3Paragraph 2:Chemical reactions involve inputs and outputs (also called products and reactants).When a chemical reaction occurs, the new chemicals created have different properties than the original chemicals.An example is when a candle burns. A candle is made of paraffin. When it is burned, it uses up oxygen, and produces carbon dioxide and water. The properties of paraffin and oxygen (inputs) are very different than the properties of carbon dioxide and water (outputs).Another example is when you mix salt and water. Saltwater (output) is very different than either salt alone or pure water.
4Paragraph 3:Conservation of matter means that matter is neither created nor destroyed…it just changes form.For example, when I mix salt and water, the salt seems to disappear. But the sodium and chloride atoms are still present, just in a different form.Energy is also present and it changes form, too. Some reactions break chemical bonds apart. In this case, the energy is used to form the new chemicals, or released as heat or light or sound.
6Density Layers You already know quite a bit about density. Density = mass/volumeMaterials with lower density ‘float’ on top of materials with greater density. This is truein liquids in solids & in gases
7Examples: Helium balloons float in air. Hydrogen filled zeppelins (the Hindenburg) float in air.*Helium and Hydrogen are both gases that are less dense than air when all are at the same temperature and pressure.1) Can you think of any gases that are more dense than air?2) What is wrong with the phrase:“can you think of any gases that are heavier than air?”
8Your turn: the density of air vs. Carbon Dioxide Remember that dry ice is frozen carbon-dioxide. It goes directly from it’s solid (frozen) state into a gaseous (vapor) state. This is called sublimation.Dry ice sublimates at −78.5 °C (−109.3 °F) at atmospheric pressure. In other words, if dry ice exists as a solid, it is at LEAST minus °F . This extreme cold makes the solid dangerous to handle without protection due to burns caused by freezing (frostbite).Compare this to water ice, which melts at positive-32 °F.In other words… don’t touch the stuff!
9The Periodic Table & Molecular Masses: The atomic mass is the total mass of protons, neutrons and electrons in a single atom. It is usually expressed in units called atomic mass units (amu).
10The Periodic Table & Molecular Masses: Examples:NaClNa =Cl =NaCl =H2OH =O=H2O =Your TurnCH4C =H =CH4 =CO2O=CO2 =NO (~air)N =NOUsing this information, which bubbles should float in air, and which should sink???
11The Periodic Table & Molecular Masses: Density Layers in stars indicate the presence of several elements…
13Atomic Math Handout Protons and neutrons have roughly the same mass. But first… Remember 5 important points:Protons and neutrons have roughly the same mass.Electrons are relatively light compared to protons and neutrons (it takes about 1836 electrons to equal the mass of a single proton).The atomic number tells us how many protons are in an atom.An atom that is “electrically neutral” has the SAME number of electrons as it has protons.The number of PROTONS determines the type of element.
14Several Isotopes of Selenium Notice they can have different colors and textures!
15Isotopes Worksheet But first… Remember 3 important points: An atom does NOT have to have an equivalent number of neutrons for each proton.The number of NEUTRONS determines the “flavor” of element… called an ISOTOPE.We can determine the number of neutrons by looking at the atomic mass and the atomic number of an element:For instance, carbon has an atomic number of 6. This means it has SIX protons. It also has an average atomic weight of This means it’s average weight is 12 “amu”. Which implies that carbon has about 6 neutrons on average.
16Summary, p. 1/3 Hydrogen is the lightest element. Astronomers have determined that stars have elements other than just hydrogen.H is used in nuclear reactions to make new elements (with different properties!)Helium is next most common.The energy released by stars is a result of nuclear reactions.Stars are NOT big bonfires, like a planet on fire! They are a ball of gas, held together by gravity and releasing energy because of all the nuclear reactions occurring. If they were really “on fire”, they would need a lot of oxygen. Check out the table on the next page to see how much oxygen there is in our Sun…
17Here is a table of the 10 most common elements in our Sun: Summary, p. 2/3Here is a table of the 10 most common elements in our Sun:ElementAbundance(% of totalnumber of atoms)mass)Hydrogen91.271.0Helium8.727.1Oxygen0.0780.97Carbon0.0430.40Nitrogen0.00880.096Silicon0.00450.099Magnesium0.00380.076Neon0.00350.058Iron0.0300.014Sulfur0.0150.040
18Charges Must be Balanced Too! Take-home message:A positive charge plus a negative charge = neutral charge.In the high-energy environment of the stars, protons and electrons can combine!
