WUHS Chemistry: The Atom & Its Parts Unit

Week 1 – How can substances be composed of the same elements but be different? What makes up an atom?

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The Atom & Its Parts Unit – Week 1

• Driving Question: How can substances be composed of the same elements but be different? What makes up an atom?
• Why does CO kill but CO₂ is rarely harmful?
• Why do compounds always have the same percentages of each element?
• What are atoms made from?
• How big are atoms?

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Week 1 – How can substances be composed of the same elements but be different? What makes up an atom?

The Atom & Its Parts Unit

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What Chemists Do

• Chemists investigate the properties of substances and predict how these properties change during chemical reactions.
• For example, if chemists know that a molecule contains carbon and oxygen atoms, they can make some predictions about how that molecule behaves.
• Molecular properties depend not only on the types of atoms but also the exact quantities of each element.
• For example, carbon dioxide (CO₂) is generally harmless, but carbon monoxide (CO) can quickly kill a person.
• While both carbon dioxide and carbon monoxide contain only two elements – carbon and oxygen - each type of molecule has very different properties.

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Determining Elemental Composition

• All substances have a predictable and consistent �elemental composition.
• For example, carbon dioxide always contains one �carbon atom and two oxygen atoms.
• Regular water molecules always contain one �oxygen atom and two hydrogen atoms. �
• The Law of Definite Proportions indicates that a �compound always has the same ratio of types of atoms.
• The number of atoms & the weight of each element in a molecule will always be the same for any pure sample of a compound.
• For example, pure water always contains 11.19% hydrogen and 88.81% oxygen by mass regardless of the source of that water.

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CO₂ always contains one carbon and two oxygen atoms regardless of its source. Similarly, water always contains one oxygen and two hydrogen atoms.

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Carbon Monoxide vs. Carbon Dioxide

• The Law of Multiple Proportions provides further evidence for the existence of atoms.
• The Law of Multiple Proportions states that when multiple elements combine to form different compounds they combine in simple ratios (e.g. AB, AB₂, AB₃, etc.).
• For example, carbon and oxygen can form both carbon monoxide (CO) and carbon dioxide (CO₂).
• The mass of oxygen in CO₂ is double that of CO.
• The mass of oxygen in these molecules can only increase in proportions equal to the weight of one oxygen atom.
• This provides further evidence that every molecule is comprised of individual atoms with specific predictable weights.

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CO and CO₂ follow the Law of Cons. Of Mass. The mass of oxygen in CO₂ is double the mass of oxygen in CO.

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Law of Conservation of Mass

• The existence of atoms is also supported by the Law of Conservation of Mass.
• This states that when atoms combine to form molecules, the total mass of atoms stays the same.
• I.e., atoms cannot be created or destroyed. �
• The Law of Conservation of Mass is observable when a log burns on a fire.
• The mass of the atoms in the wood and oxygen molecules is equivalent to the mass of the molecules in the carbon dioxide, water vapor, ash, and smoke that is left after the combustion reaction ends.
• Atoms cannot be created or destroyed in chemical reactions; they can only be moved from one molecule to another.

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During combustion, the atoms in the wood are moved to new molecules like CO₂ and H₂O. The mass of what is produced equals the mass of what is burned.

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Why does this matter?

• These laws provide strong evidence for the existence of atoms.
• While atoms are far too small for us to observe visually, we can use reasoning and evidence to demonstrate that they exist.
• Law of Definite Proportions: we know that all compounds have predictable properties regardless of their source.
• For example, water from your faucet is made from the same elements and in the same proportions as water anywhere else in the universe.
• Law of Multiple Proportions: when atoms combine to form different compounds they can only be added to molecules in whole numbers and even ratios.
• In other words, you cannot create H1.5O or CO2.75. This tells us that atoms exist as distinct particles that have a consistent size.
• Law of Conservation of Mass: atoms cannot appear or disappear.
• If a certain amount of atoms go into a reaction, that exact same amount must come back out as part of new molecules. �

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While we can’t directly observe atoms, we know they exist in part because of the laws discussed here.

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Matter & Atoms - Reminders

• All matter is made of atoms.
• Atoms are small indivisible particles of matter.
• Remember, matter is any gas, liquid, or solid that has mass and takes up space.
• Atoms bond to form molecules.
• For example, water is a molecule comprised of one oxygen and two hydrogen atoms.
• Atoms are one of the most important scientific discoveries in human history.
• For example, our understanding of concepts such as DNA, distant stars, and lithium-ion batteries is only possible because of our understanding of atoms.

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Atoms are the basic unit of matter (above). ��Atoms can bond to each other to form molecules (e.g., a water molecule is made of one oxygen and two hydrogen atoms).

