Chemistry - Atoms - Quick Guides

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Study GuideChemistry–Atoms1. Chemical Compounds1.1Understanding How Atoms CombineAll substances are made fromelements, and the basic unit of every element is theatom. The wordatomoriginally meant “indivisible,” meaning it was once thought atoms could not be broken down anyfurther.Atoms combine inwhole-number ratiosduring chemical reactions. This explains why chemicalcompounds form in specific and predictable ways. For example,one atom of carboncan react withone atom of oxygen, or withtwo atoms of oxygen, but not with a fraction of an atom.In chemical equations:•Reactantsare written on theleftside of the arrow.•Productsare written on therightside of the arrow.1.2What Is a Chemical Compound?Acompoundis a substance made ofatoms of more than one elementthat are chemicallycombined in adefinite ratio. This fixed ratio is what gives each compound its unique properties.The composition of a compound is shown by itschemical formula.For example:

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Study Guide•Carbon dioxide (CO₂)always has1 carbon atomand2 oxygen atoms.•This means the atomic ratio of carbon to oxygen is1:2.1.3Molecules: Atoms Acting as a UnitWhen atoms bond together so tightly that they move and behave as a single unit, they form amolecule.Bothelementsandcompoundscan exist as molecules:•O₂is a molecule made oftwo oxygen atoms.•CO₂is a molecule made ofthree atoms total:o1 carbon atomo2 oxygen atoms1.4Atoms and Molecules in ReactionsDuring the reaction that forms carbon dioxide:•Oxygen exists naturally as anO₂molecule(two oxygen atoms bonded together).•Carbon combines with this oxygen molecule to formCO₂, a compound with a fixed structure.This shows how atoms rearrange during reactions but always followspecific whole-number ratios.

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Study Guide1.5The Oxides of NitrogenNitrogen and oxygen can combine in several different ways to formfive major compounds, calledtheoxides of nitrogen. Each compound has a differentatomic ratio of nitrogen to oxygen, whichgives it different properties.Key points from the table:•Nitrous oxide (N₂O)isrichest in nitrogen.•Dinitrogen pentoxide (N₂O₅)isrichest in oxygen.•Even though these compounds contain the same two elements, theirdifferent ratiosmakethem completely different substances.Summary•Atoms are the basic units of elements.•Compounds form when atoms of different elements combine in fixed ratios.•Molecules are groups of atoms bonded together that act as one unit.•Chemical formulas show the exact number of each type of atom in a compound.•Nitrogen and oxygen form several different compounds depending on their atomic ratios.

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Study Guide2. Quiz: Chemical Compounds1.QuestionThe simplest unit of an element is a(n)Answer Choices•atom•molecule•protonCorrect AnsweratomWhy This Is CorrectAnatomis the smallest unit of an element that still keeps all the chemical properties of that element.•Amoleculeis made of two or more atoms bonded together.•Aprotonis only a part of an atom and does not represent the whole element by itself.For example:A single atom of gold is still gold, but a proton alone is not.2. QuestionAn amount of an element equal to its atomic mass in grams isAnswer Choices•one atom of the element•one mole of the element•one molecule of the elementCorrect Answerone mole of the element

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Study GuideWhy This Is CorrectTheatomic massof an element (found on the periodic table) tells you themass of one moleof thatelementin grams.Amoleis a counting unit that represents6.022 × 10²³ atomsof an element (Avogadro’s number).For example:•Carbon has an atomic mass of12 amu•12 grams of carbon = 1 mole of carbon•This amount contains6.022 × 10²³ carbon atoms•Asingle atomhas an extremely tiny mass.•Amoleculeapplies only to substances made of two or more atoms bonded together.3.QuestionA substance containing atoms of more than one element in a definite ratio is called a(n)Answer Choices•compound•element•mixtureCorrect AnswercompoundWhy This Is CorrectAcompoundis a pure substance made oftwo or more different elements chemically combinedin a fixed (definite) ratio.Because the ratio is constant, a compound always has the same chemical formula and properties.For example:•Water (H₂O) always has2 hydrogen atoms for every 1 oxygen atom

