Chapter 11: The Atomic Nature of Matter

Make-Up of the Universe Studies of distant galaxies first made in the 1920s showed that the Universe was expanding, and that it has been expanding for the past 13.7 billion years. More recently, in the 1990s, a game-changing fact about the expansion rate of the Universe was discovered; the expansion rate of the Universe is increasing with time. The expansion of the Universe is speeding up with time! This remarkable result was the first suggestion that the dominant component of the Universe was some mysterious material known as Dark Energy (which makes up ~ 72 % of the Universe). This means that matter makes up only 28 % of the Universe. Furthermore, based on other studies, we also know that only 15 % of the matter is normal matter (4.6 % of the Universe). The rest of the matter known as Dark Matter (23 % of the Universe), is thought to be exotic material. Remarkably, we do not know what the Dark Matter or Dark Energy are, and so we know the make-up of less than 5 % of the Universe! We will spend the next two weeks talking about this 5 % as it is the stuff relevant to our daily lives and routines. Also, our Universe does not appear to contain large amounts of anti-matter. Every matter particle is thought to have an anti-matter twin where its anti-matter twin is similar to the normal matter counterpart except for its electrical properties. Recently at CERN, researchers were able to form and trap anti-matter for more than 15 minutes (a big deal) and there was a suggestion that perhaps anti-matter had negative mass (not likely).

Atoms In the fifth century B.C., Democritus and Leucippus proposed that if one took a rock and cut it in half, and then cut one of the halves in half, and then cut the half of the halve in half, and so on. Eventually, one would reach the point where you could no longer cut the piece in half and you would have arrived at the smallest piece of matter. Democritus referred to this unit as an Atom (in the upper left panel is shown an Argon39 atom in a trap). This view was not accepted at the time; Aristotelian physics (a world of earth, water, air, and fire) was dominant. The idea was revived in the 1800s (Dalton), the discovery of Brownian motion (Brown (1827) shown in bottom left panel) demonstrates the existence of atoms. Atoms are invisible in the pictures (of pollen and smoke particles) but that they continually jostle the visible particles (a theory first worked out by Einstein 1905). Atoms are tiny with sizes on the order of 0.0000000001 meter (=10-8 centimeters) and are everywhere, the air in this room contains more than 1018-19 atoms in every cubic centimeter (roughly cubes the size of a sugar cube). In a cubic centimeter of water (about 1 gram), there are about 1023 atoms.

Atomic Structure Atoms are composed of three types of fundamental particles, protons, neutrons, and electrons (however, note that the nucleans [protons and neutrons] are actually composed of three quarks). The nucleus of an atom is composed of the positively charged protons and the neutral neutrons. The negatively charged electrons are contained in structures which surround the nucleus. The positively charged protons attract and hold onto the electrons (bind the atom together); atoms are held together by the electrical force. The size of the atom is determined by the size of the electron cloud. The nucleus of the atom is tiny, its diameter is only around 0.00001 times the size of the atom. This is made further interesting by the fact that neutrons and protons are around 2,000 times the mass of an electron. Usually an atom contains equal amounts of protons and electrons (but not always) so that most of the mass of an atom is in its nucleus and since the size of the atom is given by the size of the electron cloud, we infer that atoms are, in a sense, mostly empty space. This huge difference in the masses of protons and electrons does not carry-over to the electrical charges they carry. Protons and electrons carry opposite electrical chagres (and so they attract), but the charges are of the same size.

Chemical Elements Different chemical elements are different types of atoms. The type of element is determined by: The number of protons contained in the atom's nucleus The type of element is determined by only the number of protons, it is independent of the number of neutrons and electrons. The number of neutrons determines the isotope of an element and the number of electrons determines the type of ion with which we deal. The atomic number of an atom, Z = # of protons, determines the chemical element;the mass of the atom, A = Z + N (number of neutrons), determines the atomic weight of the nucleus. The placement of elements on the Periodic Table tells us about their chemical properties, e.g., the elements are the far right of the table, He, Ne, Ar, Kr, Xe, and Rn, are not chemically active and are referred to as Noble Gases (inert gases). Understanding of the arrangement of the Periodic Table comes from the electron shells (clouds).

Compounds, Molecules, and Mixtures Different chemical elements can be combined in different ways. Compounds: Different elements are chemically bonded (ionic of covalent bonds) Molecules: A group of elements (perhaps the same) where the elements are bonded by the sharing of an electron (a covalent bond). Mixtures: Groups of elements or molecules that are grouped but not bonded chemically. There can be overlap between compounds and molecules.