19Isotopes: Nuclear Symbols Take-home message:The # of electrons = # protons in a neutrally charged atom.Chemical reactions involve electrons, and do not depend upon the number of neutrons.So, the chemical properties of isotopes are the same.So, carbon-13 reacts chemically the same way that carbon-14 reacts. But carbon-14 is still heavier.These “isotope symbols” are most often called“Nuclear Symbols”.
22My Notes: Fusion pages 223-225 Fusion reactions fuel stars.The carbon on Earth comes from those fusion reactions in the stars.Centers of stars can be 10 million degrees Celsius. A candle, by comparison, is about 400 degrees Celsius.Four Hydrogen protons fuse to make one helium. Two of the protons from the hydrogen atoms join with 2 electrons and become 2 neutrons. So, ONLY 1 helium atoms are formed and the additional neutrinos are emitted along with a lot of energy!Some mass is converted into energy.For example, 454 g of hydrogen fuses to make only 451 g of helium. The “lost” 3 grams (1%) is converted into energy… like heat and light from the sun.Continued, next slide…
23My Notes: Fusion pages 223-225 The strong force holds protons and neutrons together.8) The electrostatic force is the force between electrons and protons. It is not as powerful as the Strong force. Scientists know it is weaker because the electrostatic force is responsible for chemical reactions, which require less energy than nuclear reactions (influenced by the strong force).
24My Notes: Radioactive Decay pages 225 - 227 Fusion is not the only way nature alters a nucleus.Some nuclei break apart on their own. This is called radioactive decay.An alpha particle is a helium nucleus, with 2 protons and 2 neutrons.Charge is conserved, and mass numbers are the same.Beta and gamma particles are also types of radioactive decay particles.All radioactive decay processes are random… you cannot tell exactly which particle will decay at any given moment. But you can tell the RATE of decay of a group of particles.Continued on next slide…
25My Notes: Radioactive Decay pages 225 - 227 The half-life is unique to each type of particle. It tells us how much radioactive material will be present at a given time. For instance, it 100 g sample of Iodine-131 has a half-life of 8 days, then after 8 day there will be 50 grams left.Half-lives vary from 4.5 billion years for Uranium-238 to just fractions of a second for some elements such as most isotopes of gold(
26My Notes: Fission pages 228 - 229 Fusion involves putting nuclei together.Fission involves taking nuclei apart.Both involve changing the nuclei.In fission, nuclei split into approximately equal parts.Nuclear power plants use fission reactions to produce energy.Sample reaction:Notice that the mass numbers (top boxes) equal the same numbers on the left as on the right side. The mass is conserved.Notice that the atomic numbers also balance in fission reactions.Continued on next slide…
27My Notes: Fission pages 228 - 229 Chain reactions in power plants occur when a reactant collides with another particle, causing a 2nd fission, etc.The minimum amount of material needed to produce an atomic explosion is called the critical mass. For some bombs, it only takes about 45 lbs of material to set off an atomic bomb.Controlled fission does not explode.Water coolants surrounding the material also absorb some of the extra neutrons.Energy from the fission is used to boil water, which turns turbines and produce electricity.Some of the products are radioactive isotopes with very long half-lives; they last a long time. They are difficult to dispose of safely. We currently bury them in containers.
28My Stop & Think Answers: Fission(p. 229) Fission and fusion are alike because:* both involve nuclei changing* both require fast moving particles to collide* both involve the conversion of matter into energy2) Fission and fusion differ in the following ways:* fusion involves combining two or more nuclei into larger, more massive nucleus; fission breaks up a nucleus into 2 smaller nuclei or 1 smaller nucleus & other particles.* fusion powers the Sun; fission does not!3)Continued on next slide……
29My Stop & Think Answers: Fission(p. 229) 4) Fusion reactions in stars are not necessarily chain reactions, as they are in nuclear power plants.In power plants, humans initiate a chain reaction by getting one particle to fuse with another. Then the reactant produces a particle that subsequently initiates the next reaction [kind of like in billiards when one ball hits several other balls, then they all go off hitting even more]. This occurs until the reactant concentrations become very small or zero.In stars, the fusion products might go off and initiate it’s own chain, but there is so much energy in a star that thousands of fusion reactions are starting spontaneously. Many of these reactants are involved in other nuclear processes, not just the “chain” of fusion events.5) 72 days = 3 half lives. So, we start with 100 grams, then “half” it three times: 100 50 25 12.5 grams of Thorium remaining.