Chemistry - Elements - Quick Guides | Chemistry

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Study GuideChemistry–Elements1. Discovery and Similarity1.1How Chemistry Began: Discovering ElementsModern chemistry began in the18th century, when scientists tried to understand what substancesare really made of. They studied how materials break down when heated or when electricity is passedthrough them.In1774, an English scientist namedJoseph Priestleymade an important discovery. When he heateda red powder calledmercuric oxide, it broke apart into:•aliquid metal(mercury), and•acolorless gasthat helped things burn.This gas was later namedoxygen.Scientists found that many substances can be broken down into simpler parts using heat or electricity.However, some substancescannot be broken down any further, no matter how much energy isused. These simplest substances are calledchemical elements. They are the basic building blocksof all matter.1.2How Many Elements Are There?In1789, the French chemistAntoine Lavoisierpublished one of the first lists of elements. At thattime, only33 elementswere known—and some of them turned out to be incorrect.By1930, careful work by many chemists increased the number of naturally occurring elements to90.Later, scientists working in high-energy laboratories created about20 additional elements. Theseelements areradioactive and unstable, and they do not occur naturally on Earth, although they arelikely formed in the hot centers of some stars.Today, the total number of known elements has reached112, and the list may continue to grow.The good news for students is thatonly about 40 elementsare important for basic chemistry.

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Study Guide1.3What You Need to Know as a StudentTake a look at theperiodic tableat the end of the chapter and findcalcium (element 20).You should become familiar with:•thesymbolsandgeneral propertiesof thefirst 20 elements(up to calcium), and•about20 more elementsthat are especially important in chemistry.Important chemistry ideas are printed inboldthe first time they appear in the text. These are keyconcepts that you will see again and again. Their definitions can be found in theglossaryat the endof the book.1.4Groups of Elements with Similar PropertiesFigure 1Some elements behave in very similar ways. Because of this, they are grouped together on theperiodic table.

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Study GuideInert (Noble) Gases•Helium, neon, argon, krypton, and xenon•These arecolorless gases•Theyrarely reactwith other elements•Because they are so unreactive, they are calledinert gases(also known asnoble gases).Halogens (Salt Formers)•Fluorine, chlorine, bromine, and iodine•Fluorine and chlorine aregreenish, corrosive gases•They react easily with metals to formsalts•This is why they are calledhalogens, which meanssalt formers.Alkali Metals•Lithium, sodium, and potassium•These metals arevery lightand canfloat on water•They areextremely reactiveand mayburst into flameswhen they touch water•They form strong alkalis, so they are calledalkali metals.1.5Where Similar Elements Are Found in NatureElements with similar properties are often found in thesame natural environments.For example:•Halogensare common inseawater•The main salt in ocean water issodium chloride•Bromineis obtained from salt beds formed after seawater evaporates•Iodineis collected fromkelp, a type of seaweed that grows in oceans

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Study Guide2. Quiz: Discovery and Similarity1. QuestionThe modern periodic table is arrangedAnswer Choices•by increasing atomic mass•by increasing atomic number•alphabeticallyCorrect Answerby increasing atomic numberWhy This Is CorrectThe modern periodic table is organized according toincreasing atomic number, which meanselements are arranged based on the number of protons in their atoms.This arrangement explains repeating (periodic) patterns in the chemical properties of elements moreaccurately than atomic mass.For example:Hydrogen (atomic number 1) comes before Helium (atomic number 2) because it has one fewerproton.2. QuestionThe group of most reactive metals that form strong alkaline solutions with water:Answer Choices•inert gases•halogens•alkali metals

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Study GuideCorrect Answeralkali metalsWhy This Is CorrectAlkali metals are extremely reactive metals found in Group 1 of the periodic table. They reactvigorously with water to formalkaline (basic) solutionsand release hydrogen gas.For example:•Sodiumreacts with water to form sodium hydroxide (a strong alkali) and hydrogen gas.In contrast:•Inert gasesare very unreactive.•Halogensare reactive nonmetals, not metals, and do not form alkaline solutions with water.3.QuestionThe group of most reactive nonmetals that readily combine with metals to form salts:Answer Choices•inert gases•halogens•alkali metalsCorrect AnswerhalogensWhy This Is CorrectHalogens are the most reactive nonmetals on the periodic table. They have seven valence electronsand readily gain one electron to form stable negative ions. Because of this, they easily react withmetals to form salts.For example:•Sodium (a metal) + Chlorine (a halogen) → Sodium chloride (table salt)

