science 2

What is the probability that any location chosen at random under one of the sectors listed in the graph are using renewable energy? US Primary Energy Consumption by Source and Sector, 2016 This graphic shows US primary energy consumption in 2016. Energy sources are listed on the left side of the graph and the sectors that use these energy sources are listed on the right side of the graph. The lines and arrows connecting the two sides of the graphic break down how each source was used by each sector.

10% Explanation: The overall percentage of renewable energy being consumed is 10%, compared to 37% for petroleum, 29% for natural gas, 15% for coal, and 9% for nuclear.

In which year did the total number of lionfish sightings reported at the Flower Garden Banks National Marine Sanctuary (FGBNMS) first exceed 500? Invasive Lionfish The common name "lionfish" refers to two closely related and nearly indistinguishable species that are invasive in US waters. Lionfish, which are native to the South Pacific and Indian Oceans, were first detected along Florida coasts in the mid-1980s, but their populations have swelled dramatically in the past 15 years. Lionfish are popular aquarium fish, so it is plausible that repeated escapes into the wild via aquarium releases are the cause for the invasion. Lionfish now inhabit reefs, wrecks, and other habitat types in the warm marine waters of the greater Atlantic. As lionfish populations grow, they put additional stress on coral reefs already struggling from the effects of climate change, pollution, disease, overfishing, sedimentation, and other stressors. For example, lionfish eat herbivores and herbivores eat algae from coral reefs. Without herbivores, algal growth goes unchecked, which can be detrimental to the health of coral reefs. Adult lionfish are primarily fish-eaters and have very few predators outside of their home range. Because lionfish feed on prey normally consumed by snappers, groupers, and other commercially important native species, their presence could negatively affect the well-being of valuable commercial and recreational fisheries. Lionfish continue to expand at astonishing speeds and are harming native coral reef ecosystems in the Atlantic, Gulf of Mexico, and Caribbean. The first lionfish recorded in the Western Atlantic (east coast of the United States, Caribbean Sea and Gulf of Mexico) was a specimen captured near Dania, Florida in 1985. No other lionfish sightings were reported until 1992. The most likely source of these fish was the home aquarium trade. At first, the spread of the lionfish population was rather gradual, but in 2000 the number of sightings began to increase exponentially. By 2009, lionfish were pretty well established along the Atlantic coast and throughout the Caribbea In 2010, sightings were also recorded in the northwestern Gulf of Mexico, along the coasts of Alabama, Mississippi and Louisiana. In July 2011, the first lionfish was observed in the Flower Garden Banks National Marine Sanctuary (FGBNMS). By the end of 2015, over 2,600 lionfish were observed within the sanctuary. About 1,500 of those were successfully removed and analyzed for important data. Experts say it is unlikely that we will ever be able to completely eliminate lionfish from the Western Atlantic. So, the objective now is to minimize their impact on sanctuary resources. At this time, sanctuary policy is to remove any lionfish encountered. Research has shown that targeted removals in localized areas can be an effective control mechanism. 2013 89 172 265

2013 Explanation: In 2013, a total of 526 lionfish were observed at FGBNMS. 89 were seen at Stetson Bank, 172 at East Flower Garden Bank, and 265 at West Flower Garden Bank. 89+172+265 = 526

Veronica is sick and her doctor is trying to determine the cause of her illness. Her doctor has recommended 9 different tests. He wants to complete 4 of the tests today and 5 of the tests tomorrow. The order of the tests is important, as some tests have to be complete before others. How many ways can the doctor pick the 4 tests to be run today?

3,024 Explanation: A permutation is the number of ways objects or items can be chosen from a group when order is important. In this case, the order of tests is important to the doctor because each test provides him with more information. It is possible that not all 9 tests are not necessary.

