Atoms, Elements, and Molecules

Atoms, Elements, and Molecules

Middle School

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Learning Objectives

• Define the key terms atom, element, molecule, and compound.

• Differentiate between an element and a compound based on their composition.

• Use the periodic table to determine the composition of an atom.

• Identify substances as elements, molecules, or compounds based on their chemical makeup.

Key Vocabulary

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Atom

The smallest unit of matter, the atom is the basic building block of a chemical element.

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Element

An element is a pure substance made of only one type of atom that cannot be broken down.

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Molecule

A molecule is a group of two or more atoms that are chemically bonded together.

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Compound

A compound is a substance made of two or more different elements chemically bonded together.

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The Atom: Building Block of Matter

• An atom is the smallest unit of an element that has its chemical properties.

• Its nucleus has positive protons and neutral neutrons, circled by negative electrons.

• An element’s atomic number is equal to its total number of protons.

• Mass number is protons plus neutrons; you can find neutrons by subtracting numbers.

Multiple Choice

An atom has an atomic number of 17 and a mass number of 35. How many neutrons does this atom contain?

• 18

• 35

• 17

• 52

Elements, Molecules, and Compounds

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Element

• An element is a pure substance made of only one type of atom.

• It cannot be broken down into simpler substances by chemical means.

• Examples are Gold (Au), Iron (Fe), and Oxygen gas (O₂).

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Molecule

• A molecule is formed when two or more atoms are chemically bonded.

• It is the smallest unit of a pure substance.

• Examples include Hydrogen gas (H₂) and water (H₂O).

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Compound

• A compound is formed from two or more different elements chemically bonded.

• Water (H₂O) is a compound of hydrogen and oxygen atoms.

• Examples are salt (NaCl) and glucose (C₆H₁₂O₆).

Multiple Choice

Based on its formula, H₂O (water), how would you classify this substance?

• As a mixture

• As a compound

• As an element

• As an atom

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Common Misconceptions

Misconception	Correction
All molecules are compounds.	Molecules like O2 have one element. Compounds must have different elements.
Atoms are the smallest particles that exist.	Atoms are made of smaller particles: protons, neutrons, and electrons.
Water (H2O) is an element.	Water is a compound of two elements: hydrogen and oxygen.

Multiple Choice

How do atoms relate to elements?

• Atoms and elements are completely unrelated.

• Atoms are larger than elements.

• An atom is a mixture of different elements.

• An element is a pure substance made of only one type of atom.

Multiple Choice

A substance is composed of a single type of atom, but the atoms are bonded together in pairs (like O₂). How would you classify this substance?

• As a compound only.

• As an atom only.

• As both an element and a molecule.

• As a mixture of atoms.

Multiple Choice

Methane (CH₄) is a substance made of one carbon atom and four hydrogen atoms. Which statement accurately analyzes its classification?

• It is a mixture of two different elements, carbon and hydrogen.

• It is a single atom called Methane.

• It is a molecule of a compound because atoms of different elements are chemically bonded.

• It is an element because it is a pure substance.

Multiple Choice

If you discover a new substance that cannot be broken down into simpler substances by chemical means, what have you most likely found, and why?

• A compound, because it is a complex substance.

• A molecule, because all substances are made of molecules.

• An element, because its defining property is that it cannot be chemically decomposed.

• A mixture, because it might contain different things.

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Summary

• Atoms are the basic units of matter, made of protons, neutrons, and electrons.

• An element is a pure substance made of only one type of atom.

• Molecules are bonded atoms; compounds are molecules made from different elements.

• An element's atomic number is the number of protons in its atoms.

Poll

On a scale of 1-4, how confident are you in distinguishing between atoms, elements, molecules, and compounds?

• 1

• 2

• 3

• 4