Organisms are made up of molecules, which are collections of atoms bound to one another. The most important molecule to life is water.

Biology is the study of life and a lot of life involves chemistry.

All matter is composed of a rather limited number of basic substances that we refer to as elements. A. Lavoisier is credited as having made the first reasonable list of elements in the 1780s.

There are 110 identified elements. 92 of them are naturally occurring; the others are man-made, many of them at Berkeley, California. These naturally occuring elements combine in various ways to make up everything in the universe. Four of these elements: carbon, hydrogen, oxygen and nitrogen, make up over 90 percent of your body weight.

You should become familiar with the names and symbols of perhaps 12 elements. These are the most common elements found in both plants and animals.

You may be interested in a complete Periodic Table of the Elements. This hyperlink provides a great deal of information about each of the elements that might be useful to the average citizen.

An atom is the smallest unit of matter unique to a particular element and an atom has a substructure. Many sources credit John Dalton with formulating the atomic theory of the structure of matter, where matter is any material substance that occupies space and has weight. Dalton thought of matter as being composed of tiny solid spheres, or atoms.

Actually the Greek philosophers spoke of atoms much earlier. Leucippus and his pupil Democritus, in the fifth century BC were fascinated by the divisibleness of matter. A coherent stream of water can be broken into drops; the apparently solid beach can be seen and felt ultimately as grains of sand. Later (first century BC) Lucretius wrote of standing on a high hill overlooking a plain filled with a vast array of soldiers and horses which at that distance seemed to be one glittering body in the sun.

These observations led them by sheer intuition to the idea that there must be some limit to the number of subdivisions that can be performed on any bit of matter. Thus the idea of the ultimate indivisible and indestructible unit of matter, the atom, was conceived.

The word atom comes from the Greek negative prefix, a, coupled with the Greek word, tom, to cut. Their speculations alone led them to a view remarkably consistent with our modern sophistication about atoms. Their atoms moved; "all is a flux" was a far-reaching dictum. The Greeks did nothing, however, with questions of size, mass, shape, etc. This had to wait for more sophisticated techniques and tools.

Ernest Rutherford and his colleagues, in the early 1900s, were interested in the properties of solid materials, particularly metals and began conducting experiments to reveal their nature. They used a beam of alpha particles from a radioactive source. Alpha particles are positively charged particles produced by the radioactive decay of certain elements. They aligned their beam to strike a thin metal foil and then measured the deflection, if any, of the particles.

This was essentially like using positively charged bullets. Rutherford could measure the number of particles and their kinetic energy (energy of motion). When a beam hit a piece of gold foil only 0.0005 inches thick, it went right on through almost undiminished. What a puzzle. How could this be?

Obviously there was matter in the foil but it must be mostly vacant space much as a chicken wire fence made of wire thread must look.

One of Rutherford's students, Ernst Marsden, discovered and then accurately measured the small number of alpha "bullets" that were deflected by the gold foil.

By 1912, Rutherford fit his observations into what he called the nuclear model of atomic structure. His calculations showed that the overall diameter of the atom was about 10^-8 cm which he computed based on the assumption that the gold atoms in the foil were touching each other much as piled up tennis balls.

However, since most of the alpha beam particles went through the pile of gold atoms without encountering opposition, it suggested that the mass of the atom was concentrated in a nucleus.

The results of the experiments could be accounted for by calculations which gave the nucleus a diameter of only 10^-12 cm, about 1/10,000 that of the atom.

The scattering of particles from the beam could be accounted for only by assuming that the nucleus contained a large positive charge which tended to repel the positive charges of the alpha particles.

When more data was accumulated with other metal foils, approximately the same nucleus to atom ratio of dimensions showed up but the positive charge on the nucleus was characteristically different for each element investigated.

Soon, the magnitude of this charge, expressed as multiples of the unit charge came to be identified as an atomic number. The magnitude of this unit charge depends on the number of protons present.

The electric neutrality of the atom, according to Rutherford, was established by the atom's having a number of negatively charged subatomic particles, electrons, outside the nucleus and sufficient to balance the positive charge of the nucleus.

The space occupied by these electrons was enormous, since the whole atom has 10,000 times the diameter of the nucleus and hence 10¹² times the volume.

Rutherford did not know that the nucleus also contained another kind of subatomic particle that had no charge--neutrons. Someone else discovered them about 20 years later.

Protons and neutrons both have about the same mass (weight). A proton masses 1.6 x 10^-24 grams. These particles are small and don't weigh very much!

Electrons are even smaller than protons or neutrons (about 1/1,800 the mass of a proton).

When you add the mass of protons and neutrons, you get the atomic weight of an element.

This also lets us deal with isotopes. Isotopes of an element have identical atomic numbers but different atomic weights because they have different numbers of neutrons. And by the way, not all isotopes are radioactive!

Rutherford gave us a model for an atom that had a nucleus (a concentration of mass and positive charge) which was surrounded by a cloud of electrons. While this theory accounted for a lot of atomic properties, it failed to answer the most fundamental of all questions-- how can such an aggregate of positive and negative charges be stable?

Another physicist, James Maxwell, predicted that if electrons oscillated in the atom, since they were charged particles, they would by this very motion, radiate energy. And, because this energy cannot simply be created, the emission of radiant energy would have to correspond to a loss of kinetic energy (energy of motion) of the electrons. This loss of energy should continue until the electrons collapse into the nucleus. Maxwell's theory would also predict that atoms would emit light energy of all frequencies (colors).

The facts were against Maxwell. Each atom emits its own characteristic spectrum of light frequencies or colors. Consider neon lights! And atoms don't collapse upon themselves. Nature must have atoms working with different rules.

A newcomer to Rutherford's laboratory was a Dane, Niels Bohr. He saw in the failure of Maxwell's theory to predict the emission spectrum of atoms the need for a departure from existing theories.

Bohr's start on the problem was to argue that the energy of the electrons within atoms could be kinetic energy of motion but that this motion of a charged particle did not radiate electromagnetic energy. He called this circumstance a "stationary state" in terms of energy exchange (but not of motion).

He then made a second postulate and said that only certain stationary states are possible.

No one would have noticed Bohr's ideas except that he was pretty good with mathematics and he stated his theory with a strong mathematical argument. He described his model of stationary states in mathematical language consistent with the known behavior of particles in motion. He consigned electrons to motion in circular orbits just as the moon is in orbit around the earth.

When a satellite such as the moon is in stable orbit, its motion is a straight line tangential to its orbit and it is exactly offset by the pull of gravity back towards the earth. In the case of the electron, the pull back toward the nucleus is not gravitational but is the electrostatic attraction of the positive nucleus on the negative electron.

By the 1920s, it was apparent that Bohr's model was overly simplistic but it works well enough for our purposes.

A more precise notion of the nature of the atom was added in 1925 by Schrödinger who proposed that electrons be treated as waves rather than particles. This led to a new view of the workings of atom called quantum mechanics. Quantum mechanics describes electrons in terms of wave functions which indicate the path of undulations that an electron makes in 3-dimensional space around an atomic nucleus. If one maps these shapes, they then represent probabilities where we might find an electron.

We'll continue with some more chemistry in the next module or you can return to the Syllabus Page.