In 1963 Richard Feynman, a Nobel Prize winning physicist, said that if only one sentence could survive a world-wide cataclysm, the sentence that would be most valuable for restarting science is: “All things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.”
The size of atoms is about 10 -10 meters in diameter. This corresponds to about
100,000,000 atoms across the width of your smallest fingernail.
The 3 components of an atom are protons and neutrons (which combine to form the atom’s nucleus ), and electrons (which form the atom’s shells ).
Protons, neutrons and electrons are subatomic particles. Protons and neutrons are nucleons , which are themselves made up of other subatomic particles called quarks .
Quarks combine in 3’s to form nucleons. Quarks are never seen alone. All subatomic particles have an antimatter counterpart. If these two every meet they annihilate one another in a gamma ray burst (E = mc 2 )!
Protons, neutrons and electrons belong in a class of subatomic particles called fermions , which are particles that can not share the same quantum mechanical state . This is why electrons form distinct orbital shells around an atomic nucleus —they can’t pile up on one another —which is ultimately responsible for the chemical properties of an element.
Generally, a particle’s quantum mechanical state is “the probability of where it is and where it is moving.” The alternative class of particles is called bosons , which include photons and gravitons ; and these can share the same quantum mechanical state.
The number of protons in an atomic nucleus ( ie.
the atomic number ) determines what type of element it is. The neutrons in the nucleus help stabilize the nucleus. The nucleus is held together and its behavior is governed by the strong and weak nuclear forces.
Nuclei of the same element can have different numbers of neutrons (forming different isotopes of the same element) —but some combinations are less stable and tend to undergo radioactive decay . Large nuclei, like those in uranium, tend to be unstable and also undergo radioactive decay.
The sum total of protons and neutrons in an atom is its mass number . The total mass of the atom, including its electrons, is its atomic mass . Because elements have different isotopes, the average atomic mass —accounting for the different fractions of mass numbers —is generally a non-integer number of amu ’s (ie. atomic mass units. 1 amu =
1.66 * 10 -27 kg ; 1 Jupiter = 1.9 * 10 27 kg ).
Radioactive decay means a nucleus breaks up (ie. undergoes fission ) or otherwise transforms ( eg.
a neutron changes to a proton), releasing subatomic particles and energy. Small nuclei can also combine to form more stable configurations; these fusions also release energy. Elements except hydrogen and helium are the products of stellar fusion (helium can also be formed by fusion.). All elements above iron have nuclei too large to be fusion products, these are formed by supernova explosions or high energy collisions.

An electrically-neutral atom has the same number electrons forming its shells as there are protons in the nucleus. Multiple electrons can fit in each atomic shell, but within a given shell, each electron has its own unique quantum mechanical state. The number of electrons that can occupy the 1 st atomic shell is 2, eight electrons can occupy the 2 nd shell, and 18 electrons can occupy the 3 rd shell. The number of electrons in each shell, and the shapes of their orbits (that is, their probability distributions ) are determined by the nature of quantum mechanics.
As elements increase in atomic number, electrons do not fill up atomic shells in sequential order. Quantum mechanically, atoms are “temporarily satisfied” with 2 electrons in its valence ( ie. outermost) shell. Further electrons will tend to fill in gaps in lower shells until they are filled. Then additional electrons will fill out the valence shell, up to the number 8, which is very satisfactory for the atom in a quantum mechanical point of view. Elements with 8 valence electrons form the class of noble gases , which are chemically inert. The quantum mechanical drive for atoms to fill out their valence shells with 8 possessed, “stolen” or “borrowed” electrons, is the primary determiner of how atoms combine to form compounds and molecules .
The Periodic Table is an arrangement of the elements. Each row represents the number of electron shells the element has. Each column represents the number of valence electrons (as well as the arrangement of inner shell electrons), so that elements of similar chemical properties are grouped in columns.
Common salt is an example of an ionic compound (Chlorine “steals” one of sodium’s electrons. Chlorine becomes a negative ion , sodium becomes a positive ion , and these two attract to form the compound.)
Metals like copper and iron freely share valence electrons, forming an “electron sea” which allows electrons ( ie. electricity ) to flow easily through compound. On the other hand, molecules that hold tight to their valence electrons tend to be electrical insulators.
Most molecules form covalent bonds , their constituent atoms share valence electrons.
But the sharing is usually not equal. Some atoms will tend to grab electrons
—to fill out their valence shells to 8, while other types of atoms will lend electrons —to empty out their valence shells. The result is that electrons will congregate at one end of the molecule more than at the other end, creating a dipole — an electric field across the molecule. Dipoles have strong intermolecular attractions. Water is a dipole molecule, whose interactions with itself and other molecules form a key foundation for life.
There are four states of matter: solid, liquid, gas & plasma. Solids have a fixed volume and shape. Its molecules vibrated in fixed positions, held in place by intermolecular attraction. Liquids have a fixed volume, but no fixed shape. Its molecules slide around one another, but stay close.
Gases have no fixed volume or shape. Its molecules bounce off one another, only slightly affected by intermolecular forces. At room temperature, typical gas molecules travel at 1000mph.
Complex molecules can have multi-pole electrical properties, which can make them bend in intricate shapes. The multi-pole interactions of complex organic molecules, like proteins and
DNA, govern the machinery of life. Cell membranes are formed by phospholipids , which have

dipole properties ( hydrophilic ) at one end, while the other end does not ( hydrophobic ); these bunch together to form a surface separating water-borne molecules inside of a cell from waterborne molecules outside the cell.
Astronomers have noted there is not enough observable matter in the universe to account for the gravitational attraction within galaxies. There must be a mysterious dark matter which gravitationally interacts with regular matter. Also, astronomers observe that the universe’s expansion is accelerating—there must be a mysterious antigravity force
( dark energy ) acting on inter-galactic scales.
The latest estimate by NASA of the composition of the universe: 0.03% heavy elements,
0.3% neutrinos (a subatomic byproduct of solar fusion processes), 0.5% stars, 4% free
H
2
and He, 30% dark matter and 65% dark energy. 95% OF THE UNIVERSE IS A
MYSTERY!