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Hwa Chong InstitutionSecondary 2 Lower Secondary Science

Science Term 2 ACE AssignmentOnline Lesson Periodic Table

Name: Keven Loo Yuquan (14) Class: 2I1 Date: 14 January2011

1. Scientists have created all the elements beyond element 92. Find out howthey have been made.

Currently, scientists have been able to artificially create all theelements from atomic number 93 to atomic number 118 (the latest, anisotope of element 118, ununoctium, was created in 2006 by firing theprotons of48Ca into the nucleus of249Cf) in the laboratory. However,neptunium (atomic number 93) and plutonium (atomic number 94), can alsobe found in trace amounts in uranium-rich rock.

Elements over atomic number 93 mostly have isotopes, which differ inthe number of neutrons, but still have the same number of protons. Thus,they can be differentiated by their characteristics, for example, the kinds ofradiation they release (alpha, beta, the element isotope that they decay into),their half-lifes, their stability, et cetera. Often however, due to theirradioactivity and cost, they are rarely used for commercial purposes.

It is commonly known that the atomic number of an element is definedby the number of protons that are present in its nucleus. Therefore, in orderto create elements of a higher atomic number, they would have to figure outa way to increase the number of protons present in the nucleus. However,they had already earlier realized that the more protons there were in thenucleus, the more radioactive the element would be, and the shorter the half-

life of the element would be (the half-life refers to the amount of time it takesfor an element undergoing decay to decrease by half), making studying theelement difficult, as they could easily decay into other elements within ashort period of time.

In the early 1930s, Ernest Lawrence at the University of California atBerkeley invented the cyclotron, which could speed up protons or ions(charged atoms) of specific elements to high energies and fire them at theatoms of uranium (or any other chosen element) like machine-gun bullets ata target. In the resulting nuclear reaction, some of the protons from the bulletnuclei would stick to the target nuclei, transforming them into nuclei ofhigher atomic numbers. Shooting light atoms at heavy atoms has turned outto be one of the main methods for producing even heavier atoms far beyond

uranium.Now however, the cyclotron is no longer commonly used and particleaccelerators and nuclear reactors replaces its job. The particle accelerator isa device that makes use of electromagnetic fields in order to propel chargedparticles to high speeds and to contain them in well-defined beams.

2. It is said that the stars are the element factories of the universe, that is,stars make the elements. Do some research and find out how the stars makeelements.

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The method in which stars make virtually all the natural elements isthrough a process called nucleosynthesis. However, to understand how thisworks out, we would have to first understand a bit of history of elements inthe universe.

After the Big Bang, when the first stars began to form and the galaxies

took shape, the only three elements in the universe were hydrogen, heliumand some lithium. However, through a process of nuclear fusion, the starsconsumed hydrogen and slowly created heavier elements, and as moregeneration of stars passed, the elements created became heavier andheavier. This process of making the heavy elements and then ejecting theminto the Universe takes place over the lifespan of a star, and it could rangefrom 2 billion to 13 billion years. The heavier elements we see in the worldtoday were all ejected from stars that had reached the end of their lifespanand exploded into supernovas before settling into old age as a white dwarf, aneutron star or a black hole.

For nuclei to fuse, even of those of hydrogen, it requires a hugeamount of energy. The reason for this is because all nuclei have a positive

charge due to their protons, and since like charges repel, they strongly resistbeing put too close together.

The way in which stars create heavier elements is through the nuclearreactions that happen in them. These nuclear temperatures release largeamounts of thermal energy, which accelerates the atoms to high speeds.

Thus, they can overcome this electromagnetic repulsion and get closeenough for the nuclear force to be sufficient to result in fusion. Additionally,the fusion of lighter elements in stars releases energy, production ofelements heavier than iron however, absorbs energy, as it requires moreenergy to force the nuclei together. For nuclei to react, conditions areestimated to be a minimum of 10 million degrees Celsius.

Also, in our universe, these temperatures existed in the very early

stages of the Universe (just after the Big Bang, which released a hugeamount of energy that was changed to thermal energy, eventually it is stillthe same method in which stars create elements).

3. Choose an element and research the following information about it: Whenit was discovered, by whom it was discovered, how it is found in nature, itsproperties and uses.

The element that I have chosen to research and compile informationabout is chlorine, the element with an atomic number of 17.

Chlorine was discovered in 1810 by Sir Humphry Davy, who repeated afailed experiment earlier to determine if it was an element, and eventuallysucceeded in proving that it is an element and not a compound. He named itfrom the Greek word (chlros), meaning green-yellow.

In nature, Chlorine is rather abundant and is found primarily as thechloride ion, a component of the salt that is deposited in the earth ordissolved in the oceans, and approximately 1.9% of the mass of seawaterconsists of chloride ions. Higher concentrations of chloride ions can be foundin the Dead Sea and in underground brine deposits. Most chloride salts are

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soluble in water, thus, chloride-containing minerals are usually only found inabundance in dry climates or deep underground.

The properties of chlorine are that it is a pale yellow-green gas thathas a melting point of -101.5 C and a boiling point of -34.04 C. It is ahalogen, denser than air (and thus tends to accumulate at the bottom ofpoorly ventilated spaces) and it is also highly reactive. It is a toxic gas that

causes irritation to the respiratory system, and at 1000 ppm (parts permillion) inhaling it is fatal to humans. However, in its chloride ion form, whichmake up salts, it is necessary to most forms of life, including humans.Probably, its most important property that is utilized for commercialpurposes is that it is a powerful oxidant.

Chlorine's principal applications are in the production of a wide rangeof industrial and consumer products. For example, it is used in makingplastics, solvents for dry cleaning and metal degreasing, textiles,agrochemicals and pharmaceuticals, insecticides, dyestuffs, householdcleaning products, etc. Chlorine is also used in the manufacture of numerousorganic chlorine compounds, and also in the production of chlorates and inbromine extraction.

As it is a powerful oxidant, it is also used in the making of disinfectantsand in bleach, and also to purify water. Chlorine itself in water is moreeffective than an equivalent concentration of bromine or iodine. Chlorine isusually used (in the form of hypochlorous acid) to kill bacteria and othermicrobes in drinking water supplies and public swimming pools. However,since it has so much potential, it has also been abused by humans. Forexample, in both World War I and in the Iraq War, chlorine gas was used as achemical weapon, and from the descriptions of the soldiers who wereaffected by it, chlorine gas had a distinctive smell of a mixture betweenpepper and pineapple, and tasted metallic and stung the back of the throatand chest. On certain occasions, it can also be lethal.