AP Chemistry Unit 2 Study Notes

AP Chemistry 2.4: Structure of metals and alloys

Explain metallic bonding and how alloys alter metal properties.

Aligned to Compound Structure and Properties from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

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Structure of Metals and Alloys Overview 4.1 Structure of a Metal 4.2 Metallic Bonding 4.3 Properties Explained by Metallic Bonding 4.4 What Is an Alloy? 4.5 Why Make Alloys? 4.6 Two Important Alloy Types 4.7 Substitutional Alloy 4.8 Example of Substitutional Alloy 4.9 Interstitial Alloy 4.10 Size Requirement 4.11 Recognizing Them From Diagrams

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These detailed Unit 2 notes were organized from the provided study document. For further study, visit Khan Academy. All Khan Academy content is available for free at www.khanacademy.org.

Structure of Metals and Alloys Overview Open

  • Khan Academy's current lesson specifically covers particulate models of metals and substitutional and interstitial alloys .

4.1 Structure of a Metal Open

  • Metallic solids contain many metal atoms packed together.

  • The valence electrons are delocalized.

    • Instead of one electron pair belonging to one pair of atoms, the electrons can move through a much larger region.

4.2 Metallic Bonding Open

  • A simplified model describes metals as:

    • positive metal cores in a sea of mobile valence electrons

  • The attraction between:

    • positive metal cores

    • and:

    • delocalized electrons

    • holds the metal together.

4.3 Properties Explained by Metallic Bonding Open

  • Metallic bonding helps explain:

  • Electrical conductivity

    • mobile electrons carry charge.

  • Thermal conductivity

    • energy can move efficiently through the structure.

  • Malleability

    • metal atoms can shift without immediately destroying bonding.

  • Ductility

    • metals can often be drawn into wires.

4.4 What Is an Alloy? Open

  • An alloy is a mixture containing:

    • two or more elements

    • where:

    • at least one is a metal.

  • Alloys usually retain metallic characteristics.

  • Examples include mixtures involving metals such as:

    • iron

    • copper

    • nickel

    • zinc

4.5 Why Make Alloys? Open

  • Pure metals do not always have the desired physical properties.

  • Mixing different atoms can change:

    • hardness

    • strength

    • corrosion resistance

    • flexibility

    • conductivity

4.6 Two Important Alloy Types Open

  • Know these:

  • Substitutional alloys

  • Interstitial alloys

4.7 Substitutional Alloy Open

  • In a substitutional alloy , atoms of one element replace some of the metal atoms in the lattice.

    • This generally works when the atoms have:

    • similar atomic radii

    • Imagine:

  • A A A A A B A A A A A B B A A A

  • B atoms have substituted for A atoms.

4.8 Example of Substitutional Alloy Open

  • Copper and nickel have fairly similar atomic sizes.

    • They can form substitutional alloys.

  • Khan Academy uses copper-nickel alloys as a particulate-model example.

4.9 Interstitial Alloy Open

  • In an interstitial alloy , small atoms fit into spaces between larger metal atoms.

    • Imagine:

  • A A A b b A A A

  • The small b atoms occupy:

    • interstitial spaces

    • rather than replacing the main metal atoms.

4.10 Size Requirement Open

  • Interstitial alloy:

    • atoms have significantly different sizes .

  • The smaller atoms fit into spaces between larger atoms.

  • Substitutional alloy:

    • atoms have more similar sizes .

4.11 Recognizing Them From Diagrams Open

  • On AP questions:

  • Substitutional

  • Look for different atoms occupying normal lattice positions.

  • Interstitial

  • Look for tiny atoms located in the gaps between larger metal atoms.