According to the professor, how many protons, neutrons, and electrons does the carbon- 14 have? Carbon-14 Dating Dr. Jonnes, an expert archeologist, was giving a presentation about his most recent dig site in Egypt. He showed pictures of mummies, seed samples, and papyrus scrolls and said they were between four and five thousand years old. As Levi listened to the presentation, he wondered how Dr. Jonnes knew for sure how old the samples were. He decided to ask his chemistry professor since they had recently had a lesson about how carbon makes up all living things. Levi and his professor then had the following conversation: Levi: "Professor, how did Dr. Jonnes know that the artifacts were between four and five thousand years old?" Professor: "That is a great question! They use a process called carbon-14 dating." Levi: "Oh - we've learned about carbon in this class! Is it the same thing?" Professor:"It's very similar. The carbon we have talked about is carbon-12. It has an atomic number of six and a mass number of 12. Remember that an element's atomic number tells the number of protons in one atom and the number of protons is equal to the number of electrons. An element's mass number tells the sum of the protons and neutrons. Carbon-14 has the same atomic number, but a different mass number. In carbon-14, the mass number is 14 instead of 12." Levi: "How does carbon-14 dating work?" Professor:"Well, as we learned in our recent lesson, all living things are made of carbon. What we didn't discuss is that there are different types of carbon. Almost all of the carbon that makes up living organisms is carbon-12, but there is also a very tiny amount amount of carbon-14. As long as a plant or animal is alive, it will have a fairly constant amount of carbon-14. This is because even though carbon-14 slowly decays over time, the living organism will also ingest more carbon-14 at about the same rate. Plants absorb carbon-14 during photosynthesis and animals absorb carbon-14 when they eat plants." Levi: "But what happens when plants, animals, and people die?" Professor:"When living organisms die, they stop ingesting more carbon-14. Every 5,730 years, the amount of carbon-14 is cut in half as it slowly decays. Scientists can then measure the amount of carbon-14 left in an organism and use the half-life to approximate when it died." Levi: "Ah, so that's how Dr. Jonnes came up with that date! Thanks, Professor!"

6 protons; 8 neutrons; 6 electrons Explanation: The professor stated, "The carbon we have talked about is carbon-12. It has an atomic number of six and a mass number of 12. Remember that an element's atomic number tells the number of protons in one atom and the number of protons is equal to the number of electrons. An element's mass number tells the sum of the protons and neutrons. carbon-14 has the same atomic number, but a different mass number. In carbon-14, the mass number is 14 instead of 12." The professor says that carbon-14 has the same mass number as carbon-12, which means it still has 6 protons and 6 electrons. It also states that the mass number is equal to the sum of the protons and neutrons and that carbon-14 has a mass number of 14. Use these two pieces of information to determine the number of neutrons: mass number=protons+neutrons 14=6+n 14−6=6+n−6 8=n There are 8 neutrons in carbon-14. Carbon-14 has 6 protons, 8 neutrons, and 6 electrons.

Victoria uses 9 N of force to push a push a heavy crate across the floor. If she performs 63 J of work, how far did she move the crate? The Relationship Between Work, Force, and Distance In physics, work is calculated using the following formula: W=F⋅d in which W is work (measured in joules, J), F is force (measured in newtons, N), and d is the displacement of the object (measured in meters, m). Notice that work can only be done if: (1) a force is applied to an object AND (2) that force causes the object to move.

7 m

Calcium phosphate is an important mineral that strengthens bones and teeth. What is the most likely outcome for an individual who does NOT receive enough calcium in his or her diet? The Human Skeletal System Humans are vertebrates, animals having a vertebral column or backbone. They rely on a sturdy internal frame that is centered on a prominent spine. The human skeletal system consists of bones, cartilage, ligaments and tendons and accounts for about 20 percent of the body weight. The living bones in our bodies use oxygen and give off waste products in metabolism. They contain active tissues that consume nutrients, require a blood supply and change shape or remodel in response to variations in mechanical stress. Bones provide a rigid framework, known as the skeleton, that support and protect the soft organs of the body. The skeleton supports the body against the pull of gravity. The large bones of the lower limbs support the trunk when standing. The skeleton also protects the soft body parts. The fused bones of the cranium surround the brain to make it less vulnerable to injury. Vertebrae surround and protect the spinal cord and bones of the rib cage help protect the heart and lungs of the thorax. Bones work together with muscles as simple mechanical lever systems to produce body movement. Bones contain more calcium than any other organ. The intercellular matrix of bone contains large amounts of calcium salts, the most important being calcium phosphate. When blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for metabolic needs. When blood calcium levels are increased, the excess calcium is stored in the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously. Hematopoiesis, the formation of blood cells, mostly takes place in the red marrow of the bones. In infants, red marrow is found in the bone cavities. With age, it is largely replaced by yellow marrow for fat storage. In adults, red marrow is limited to the spongy bone in the skull, ribs, sternum, clavicles, vertebrae and pelvis. Red marrow functions in the formation of red blood cells, white blood cells and blood platelets.

