Elements, Molecules, and Compounds

Elements, Molecules, and Compounds

Middle School

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Learning Objectives

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

• Describe the basic structure of an atom and explain how an ion is formed.

• Differentiate between an element and a compound.

• Explain the difference between a molecule and a compound.

• Distinguish between pure substances and mixtures.

Key Vocabulary

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Atom

The smallest unit of matter, serving as the fundamental building block of all chemical elements.

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Element

A pure substance made of only one type of atom that cannot be chemically broken down.

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Molecule

A group of two or more atoms chemically bonded together, forming a distinct and identifiable unit.

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Compound

A substance made of two or more different elements chemically bonded together in a fixed ratio.

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Ion

An atom that has gained or lost electrons, resulting in a net positive or negative charge.

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The Building Block: The Atom

• The atom is the basic unit of all matter and has a central nucleus.

• The nucleus contains positively charged protons and neutrons with no charge.

• Tiny, negatively charged electrons orbit the nucleus.

• If an atom gains or loses electrons, it becomes a charged particle called an ion.

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Multiple Choice

What is an atom called if it gains or loses electrons, resulting in an electrical charge?

• A proton

• An ion

• A nucleus

• A neutral atom

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Elements: The Simplest Substances

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

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

• All atoms of an element have the same unique number of protons.

• Examples are Oxygen (O), Iron (Fe), Copper (Cu), and Gold (Au).

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Multiple Choice

What is the defining characteristic of an element?

• It is a pure substance consisting of only one type of atom.

• It is made of two or more different types of atoms.

• It has an unequal number of protons and electrons.

• It can be broken down into simpler substances by chemical means.

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Building Up: Molecules and Compounds

Molecules

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

• Some elements can exist as molecules made of two or more identical atoms.

• For example, an oxygen molecule (O₂) has two oxygen atoms bonded together.

Compounds

• A compound is a substance created from two or more different bonded elements.

• The elements in a compound are always present in a fixed, consistent ratio.

• For example, a water molecule (H₂O) is a compound of hydrogen and oxygen.

Multiple Choice

Which of the following best describes a compound?

• Two or more atoms of the same element bonded together.

• The smallest functional unit of all matter.

• A substance formed when two or more different elements are chemically bonded.

• A pure substance that cannot be broken down by chemical means.

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Molecules, Compounds, and Pure Substances

• All compounds are molecules, but not all molecules are compounds.

• A molecule can have one element type, like oxygen (O₂).

• A compound must have atoms from more than one type of element.

• Elements and compounds are pure substances as their atoms are chemically bonded.

Multiple Choice

Why is O₂ (Oxygen gas) considered a molecule but NOT a compound?

• Because it is a gas at room temperature.

• Because its atoms are not chemically bonded.

• Because it is made of only one type of element.

• Because it is not a pure substance.

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Common Misconceptions

Misconception	Correction
Molecules and compounds are the same.	Molecules like O
Atoms are the smallest particles.	Atoms are made of smaller subatomic particles: protons, neutrons, and electrons.
Compounds are just mixtures of elements.	Compound elements are chemically bonded; mixtures are only physically combined.

Multiple Choice

How does the structure of an atom relate to its identity as a specific element?

• The number of electrons in orbit determines the element.

• The number of protons in the nucleus determines the element.

• The total mass of the atom determines the element.

• The number of neutrons in the nucleus determines the element.

Multiple Choice

Explain why water (H₂O) is classified as both a molecule and a compound.

• It's a molecule because it's a liquid, and a compound because it contains oxygen.

• It's a molecule because it has multiple atoms bonded together, and a compound because those atoms are of different elements (H and O).

• It's a molecule because it can be broken down, and a compound because it's made of atoms.

• It's a molecule because it is a pure substance, and a compound because it has a chemical formula.

Multiple Choice

A newly discovered substance is analyzed and found to contain only one type of atom, but the atoms are always bonded together in pairs. How should this substance be classified?

• As a compound only, because atoms are bonded together.

• As both an element and a molecule, because it consists of one type of atom that is chemically bonded.

• As a mixture, because there are multiple atoms present.

• As an ion, because the atoms exist in pairs.

Multiple Choice

Which of the following is a pure substance?

• Air (a mixture of gases)

• Water (H₂O)

• Salt dissolved in water

• Soil

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Summary

• Atoms are the fundamental building blocks of all matter.

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

• A molecule forms when two or more atoms chemically bond.

• A compound is a molecule with two or more different elements.

• Not all molecules are compounds, such as O₂.

• Mixtures are physically combined, not chemically bonded.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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• 4