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Study Guide•Why the Other Options Are Incorrect•Inert gases: Very unreactive due to full valence shells•Alkali metals: Highly reactive, but they are metals, not nonmetals•Therefore, halogens is the correct answer.4.QuestionThe group of nonreactive elements in which only two of the elements form compounds:Answer Choices•inert gases•halogens•alkali metalsCorrect Answerinert gasesWhy This Is CorrectInert gases (also called noble gases) aremostly nonreactivebecause they havefull valenceelectron shells, making them very stable.Most inert gases donotform compounds at all. Onlytwo heavier inert gases—xenon andkrypton—are known to form a small number of compounds, under special conditions.For example:•Xenon can form compounds like xenon difluoride (XeF₂)•Most others (helium, neon, argon) do not reactWhy the Other Options Are Incorrect•Halogens: Highly reactive nonmetals•Alkali metals: Very reactive metals•Therefore,inert gasesis the correct answer.

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Study Guide5. QuestionThe symbol of an element often contains:Answer Choices•the first two letters of the element name•an arbitrary selection of letters•the first letter of the element nameCorrect Answerthe first letter of the element nameWhy This Is CorrectAn element’s symbol usually begins with thefirst letter of its name. If a second letter is used, it istypically another letter from the element’s name (not an arbitrary choice) and is written in lowercase.For example:•Hfor Hydrogen•Cfor Carbon•Cafor Calcium (C is the first letter, a is another letter from the name)So, the most common and consistent feature is that the symbol containsthe first letter of theelement’s name.6. QuestionThe halogens are often called:Answer Choices•salt formers•lanthanides•noble gases

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Study GuideCorrect Answersalt formersWhy This Is CorrectHalogens readily react with metals to formsalts, which is why they are known assalt formers. Thenamehalogenitself comes from Greek words meaning “salt-producing.”For example:•Sodium + Chlorine → Sodium chloride (table salt)In contrast:•Lanthanidesare a series of rare earth metals.•Noble gasesare very unreactive elements.7. QuestionThe symbol for chlorine is:Answer Choices•C•Ch•ClCorrect AnswerClWhy This Is CorrectChemical symbols useone or two letters.•Thefirst letter is always capitalized.•If there is a second letter, it islowercase.

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Study GuideFor chlorine:•Clis correct•Crepresents carbon•Chis not a valid chemical symbolSo, the correct symbol for chlorine isCl.8. QuestionThe inert elements are also known as:Answer Choices•metals•noble gases•actinidesCorrect Answernoble gasesWhy This Is CorrectInert elements are callednoble gasesbecause they are very unreactive. Their outer electron shellsare completely filled, which makes them chemically stable.Examples include:•Helium•Neon•ArgonIn contrast:•Metalsinclude many reactive elements.•Actinidesare radioactive elements found at the bottom of the periodic table.

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Study Guide9. QuestionElements with atomic number 93 and greater are all:Answer Choices•naturally occurring•man-made•known as noble gasesCorrect Answerman-madeWhy This Is CorrectElements with atomic numbers93 and higherare calledtransuranium elements. They do not occurnaturally in significant amounts and arecreated artificiallyin laboratories or nuclear reactors throughnuclear reactions.In contrast:•Naturally occurringelements are mostly found with atomic numbers 92 or lower.•Noble gasesare nonreactive elements in Group 18 and are not defined by high atomicnumbers.3. Atomic Masses3.1Understanding Why Atomic Mass MattersIn the early 1800s, chemists were trying to make sense of the growing number of chemical elements.One important thing they noticed was thatelements combine with each other in fixed mass ratioswhen they form compounds. These mass relationships were not random—they followed clearpatterns.

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