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Basics of Atoms

• Atoms are the most basic units of matter.
• Atoms are smallest particles that can take part in chemical reactions. �
• While atoms are made of tinier particles, these particles are unable to individually take part in chemical reactions.
• This is similar to how a student is the smallest unit of learning - while students are made of different organs (brain, heart, lungs, etc.), only a whole student is capable of learning.

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An atoms & molecules are analogous to students & classes. Both atoms and students are made of smaller parts that exist in one complete unit.

Atom

Molecule

Student

Class

Organs

Subatomic Particles

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Size of Atoms

• Atoms are VERY tiny!
• If you lined up a million atoms side by side, they would be roughly the width of one millimeter (the tiniest line on most meter sticks).
• If an atom was the size of a millimeter, the thickness of a sheet of paper would be roughly equivalent to the height of the Empire State Building.
• Atoms are smaller than a wavelength of light.
• This means that even with the most powerful light microscope, it is impossible to directly observe atoms (although scientists have developed indirect methods of observing atoms).

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What’s Inside Atoms

• Atoms are made of three ingredients:
• Protons: positively charged particles found in the center (or nucleus) of the atom.
• Neutrons: uncharged particles found �alongside protons in the nucleus.
• Electrons: negatively charged particles �that orbit outside the nucleus. �
• The number of protons in an atom is �equal to the number of electrons.
• E.g., if an atom has 3 protons, it will �also have 3 electrons. �

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What’s Inside Atoms

• The number of protons also determines elements.
• For example, an atom with only one proton is hydrogen.
• An atom with two protons is helium.
• The symbols for each element on a periodic table indicate how many protons and how many neutrons are found in each element.
• The mass number shows how many protons �and neutrons are in each element.
• The atomic number shows how many �protons are in each element.
• For example, helium has �2 neutrons & 2 protons.
• Its mass number is 4.
• Its atomic number is 2.�

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What are Isotopes?

• Different isotopes of the same elements have different masses.
• Isotopes are different versions of the same element that have different numbers of neutrons and different masses. �
• For example, carbon atoms must have 6 protons, but can have 6, 7, or 8 neutrons.
• While carbon-14 has a greater mass than carbon-12, all are still the same element.

There are three isotopes of carbon, each with a different number of neutrons, but all have the same number of protons.

Carbon-12

Carbon-13

Carbon-14

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Revising Our Claims

• How could you improve your responses to these questions?
• Driving Question: How can substances be composed of the same elements but be different? What makes up an atom?
• Why does CO kill is CO₂ is rarely harmful?
• Why do compounds always have the same percentages of each element?
• What are atoms made from?
• How big are atoms?

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Note: we may not have all the info we need yet – that’s ok! Collaborate to come up with your best ideas for now.

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Looking Ahead: �Part 3 Investigation

• In Part 3A you will use Oreo cookies to demonstrate your understanding of the Law of Definite Proportions, the Law of Multiple Proportions, and the Law of Conservation of Mass.
• In Part 3B you will use an online PhET simulation to build atoms.

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Key Points

• Law of Definite Proportions: a compound will always have the same ratio of types of atoms.
• E.g., pure carbon dioxide always has one carbon atom and two oxygen atoms.
• E.g., pure water always has one oxygen atom and two hydrogen atoms.
• Law of Multiple Proportions: When different elements combine to form different compounds they always combine in simple whole ratios (AB, AB₁, AB₂, etc.).
• E.g., the amount of oxygen in CO₂ is exactly double that of CO.
• Law of Conservation of Mass: When atoms are rearranged to form new molecules, the mass of the atoms in the new molecules is identical to the mass of atoms in the original molecules.
• E.g., the mass of atoms in ashes, CO₂, and H₂O is equal to the mass of the atoms in the wood before it was burned.
• Each of these laws provides evidence for the existence of atoms.
• This enables chemists to accurately reason about substances and predict how they will change when they interact with each other.

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Key Points

• Atoms are very tiny – they are smaller than a wavelength of light.
• A million atoms lined up side by side would be roughly one millimeter.�
• Atoms consist of three main ingredients.
• Protons: positively charged particles found in the center (or nucleus) of the atom.
• Neutrons: uncharged particles found alongside protons in the nucleus.
• Electrons: negatively charged particles that orbit outside the nucleus.�
• The number of protons determine elements.
• Each element has a unique number of protons.

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Key Vocab

• Protons: positively charged particles found in the center (or nucleus) of the atom.
• Neutrons: uncharged particles found alongside protons in the nucleus.
• Electrons: negatively charged particles that orbit outside the nucleus.
• Nucleus: the center of the atom; consists of neutrons and protons.
• Isotopes: different versions of the same element.�

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