# 16: Matter


We’ve concentrated primarily on electrons throughout this course. Indeed, in our everyday life, it is the interactions of electrons that, together with photons (light), drive most of what we do. In this final chapter, we’ll peer down inside the atom to see what the most fundamental particles are, and then extend our view out to bulk states comprised of large numbers of electrons.

• 16.1: The Standard Model of Particle Physics
Our best current understanding of Physics at the most basic level is that it is composed of a number of fundamental particles. These particles are, as best we can tell, points, much like electrons (which are in fact one of the fundamental particles). They have various properties associated with them, including mass, spin (angular momentum), electric charge, and others. It is from these fundamental particles that all of the matter we interact with is built. (However, matter built from these parti
• 16.2: Nuclei and Atoms
Quarks bind together to make protons and neutrons. A proton is composed of two up quarks and a down quark, and a neutron is composed of two down quarks and an up quark. Together, protons and neutrons are called nucleons. Nucleons can themselves bind together to make nuclei. These nuclei are always positively charged, with the total charge depending on the total number of protons. Nuclei are so called because they sit at the nucleus of atoms; an atom is a nucleus that has gathered negative electr
• 16.3: Molecules
Atoms can bond together. Sometimes, if one atom is able to completely steal an electron from another atom (as is the case with Chlorine and Sodium atoms, where a Sodium atom will donate an electron to a Chlorine atom), the resulting ions will then stick together as a result of the electrostatic attraction between their opposite net charge.
• 16.4: Solids
Roughly speaking, a solid is when a large collection of molecules are held together and fixed in place. They aren’t completely still, unless a solid is at absolute zero temperature. (And that’s not possible, as a result of Heisenberg’s Uncertainty Principle.) Most solids are at a higher temperature; the solids you interact with every day are approximately at room temperature, about 20oC or 290 K. At such a temperature, the molecules are vibrating about, each with about 1/40 eV of kinetic energy
• 16.5: Liquids, Gasses, and Plasmas
If you put enough energy into a solid, eventually it will melt. At this point, the atoms and molecules in the solid are no longer held together in the crystal, lattice, or other structure. Instead, they have enough energy to break whatever bonds (covalent, ionic, or otherwise) holding them together, and now they can flow past each other. The atoms and molecules are still largely packed together as closely as they can go, and there still are bonds of a sort holding the broadly together, but no lo
• 16.6: Planets, Stars, Galaxies, and Clusters
Once you get past the sizes of everyday solids, liquids and gasses, you enter the realm of astronomical objects. In our Solar System, such objects range from lowly asteroids, through dwarf planets such as Pluto or Ceres, through rocky planets such as the Earth or Mars, on up through the gas giants such as Saturn or Jupiter. However, the vast majority of the mass of our Solar System is in the Sun, the star about which everything else orbits. The Sun is a ball of gas, 300,000 times the mass of the
• 16.7: Dark Matter and Dark Energy

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