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Chemistry - Atoms

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    Study Guide Chemistry – Atoms 1. Chemical Compounds 1.1 Understanding How Atoms Combine All substances are made from elements , and the basic unit of every element is the atom . The word atom originally meant “indivisible,” meaning it was once thought atoms could not be broken down any further. Atoms combine in whole - number ratios during chemical reactions. This explains why chemical compounds form in specific and predictable ways. For example, one atom of carbon can react with one atom of oxygen , or with two atoms of oxygen , but not with a fraction of an atom. In chemical equations: • Reactants are written on the left side of the arrow. • Products are written on the right side of the arrow. 1.2 What Is a Chemical Compound? A compound is a substance made of atoms of more than one element that are chemically combined in a definite ratio . This fixed ratio is what gives each compound its unique properties. The composition of a compound is shown by its chemical formula . For example:

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    Study Guide • Carbon dioxide (CO ₂ ) always has 1 carbon atom and 2 oxygen atoms . • This means the atomic ratio of carbon to oxygen is 1:2 . 1.3 Molecules: Atoms Acting as a Unit When atoms bond together so tightly that they move and behave as a single unit, they form a molecule . Both elements and compounds can exist as molecules: • O ₂ is a molecule made of two oxygen atoms . • CO ₂ is a molecule made of three atoms total : o 1 carbon atom o 2 oxygen atoms 1.4 Atoms and Molecules in Reactions During the reaction that forms carbon dioxide: • Oxygen exists naturally as an O ₂ molecule (two oxygen atoms bonded together). • Carbon combines with this oxygen molecule to form CO ₂ , a compound with a fixed structure. This shows how atoms rearrange during reactions but always follow specific whole - number ratios .

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    Study Guide 1.5 The Oxides of Nitrogen Nitrogen and oxygen can combine in several different ways to form five major compounds , called the oxides of nitrogen . Each compound has a different atomic ratio of nitrogen to oxygen , which gives it different properties. Key points from the table: • Nitrous oxide (N ₂ O) is richest in nitrogen . • Dinitrogen pentoxide (N ₂ O ₅ ) is richest in oxygen . • Even though these compounds contain the same two elements, their different ratios make them 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 Guide 2. Quiz: Chemical Compounds 1. Question The simplest unit of an element is a(n) Answer Choices • atom • molecule • proton Correct Answer atom Why This Is Correct An atom is the smallest unit of an element that still keeps all the chemical properties of that element. • A molecule is made of two or more atoms bonded together. • A proton is 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 . Question An amount of an element equal to its atomic mass in grams is Answer Choices • one atom of the element • one mole of the element • one molecule of the element Correct Answer one mole of the element

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    Study Guide Why This Is Correct The atomic mass of an element (found on the periodic table) tells you the mass of one mole of that element in grams . A mole is a counting unit that represents 6.022 × 10²³ atoms of an element (Avogadro’s number). For example: • Carbon has an atomic mass of 12 amu • 12 grams of carbon = 1 mole of carbon • This amount contains 6.022 × 10²³ carbon atoms • A single atom has an extremely tiny mass. • A molecule applies only to substances made of two or more atoms bonded together. 3. Question A substance containing atoms of more than one element in a definite ratio is called a(n) Answer Choices • compound • element • mixture Correct Answer compound Why This Is Correct A compound is a pure substance made of two or more different elements chemically combined in a fixed (definite) ratio . Because the ratio is constant, a compound always has the same chemical formula and properties. For example: • Water (H ₂ O) always has 2 hydrogen atoms for every 1 oxygen atom

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    Study Guide In contrast: • An element contains only one type of atom • A mixture does not have a fixed ratio and its components are not chemically bonded 4. Question When several atoms are so tightly bonded together that they physically behave as a unit, the unit is called a(n) Answer Choices • atom • mixture • molecule Correct Answer molecule Why This Is Correct A molecule is formed when two or more atoms are chemically bonded together and act as a single unit. In contrast: • An atom is a single particle of an element. • A mixture is a physical combination of substances that are not chemically bonded . For example, a water molecule (H ₂ O) consists of hydrogen and oxygen atoms bonded together. 5 . Question The simplest unit of a compound is a(n)

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    Study Guide Answer Choices • atom • proton • molecule Correct Answer molecule Why This Is Correct A compound is made when two or more different elements chemically bond together. The smallest unit that still has all the properties of a compound is a molecule . • An atom is the smallest unit of an element, not a compound. • A proton is a subatomic particle and cannot represent a compound. 6 . Question The composition of a compound is shown in its: Answer Choices • chemical name • chemical formula • color Correct Answer chemical formula Why This Is Correct A chemical formula shows the types of elements in a compound and the number of atoms of each element present. This directly describes the compound’s composition.

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    Study Guide Example: • Water → H ₂ O (2 hydrogen atoms and 1 oxygen atom) The chemical name and color do not clearly or precisely show how many and which atoms make up the compound. 7 . Question The chemical formula for magnesium sulfate is MgSO ₄ . This formula tells us that Answer 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 oxygen Correct Answer there is one atom of Mg, one atom of sulfur, and four atoms of oxygen Why This Is Correct In a chemical formula: • If no subscript is written, it means 1 atom of that element. • The subscript 4 applies only to the element oxygen (O) . So in MgSO ₄ : • Mg → 1 magnesium atom • S → 1 sulfur atom • O ₄ → 4 oxygen atoms Therefore, the formula shows one atom of magnesium, one atom of sulfur, and four atoms of oxygen .

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    Study Guide 8. Question Two atoms of Na, one atom of S, and four atoms of O will have the chemical formula of Answer Choices • NaSO • Na ₂ (SO ₄ ) ₄ • Na ₂ SO ₄ Correct Answer Na ₂ SO ₄ Why This Is Correct Chemical formulas show the exact number of each type of atom in a compound: • Na ₂ → 2 sodium atoms • S → 1 sulfur atom • O ₄ → 4 oxygen atoms So the correct formula that matches 2 Na, 1 S, and 4 O is Na ₂ SO ₄ . The other options are incorrect because: • NaSO does not show enough sodium or oxygen atoms. • Na ₂ (SO ₄ ) ₄ would represent 4 sulfate groups , giving too many sulfur and oxygen atoms. 9 . Question What is the atomic ratio of the elements in Ba ₃ (PO ₄ ) 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 Guide Correct Answer (Ba:P:O) 3:2:8 Why This Is Correct To find the atomic ratio, count each type of atom in the formula Ba ₃ (PO ₄ ) ₂ : • Ba ₃ → 3 barium atoms • (PO ₄ ) ₂ means the phosphate group is repeated twice o P: 1 × 2 = 2 phosphorus atoms o O: 4 × 2 = 8 oxygen atoms So the atomic ratio is: Ba : P : O = 3 : 2 : 8 3. Stoichiometry Stoichiometry helps us understand how elements combine in fixed ratios to form compounds. These ratios tell us how many atoms of each element are present and how much mass each element contributes to the compound. Let’s walk through this idea step by step using clear examples. Example 1: Nitrous Oxide (N ₂ O) In nitrous oxide, the chemical formula N ₂ O tells us: • There are 2 atoms of nitrogen • There is 1 atom of oxygen

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