Bones will weaken as calcium is released from bones into the bloodstream. Explanation: As described in the passage, bones are important for storing minerals, including calcium. When blood calcium levels drop, due to inadequate calcium in the diet, calcium is released from bones into the blood so that the blood can supply the body's tissues with calcium needed for metabolic functions. If this calcium is not replenished, the bone mass will decrease and the bones will gradually become weak and brittle. The fourth paragraph of the passage states, "Bones contain more calcium than any other organ. The intercellular matrix of bone contains large amounts of calcium salts, the most important being calcium phosphate. When blood calcium levels decrease below normal, calcium is released from the bones so that there will be an adequate supply for metabolic needs. When blood calcium levels are increased, the excess calcium is stored in the bone matrix. The dynamic process of releasing and storing calcium goes on almost continuously."

Given this table of melting points, the graph most likely represents the heating curve of which element? The graph shows a science experiment in which a sample of an unknown material is heated at a constant rate. During the experiment, the following observations were made: (1) the sample started as a solid, (2) the sample then melted into a liquid, (3) after a period of time the sample began to boil until, (4) the entire sample disappeared

copper The passage states, "During the experiment, the following observations were made: (1) the sample started as a solid, (2) the sample then melted into a liquid, (3) after a period of time the sample began to boil until, (4) the entire sample disappeared." The entire sample existed as a solid during region E until it started to melt into a liquid in region D. This melting occurs at approximately 1,100°C. Look at the table to find an element that has a melting point around 1,100°C. Copper has the melting point closest to 1,100°C.

Based on the graph, which of the following areas received the most help? US Post-Earthquake Assistance to Haiti

health and disabilities Explanation: The largest section of the graph is marked "health and disabilities," meaning that the greatest portion of aid went toward this area of need.

What event led to the acceptance of AC power over DC power in the battle to set a standard for the United States and the world? The War of the Currents Starting in the late 1880s, Thomas Edison and Nikola Tesla were embroiled in a battle now known as the War of the Currents. Edison developed direct current -- current that runs continually in a single direction, like in a battery or a fuel cell. During the early years of electricity, direct current (shorthanded as DC) was the standard in the US But there was one problem. Direct current is not easily converted to higher or lower voltages. Tesla believed that alternating current (or AC) was the solution to this problem. Alternating current reverses direction a certain number of times per second -- 60 in the US -- and can be converted to different voltages relatively easily using a transformer. Edison, not wanting to lose the royalties he was earning from his direct current patents, began a campaign to discredit alternating current. He spread misinformation saying that alternating current was more dangerous, even going so far as to publicly electrocute stray animals using alternating current to prove his point. The Chicago World's Fair -- also known as the World's Columbian Exposition -- took place in 1893, at the height of the Current War. General Electric bid to electrify the fair using Edison's direct current for $554,000 but lost to George Westinghouse, who said he could power the fair for only $399,000 using Tesla's alternating current. That same year, the Niagara Falls Power Company decided to award Westinghouse -- who had licensed Tesla's polyphase AC induction motor patent -- the contract to generate power from Niagara Falls. Although some doubted that the falls could power all of Buffalo, New York, Tesla was convinced it could power not only Buffalo but also the entire Eastern United States. On Nov. 16, 1896, Buffalo was lit up by the alternating current from Niagara Falls. By this time General Electric had decided to jump on the alternating current train, too. It would appear that alternating current had all but obliterated direct current, but in recent years direct current has seen a bit of a renaissance. Today our electricity is still predominantly powered by alternating current, but computers, LEDs, solar cells and electric vehicles all run on DC power. And methods are now available for converting direct current to higher and lower voltages. Since direct current is more stable, companies are finding ways of using high voltage direct current (HVDC) to transport electricity long distances with less electricity loss. So it appears the War of the Currents may not be over yet. But instead of continuing in a heated AC vs. DC battle, it looks like the two currents will end up working parallel to each other in a sort of hybrid armistice. And none of that would be possible without the genius of both Tesla and Edison.

the Chicago World's Fair of 1893 Explanation: When George Westinghouse won the bid to supply electricity for the Chicago World's Fair in 1893 it set the stage for Tesla's AC power to overtake Edison's DC power. The fact that Westinghouse was able to submit a bid to supply electricity to the World's Fair at a significantly lower price was a good indicator of the advantages of AC power.

