🧪AP Chemistry
5 min read•Last Updated on June 18, 2024
Dalia Savy
Anika P
Dalia Savy
Anika P
This unit is all about molecular and ionic compound structure and properties! Now that we've discussed the types of bonds, intramolecular forces, and the structure of ionic solids, it is time to discuss metals and alloys! 🪙
Although ionic and covalent substances are more common, you should be familiar with metallic substances as well and their lattice of cations surrounded by a ‘sea’ of valence electrons. 🌊
When metals ionize, they lose a valence electron and become positive ions or cations. Metallic bonding can therefore be represented as an array of cations surrounded by this "sea" of valence electrons.
What this means is the nucleus and core electrons of the metal stay in place, but the valence electrons are very mobile, allowing them to be thought of as a sea.
Electrons usually belong to a certain atom but in metals, they move so much that they don't belong to one, single atom.
Since the valence electrons are free to move throughout the entire metallic structure, metallic substances have very unique properties:
When comparing properties among the different solids, remember this chart:
Type of Solid | Form of Unit Particles | Forces Between Particles | Properties | Examples |
Molecular🧊 | Atoms or Molecules | LDFs, dipole-dipole, hydrogen bonds | fairly soft, low melting point, bad conductor | Argon, methane, sucrose, dry ice |
Covalent-Network💎 | Atoms connected in a network of covalent bonds | Covalent bonds | Very hard, very high melting point, bad conductor | diamond, quartz |
Ionic🧂 | Positive and negative ions | Electrostatic attractions | Hard and brittle, high melting point, bad conductor | salts (NaCl) |
Metallic✨ | Atoms | Metallic bonds | varying hardness and melting points, good conductor, malleable, ductile | metals! Cu, Fe, Al |
Right now, you should only be very familiar with the two bolded rows. The others are covered in unit three in more depth when we discuss intermolecular forces!
Metals can also bond with other elements and create alloys. Alloys can be formed when two or more elements, where at least one is a metal, are in their liquid form being mixed together. When this mixture cools, the alloy is created. In order to produce a certain alloy, pure metals and elements must be mixed in a specific ratio. Each combination of substances produces an alloy with very unique characteristics.
There are two types of alloys you should be familiar with.
Interstitial alloys form when smaller atoms fill the interstitial spaces between larger atoms. The best example is steel, a substance that you probably haven't realized is an alloy! ⚙️
Steel is made up of iron and carbon atoms. Iron is a common base element for interstitial alloys, and carbon is a common alloying element. These properties reflect their atomic radii. In steel, carbon fits into the interstices of iron.
The properties of a sample of steel all depend on the ratio of carbon to iron. The amount of carbon that one uses to make steel can vary, allowing one to make different samples of steel.
Interstitial alloys, like steel, are known for their strength and hardness. This is because of the small size of the interstitial atoms (like carbon) that give the resulting alloy a high density. Think about it this way: you're stuffing small pieces into an array of atoms. This only makes the substance more dense and stronger!
Substitutional alloys form when an atom of one element substitutes an atom of another element of similar size. This is different from interstitial because there are no additional atoms being added to the array, atoms are rather replaced.
The most well-known example of a substitutional alloy is brass. Copper is a common base element for substitutional alloys, and zinc is a common alloying element. In brass, zinc is substituted for copper. 🎷
Substitutional alloys, like brass, are known for their good electrical and thermal conductivity. These two properties result from the presence of delocalized electrons in the crystal lattice.
The biggest difference is that with an interstitial alloy, you are adding smaller atoms, but with a substitutional alloy, you are replacing atoms with ones that are similar in size.
Alloys are generally harder and stronger than pure metals because the added elements distort the structure and properties. Alloys are also less malleable than pure metals.
The following practice question is based on one posted by the Advanced Placement Youtube channel, and it goes over content reviewed in this guide and the previous guide.
(1) A student ran an experiment to see if the following solids conduct electricity.
Solids | Does it conduct electricity? |
Fe (s) | yes |
FeCl2 (s) | no |
(a) Explain the results the student saw.
(b) Is there anything that could have been different in this experiment to see the FeCl2 sample conduct electricity?
The following are sample responses for part a:
Recall that as long as there are mobile valence electrons, the sample will conduct electricity. There are two ways to ensure mobile valence electrons are present. Either of the following responses is acceptable:
Alloys are mixtures composed primarily of metallic elements combined with at least one other element. They exhibit enhanced properties compared with their constituent elements.
Term 1 of 14
Alloys are mixtures composed primarily of metallic elements combined with at least one other element. They exhibit enhanced properties compared with their constituent elements.
