Forces in Materials Revision

Study Notes

An atom is electrically neutral if the number of electrons in its orbitals equals the number of protons in the nucleus.
When an atom has a net charge, it is known as an ion and interacts with other charged particles via the Coulomb force.
Ion interactions form the basis of ionic bonds, which can form lattice patterns of alternately charged particles.

In a covalent bond, at least one electron from the two atoms forming the bond interacts with both nuclei.
Covalent bonds form molecules, with a defined bond length between the two nuclei.
The attraction of bonding electrons to each nucleus pulls the nuclei closer together, while the nuclei also experience repulsion due to their positive charges.
The potential energy curve of a covalent bond describes the force between the two atoms, with a steep slope indicating attraction and a negative slope indicating repulsion.

Chiral molecules have the same chemical formula but a different physical arrangement, with a mirror image of one another.

Metallic bonds are similar to covalent bonds, but valence electrons interact with multiple nuclei.
In a bulk metal sample, valence electrons can move relatively freely among all nuclei.
Metallic bonds do not form molecules and do not have a specific elemental ratio or formula.

The strength of interatomic or intermolecular interactions in a material defines its state (solid, liquid, or gas).
Atoms or molecules in a solid state are constantly experiencing close interactions with one another.
In a liquid state, atoms or molecules are still in constant close interaction, but with enough energy to move past one another.
In a gas state, particles have little to no interaction and are spaced far apart.

When a solid is subjected to an external force, its shape changes as its individual atoms or molecules react to the force and their bonds with one another.
Stress is an expression of an object's internal intermolecular forces per unit of cross-sectional area.
Strain is the fractional change in an object's length in each spatial dimension when a stressed object deforms.

Bulk modulus is the measure of a substance's resistance to compressive environments and applies to both solids and fluids.
Young's modulus is useful for understanding a material's response to an external force applied in a single direction, but not applicable to an object subjected to a three-dimensional compressive environment.

Metallic bonds do not require a specific elemental ratio or formula.
An alloy is a mixture of two or more metallic elements, with atoms of different sizes forming interstitial or substitutional alloys.
The alternating crystal pattern in alloys makes it more difficult for bond dislocations to occur, making the material stronger, but also less conductive and less ductile than its constituent elements.

Ceramics are a broadly useful category of materials due to their:
- Resistance to deformation under stress
- Ability to withstand high temperatures
- Low susceptibility to corrosion
- Good thermal and electrical insulation

Carbon atoms can bond with each other to form carbon-based chains, with each carbon atom bonding with two other carbons.
Carbon is not the only element that forms chains, with boron, silicon, and sulfur also having this ability.

Polymers are long chains formed from monomers, with entire side chains branching off from the main chain.
Polymers can be natural (e.g. proteins, carbohydrates, nucleic acids) or synthetic (e.g. nylon, neoprene).
Inorganic polymers can have very different properties from organic polymers.

Elastomers are certain polymer materials that display elasticity at ambient temperatures and have a wide range of uses.
Natural rubber is made from rubber tree extracts and becomes an elastic solid through vulcanization.
Vulcanization is a process of heating with sulfur that loosely bonds the long-chain molecules together with cross-links.

The complex shapes and varied reactivity patterns of large polymers contribute to the reactions and behaviors of each molecule.
The Lock and Key model helps explain how hormones can stir into action certain cells in the body while leaving other cells alone.
Agonists are key-like chemicals that activate receptor molecules, while antagonists are blocking chemicals that can fit receptors without activating them.