What must happen for allopatric speciation to occur? Speciation Speciation is the evolutionary process by which biological populations evolve to become distinct species. The biologist Orator F. Cook coined the term speciation in 1906, but Charles Darwin was the first to describe the role of speciation in his 1859 book The Origin of Species. There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric, parapatric, and sympatric. Allopatric and peripatric are the two most observed modes of speciation. During allopatric speciation, a population splits into two geographically isolated populations. The isolated populations then undergo genotypic or phenotypic divergence as they become subjected to dissimilar selective pressures and as different mutations arise. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes. The Galápagos Islands are particularly famous for their confirmation of allopatric speciation. During Charles Darwin's five weeks on the Galápagos Islands, he noticed that finches differed from one island to another. When he returned to England, his speculation on evolution deepened after experts informed him that these were separate species, not just varieties. Darwin's finches are now one of the most well-known examples of adaptive evolution and allopatric speciation discussed in biology today. In peripatric speciation, a sub-form of allopatric speciation, new species are formed in isolated, smaller peripheral populations that are prevented from exchanging genes with the main population. Since this is a sub-form of allopatric speciation, it is often difficult to determine if peripatric speciation occurs. The biggest indicator of peripatric speciation is that the isolated species will be a much smaller group than the non-isolated population.

A group within a species must move to an isolated area so it can adapt independently from the rest of the species. Explanation: The passage states, "During allopatric speciation, a population splits into two geographically isolated populations." For allopatric speciation to occur, the populations have to be separated so that they become two distinct species. Eventually, they will evolve to the point where they are no longer able to reproduce with the other population.

If a rock contained fossil evidence of both a dinosaur species and a flowering plant species, what would further narrow down the range of time when dating the rock? The Evolution of Life on Earth A very selective history of life on Earth over the past 600 million years is provided in the figure below. The major groups of organisms that we are familiar with evolved between the late Proterozoic and the Cambrian (~600 Ma to ~520 Ma). Plants, which evolved in the oceans as green algae, came onto land during the Ordovician (~450 Ma). Insects, which evolved from marine arthropods, came onto land during the Devonian (400 Ma), and amphibians (i.e., vertebrates) came onto land about 50 million years later. By the late Carboniferous, trees had evolved from earlier plants, and reptiles had evolved from amphibians. By the mid-Triassic, dinosaurs and mammals had evolved from very different branches of the reptiles; birds evolved from dinosaurs during the Jurassic. Flowering plants evolved in the late Jurassic or early Cretaceous. The earliest primates evolved from other mammals in the early Paleogene, and the genus Homo evolved during the late Neogene (~2.8 Ma).

An additional fossil of an organism appearing partway through the Cretaceous is discovered in the rock. Explanation: To obtain a more specific date from the rock, the fossil must be more specific than the Cretaceous period. An insect could be found in any fossil after the Devonian; an organism appearing partway through the Triassic could be any dinosaur, which is not more specific than what we already know. Since primates appear after dinosaurs are extinct, they would not be found in the same rock. An organism appearing partway through the Cretaceous, however, would appear after flowering plants appear and, therefore, indicate a more narrow band of time: partway through the Cretaceous to the Cretaceous-Paleogene boundary (rather than the entirety of the Cretaceous).