Term 1 of 14
Alloys are mixtures composed primarily of metallic elements combined with at least one other element. They exhibit enhanced properties compared with their constituent elements.
Term 1 of 14
Alloys are mixtures composed primarily of metallic elements combined with at least one other element. They exhibit enhanced properties compared with their constituent elements.
Bronze: An alloy consisting primarily of copper, usually with tin as the main additive.
Steel: An alloy of iron and carbon, often combined with other elements to achieve desired properties.
Solder: A fusible metal alloy used to create a permanent bond between metal workpieces.
Materials that allow electric charges to flow through them easily.
Resistivity: The measure of a material's ability to oppose the flow of electric current.
Insulator: A material that does not allow electric charges to move freely, acting as a roadblock on our 'electricity highway'.
Semiconductor: A material whose conductivity falls between that of conductors and insulators. It's like a smaller road or street in our 'electricity highway' analogy.
The temperature at which a liquid turns into vapor under standard atmospheric conditions.
Evaporation: Process by which molecules in a liquid state (like our water before it boils) spontaneously become gaseous (like the steam).
Condensation: The process where gas turns back into a liquid. It's like when the steam from your boiling pasta water cools down and forms droplets on the kitchen window.
Heat of Vaporization: The amount of energy required to change one gram of a substance from a liquid to a gas at constant temperature and pressure, just like turning our calm pot of water into bubbling chaos!
Malleability is a property of metals that allows them to be hammered, rolled or pressed into thin sheets without breaking.
Hardness: This refers to a material's resistance to deformation, indentation or scratching.
Elasticity: This is the ability of a material to return to its original shape after being stretched or compressed.
Plasticity: This is the ability of a material to permanently deform under stress without breaking.
Ductility is the property that allows a substance (usually metal) to be drawn out into a wire.
Tensile Strength: The maximum amount of tensile (pulling) stress that an object/material can take before failure i.e., before breaking or permanent deformation occurs.
Brittleness: A characteristic of materials that break or shatter instead of bending when subjected to stress; opposite of ductility.
Strain Hardening (Work Hardening): The process by which metal becomes stronger and harder due to plastic deformation caused by heat treatment or through working such as hammering or bending.
The temperature at which a solid turns into a liquid under standard atmospheric conditions.
Freezing Point: The temperature at which a liquid turns into a solid. It's like when your melted ice cream gets put back in the freezer and becomes solid again.
Phase Transition: A change from one state of matter (solid, liquid, gas) to another without changing its chemical composition. Like going from an ice cube (solid water) to water (liquid) or steam (gas).
Heat of Fusion: The amount of energy required to change one gram of substance from the solid phase to the liquid phase at its melting point.
Interstitial alloys are a type of metallic alloy where the atoms of one element fill in the gaps or interstices between the atoms of another.
Interstitial Compound: A compound formed when an atom with a small enough radius sits in an interstice (gap) of another crystal lattice.
Metallic Bonding: The electrostatic attraction between positively charged ions and delocalized electrons in metals.
Crystal Lattice: A 3D arrangement of particles (atoms, ions, or molecules) in metals and many minerals.
Steel is an alloy made primarily from iron and carbon, often along with other elements like manganese or nickel to enhance certain properties like strength or resistance to rust.
Carbon Steel: Steel that contains mainly iron and carbon, with smaller amounts of other elements.
Stainless Steel: A type of steel alloy that contains chromium which makes it resistant to tarnish and rust.
Alloying Element: An element added to a metal to improve its properties.
Carbon atoms are the basic units of carbon, a nonmetallic element that is the basis of all organic compounds. Each carbon atom has six protons and can form four covalent bonds with other atoms.
Hydrocarbon: A compound consisting entirely of hydrogen and carbon atoms.
Organic Chemistry: The study of compounds containing carbon atoms.
Covalent Bond: A type of chemical bond where two atoms share a pair of electrons.
Substitutional alloys are a type of metallic alloy where some of the host metal atoms are replaced by other metal atoms of similar sizes.
Solid Solution Alloying: The process by which metals are mixed together to form an alloy at specific temperatures.
Atomic Radius: The total distance from an atom’s nucleus to its outermost electron shell.
Lattice Structure: The regular geometric arrangement of points in crystal space; these points represent individual particles within the structure.
Brass is an alloy made primarily from copper and zinc. It's known for its bright gold appearance and resistance to corrosion.
Alloy: A substance made by melting two or more elements together, at least one of them a metal.
Copper: A reddish-brown ductile malleable metallic element that is an excellent conductor of heat and electricity.
Zinc: A bluish-white lustrous metal that is brittle at room temperature but malleable with heating. It's used to form a wide variety of alloys including brass.