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Study GuideIn contrast:•Anelementcontains only one type of atom•Amixturedoes not have a fixed ratio and its components are not chemically bonded4.QuestionWhen several atoms are so tightly bonded together that they physically behave as a unit, the unit iscalled a(n)Answer Choices•atom•mixture•moleculeCorrect AnswermoleculeWhy This Is CorrectAmoleculeis formed when two or more atoms arechemically bondedtogether and act as a singleunit.In contrast:•Anatomis a single particle of an element.•Amixtureis a physical combination of substances that arenot chemically bonded.For example, a water molecule (H₂O) consists of hydrogen and oxygen atoms bonded together.5. QuestionThe simplest unit of a compound is a(n)

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Study GuideAnswer Choices•atom•proton•moleculeCorrect AnswermoleculeWhy This Is CorrectAcompoundis made when two or more different elements chemically bond together.Thesmallest unitthat still has all the properties of a compound is amolecule.•Anatomis the smallest unit of an element, not a compound.•Aprotonis a subatomic particle and cannot represent a compound.6. QuestionThe composition of a compound is shown in its:Answer Choices•chemical name•chemical formula•colorCorrect Answerchemical formulaWhy This Is CorrectAchemical formulashows thetypes of elementsin a compound and thenumber of atomsof eachelement present. This directly describes the compound’s composition.

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Study GuideExample:•Water → H₂O (2 hydrogen atoms and 1 oxygen atom)The chemical name and color do not clearly or precisely show how many and which atoms make upthe compound.7. QuestionThe chemical formula for magnesium sulfate isMgSO₄. This formula tells us thatAnswer Choices•there is one atom of Mg, one atom of sulfur, and four atoms of oxygen•there are four atoms of Mg, four atoms of sulfur, and four atoms of oxygen•there is one atom of Mg, four atoms of sulfur, and four atoms of oxygenCorrect Answerthere is one atom of Mg, one atom of sulfur, and four atoms of oxygenWhy This Is CorrectIn a chemical formula:•If no subscript is written, it means1 atomof that element.•The subscript4applies only to the elementoxygen (O).So inMgSO₄:•Mg→ 1 magnesium atom•S→ 1 sulfur atom•O₄→ 4 oxygen atomsTherefore, the formula showsone atom of magnesium, one atom of sulfur, and four atoms ofoxygen.

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Study Guide8.QuestionTwo atoms of Na, one atom of S, and four atoms of O will have the chemical formula ofAnswer Choices•NaSO•Na₂(SO₄)₄•Na₂SO₄Correct AnswerNa₂SO₄Why This Is CorrectChemical formulas show theexact number of each type of atomin a compound:•Na₂→ 2 sodium atoms•S→ 1 sulfur atom•O₄→ 4 oxygen atomsSo the correct formula that matches2 Na, 1 S, and 4 OisNa₂SO₄.The other options are incorrect because:•NaSOdoes not show enough sodium or oxygen atoms.•Na₂(SO₄)₄would represent4 sulfate groups, giving too many sulfur and oxygen atoms.9. QuestionWhat is the atomic ratio of the elements inBa₃(PO₄)₂?Answer Choices•(Ba:P:O)3:1:8•(Ba:P:O)1:2:8•(Ba:P:O) 3:2:8

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Study GuideCorrect Answer(Ba:P:O) 3:2:8Why This Is CorrectTo find the atomic ratio, count each type of atom in the formulaBa₃(PO₄)₂:•Ba₃→ 3 barium atoms•(PO₄)₂means the phosphate group is repeated twiceoP: 1 × 2 = 2 phosphorus atomsoO: 4 × 2 = 8 oxygen atomsSo the atomic ratio is:Ba : P : O = 3 : 2 : 83. StoichiometryStoichiometry helps us understand how elements combine in fixed ratios to form compounds. Theseratios tell ushow many atomsof each element are present andhow much masseach elementcontributes to the compound. Let’s walk through this idea step by step using clear examples.Example 1: Nitrous Oxide (N₂O)In nitrous oxide, the chemical formulaN₂Otells us:•There are2 atoms of nitrogen•There is1 atom of oxygen

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