Who developed the first list of geologic ages based on radioactive decay? Using Radioactive Decay to Determine Geologic Age The discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, the French physicist, opened new vistas in science. In 1905, the British physicist Lord Rutherford -- after defining the structure of the atom -- made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly; shortly thereafter, in 1907, Professor B. B. Boltwood, radiochemist of Yale University, published a list of geologic ages based on radioactivity. Although Boltwood's ages have since been revised, they did show correctly that the duration of geologic time would be measured in terms of hundreds-to-thousands of millions of years. The next 40 years was a period of expanding research on the nature and behavior of atoms, leading to the development of nuclear fission and fusion as energy sources. A byproduct of this atomic research has been the development and continuing refinement of the various methods and techniques used to measure the age of Earth materials. Precise dating has been accomplished since 1950. A chemical element consists of atoms with a specific number of protons in their nuclei but different atomic weights owing to variations in the number of neutrons. Atoms of the same element with differing atomic weights are called isotopes. Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter). The rate of decay is expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope in a sample to decay. Most radioactive isotopes have rapid rates of decay (that is, short half-lives) and lose their radioactivity within a few days or years. Some isotopes, however, decay slowly, and several of these are used as geologic clocks. The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed in the table:

B.B. Boltwood Explanation: Yale Professor B.B. Boltwood published the first list of geologic ages based on radioactive decay in 1907. "Although Boltwood's ages have since been revised, they did show correctly that the duration of geologic time would be measured in terms of hundreds-to-thousands of millions of years."

Sedimentary rock is formed from a combination of sediments of other rock and subsequent burial and cementation. Why might radioisotopic dating be a poor or limited tool for determining the age of a sedimentary rock? Isotopic Dating Methods When fossils were first discovered, scientists could only predict their relative ages. Although early paleontologists understood biological succession, there was no way for them to determine the absolute ages of fossilized organisms. It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils. Isotopic techniques cannot be directly used to date fossils or the sedimentary rocks in which they are found. However, isotopic techniques can be used to date igneous rocks that cut across sedimentary rocks or volcanic ash layers that lie within sedimentary layers. Isotopes of the same element differ in neutron number and therefore atomic mass. Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time. It is also based on the premise that when the atoms of an element decay within a mineral or a rock, they stay there and don't escape to the surrounding rock, water, or air. One of the isotope pairs widely used in geology is the decay of 40K to 40Ar (potassium-40 to argon-40). 40K is a radioactive isotope of potassium that is present in very small amounts in all minerals that have potassium in them. It has a half-life of 1.3 billion years, meaning that over a period of 1.3 Ga one-half of the 40K atoms in a mineral or rock will decay to 40Ar, and over the next 1.3 giga-annum (Ga) one-half of the remaining atoms will decay, and so on (Figure 8.14).

Different components of a sedimentary rock might have different ages. Explanation: Sedimentary rocks are formed from fragments of other rocks, not through heat and pressure (answer choice B), which refers to metamorphic rock. Additionally, it is untrue that sedimentary rocks never contain potassium deposits (answer choice A); they are formed from igneous and other rocks, so they can contain many different types of deposits. However, this means that (answer choice C) different components of any given sedimentary rock might have different ages, and different proportions of radioactive isotopes, giving disparate age ranges when subjected to isotopic dating.

What is the most likely reason that the marble reached a higher distance above ground in test #4 than in all of the other tests? Marble on a String Experiment A marble is tied to a string and taped to the underside of a table so it can swing freely. The marble is pulled back to Point A, 36 cm above the ground, and then released. On the first swing, the marble passes points B and C. It continues to point D, at which point the marble pauses before starting to swing back towards its starting point. Students measure and record the height of the marble at position D, where it pauses before swinging back. The students repeat the test five times. Their data is shown in the table. 1 32 cm 2 31 cm 3 32 cm 4 35 cm 5 32 cm

The student who released the marble may have pushed the marble toward point D, rather than simply releasing it. Explanation: The fact that the marble reached a higher height during test #4 implies that the marble had more kinetic energy for that test. If the student who released the marble had pushed the marble, rather than simply releasing it, he or she would have increased the kinetic energy of the marble. This would allow the marble to reach a higher distance off the ground than in the other four tests. If the student had released it from a height lower than 36 cm, the marble would have had less potential energy, and would be expected to reach a lower height at point D, rather than a higher height. The time that the marble is released would not effect the marble's potential or kinetic energy.

