts. • So why does it matter what these particles are made of, or how many protons are located where? Electron Shells • The electron cloud is a large area, however, the electrons in the electron cloud are not randomly scattered here and there. • If you were to slice an atom in half, there are certain areas surrounding the nucleus where you are more likely to find electrons located. • Think of it like this, if you had a handful of bb’s and dropped them all on the floor, they would fall and scatter out. But they wouldn’t necessarily scatter randomly. There will likely be a greater concentration of bb’s that end up close to where they were dropped from, and fewer and fewer the farther you move out from that area. • That’s similar to how the electrons are spread out around the nucleus. Therefore, the area closest to the nucleus in the diagram appears darker, because there is a greater electron concentration there. • Why do you think the electrons are more likely to be concentrated close to the nucleus? • These areas where the electrons are located around the nucleus are called electron shells (or energy levels). • An electron shell diagram is simply a diagram showing where in the electron cloud the electrons are most likely to be found. • Think of the levels like a Gobstopper, with several different hard candy areas (electron shells) surrounding the center (nucleus). • Each shell can hold a certain # of electrons. Where the electrons are located is important because the location helps determine how 1 atom will react with other atoms. Electron Shells You could also think of it like the inside of a stadium. It is divided into different seating levels as you move outward from the field. Likewise, there are different electron levels (shells) as you move outward from the nucleus. Just like in a stadium, the levels closest to the nucleus (field) will fill up first. Then, once a level is full the next level out will start to fill up. The further out you move, the more electrons (people) can fit into each level (even though the concentration ,may be less) Electron Shells • In the electron cloud, the first level out from the nucleus can hold 2 electrons, the next level can hold 8, the 3rd level holds 18, the 4th - 32, and 5th - 50. • The number of electrons in the outermost shell is called the outer shell electrons. • To draw an electron shell diagram, first find the # of electrons for an element. Let's use Bromine (Br) for an example. • Bromine has an atomic # of 35, so it has 35 electrons. So let's start filling up the electron shells until all 35 of the electrons are accounted for. • The first level can hold 2 e-, so you put 2 of the 35 in there (33 left) • The second level can hold 8, so in goes 8 (25 left). The 3rd level can hold 18, which will leave you with 7. Even though the 4th level can hold 32, it doesn't have to be filled up, just stick in however many you have left, which in this case is seven. (2 + 8 + 18 + 7 = 35) • So the distribution of electrons in the electron cloud would look something like this. (note: the nucleus is greatly exaggerated in size, therefore the electron concentration around the nucleus does not appear as dense as it actually is) • However, realizing that not all of my students have the artistic ability that I do (and to save time and space), the diagram can be drawn as shown below. Now, try one on your own. Draw the electron shell diagram for Calcium. • This should be what you got. • The # of electrons located in the outer most shell for a given element ( 10 for calcium, 7 for bromine, and so on) will determine how an atom of that element will react with other atoms. But we'll get into that in a later chapter. Element Oxygen Neon Tellurium Barium-141 Plutonium Mercury Sodium Silver Radon-217 Chemical Symbol Atomic # Atomic Mass # protons # neutrons # electrons Atomic # Atomic Mass # protons # neutrons # electrons O 8 15.999 8 8 8 Neon Ne 10 20.180 10 10 10 Tellurium Te 52 127.60 52 76 52 Barium Ba 56 137.327 56 81 56 Plutonium Pu 94 244 94 150 94 Mercury Hg 80 200.59 80 121 80 Sodium Na 11 22.990 11 12 11 Silver Ag 47 107.868 47 61 47 Radon Rn 86 222 86 136 86 Element Chemical Symbol Oxygen • Name 3 different elements which all have 3 outer shell electrons. • How many neutrons would the isotope C-14 have? • How many electrons would it take to fill the first 4 electron shells? Which element would accomplish that? • U-235 and U-238 are two common isotopes of uranium. Provide the following information for each of these elements: • # of protons, neutrons, and electrons • Atomic mass Identify the following elements: • Has an atomic mass of 107.8682 • Has 22 electrons • Has 10 neutrons • Lightest element which is a liquid at room temperature • What is unusual about Iodine and Tellurium? Answers • Silver • Titanium • Fluorine or Neon • Bromine • Iodine has a larger atomic number but a smaller atomic mass • You are given a plastic bucket containing 4 different materials: iron filings, sand, salt, and very small plastic chips. The 4 materials all have about the same sized pieces and are thoroughly mixed together in equal amounts. Explain what you could do, step by step, to separate the 4 materials based on their different physical properties. Note: Picking out the separate materials piece by piece is not acceptable. • Physical properties are properties which can be easily observed or measured without changing the chemical make-up of the material. Ex.: color, density, melting point, odor • Nitrogen (78%) and oxygen (21%) are the 2 main gases which make up the air that we breathe. Based on this, and information from the Periodic Table, explain why it is often harder to breathe at higher elevations (like Denver) than it is at lower elevations. • Also explain why higher elevation locations are often favored for training by endurance athletes. • Provide the following information for argon: -# of protons, neutrons, and electrons -# of outer shell electrons -group # and period # -2 elements with similar properties as argon