Matter: Structure, Energy and Forces  External Links page.

Subatomic Particles The smallest known constituents: Protons, neutrons and similar particles are made of quarks; electrons, neutrinos and similar lighter particles may be elementary; photons (which make up light and radiation) and other particles transmit forces. What keeps those particles together/apart? So far, it boils down to four forces: Gravitational, electromagnetic, and the strong and weak nuclear forces (transmitted by different particles). Antimatter? For each kind of particle there is an anti-particle, with the same mass but the opposite sign for all of its charges.

Atoms and Molecules Atoms: The smallest units of each chemical element of matter are atoms, with protons and neutrons in the nucleus and electrons around it; Isotopes and ions are variants of atoms. A single grain of sand can contain 10 million billion atoms... How many different atoms? See the periodic table; 92 elements occurring in nature, up to Uranium, plus other unstable ones created in the lab. Molecules: The smallest pieces of each substance are molecules, made of several atoms bound together by electric forces. Internal Energy for Atoms/Molecules Mass: Changes in nuclear reactions, and different atoms (nuclei) are produced. Kinetic energy: In the motion, rotations, and vibrations of atoms and molecules. Gravitational potential energy: Plays a role only if the atom/molecule moves over a big distance. Electric potential energy: In an atom, depends on how far the electrons are from the nucleus. The big surprise: Each kind of atom or molecule can only be in certain specific states! What happens when electrons change state? Radiation is either emitted (electron loses energy) or absorbed (electron gains energy).

Phases of Matter Solid: Molecules move slowly (lower temperature) and are bound together; disordered (glass), ordered (crystals, like ice), or quasicrystals. Liquid: Molecules can move but are still close (water, oil). Gas: Molecules are free of bonds, move quickly, around 500 m/s for air at room temperature. Plasma: Energy is so high that atoms are ionized. Changes: Evaporation, melting... Phase transitions absorb or give off heat (think of heat packs!). The temperature at which they happen depends on pressure [Mars is cold, yet water on its surface would evaporate right away!]. [Is that all? There is a fifth state of matter, Bose-Einstein condensates; and... Is sand a solid? (Almost every classification is to some extent an oversimplification.)]

Relevant Forces in Astronomy Gravity: It rules the motion of all celestial objects; When it is not too strong, Newton's law of gravity describes it well. Electric force: Keeps atoms together, rules how they combine to form molecules. Magnetic force: Affects the motion of charged particles, such as the solar wind and cosmic rays. Nuclear forces: Keeps nuclei together, rules the reactions that take place inside the cores of stars.