Choose the phrase that best completes the following sentence: With the ripple tank demonstration, Young showed _____. Thomas Young's Wave Theory of Light Thomas Young was an English physician, and was known as a person of great knowledge on many subjects. Young made notable scientific contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony, and Egyptology. In Young's own judgment, his most important achievement was to establish the wave theory of light. To do so, he had to overcome the century-old view, expressed by Isaac Newton, that light is a particle. In the early-19th century Young put forth a number of reasons supporting the wave theory of light, and he developed two demonstrations to support this viewpoint. With the ripple tank, he demonstrated the idea of interference in the context of water waves. With Young's interference experiment or double-slit experiment, he demonstrated interference in the context of light as a wave. The double-slit experiment was first performed by Thomas Young in 1801. He believed it demonstrated that the wave theory of light was correct, and his experiment is sometimes referred to as Young's experiment or Young's slits. "The experiments I am about to relate ... may be repeated with great ease, whenever the sun shines, and without any other apparatus than is at hand to every one", is how Thomas Young, speaking in 1803 to the Royal Society of London, began his description of the historic experiment. His talk was published in the following year's Philosophical Transactions and was destined to become a classic, still reprinted and read today. In a paper published in 1804, Young described an experiment in which he placed a narrow card (approximately 1/30th inch) in a beam of light from a single opening in a window and observed the fringes of color in the shadow and to the sides of the card. He observed that placing another card before or after the narrow strip so as to prevent light from the beam from striking one of its edges caused the fringes to disappear. This supported the contention that light is composed of waves. He later gave credit to Grimaldi for first observing the fringes in the shadow of an object placed in a beam of light. Within ten years, much of Young's work was reproduced and then extended by Fresnel.

interference in the context of water waves The correct answer is C. Explanation: With the ripple tank demonstration, Young showed interference in the context of water waves. This is described in the passage: "In the early-19th century Young put forth a number of reasons supporting the wave theory of light, and he developed two demonstrations to support this viewpoint. With the ripple tank, he demonstrated the idea of interference in the context of water waves."

Which of the following best describes the relationship between carbon-12 and carbon-14? Carbon-14 Dating Dr. Jonnes, an expert archeologist, was giving a presentation about his most recent dig site in Egypt. He showed pictures of mummies, seed samples, and papyrus scrolls and said they were between four and five thousand years old. As Levi listened to the presentation, he wondered how Dr. Jonnes knew for sure how old the samples were. He decided to ask his chemistry professor since they had recently had a lesson about how carbon makes up all living things. Levi and his professor then had the following conversation: Levi: "Professor, how did Dr. Jonnes know that the artifacts were between four and five thousand years old?" Professor: "That is a great question! They use a process called carbon-14 dating." Levi: "Oh - we've learned about carbon in this class! Is it the same thing?" Professor:"It's very similar. The carbon we have talked about is carbon-12. It has an atomic number of six and a mass number of 12. Remember that an element's atomic number tells the number of protons in one atom and the number of protons is equal to the number of electrons. An element's mass number tells the sum of the protons and neutrons. Carbon-14 has the same atomic number, but a different mass number. In carbon-14, the mass number is 14 instead of 12." Levi: "How does carbon-14 dating work?" Professor:"Well, as we learned in our recent lesson, all living things are made of carbon. What we didn't discuss is that there are different types of carbon. Almost all of the carbon that makes up living organisms is carbon-12, but there is also a very tiny amount amount of carbon-14. As long as a plant or animal is alive, it will have a fairly constant amount of carbon-14. This is because even though carbon-14 slowly decays over time, the living organism will also ingest more carbon-14 at about the same rate. Plants absorb carbon-14 during photosynthesis and animals absorb carbon-14 when they eat plants." Levi: "But what happens when plants, animals, and people die?" Professor:"When living organisms die, they stop ingesting more carbon-14. Every 5,730 years, the amount of carbon-14 is cut in half as it slowly decays. Scientists can then measure the amount of carbon-14 left in an organism and use the half-life to approximate when it died." Levi: "Ah, so that's how Dr. Jonnes came up with that date! Thanks, Professor!"

isotopes - they are different forms of the same element Explanation: If two elements are isotopes they have the same atomic number (have the same number of protons), but different atomic masses (have different numbers of neutrons). This is true with carbon-12 and carbon-14, where both contain 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.