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Early Models

The idea that matter was made up of particles has been around for a long time. Democritus, a philosopher in ancient Greece was the first person on record to propose that matter was made up of tiny particles. In fact the name atom is derived from the ancient Greek word atomos, which means very small particles.

By the 17^th and 18^th centuries, scientists were performing experiments that provided evidence that matter was particulate in nature. Joseph Gay-Lussac discovered that gases react in simple whole number ratios. For example 2 volumes of hydrogen gas react with one volume of oxygen gas to produce 2 volumes of water vapor. The simple whole number ratios seemed to be explained best by proposing that the gases were made up of particles. The two volumes of hydrogen contain wice as pany particles as one volume of oxygen. Amadeo Avogadro interpreted these results to mean that equal volumes of any gas at the same temperature and pressure contain the same number of particles.

In the early 1800’s, John Dalton proposed four principles about the nature of matter:

1)     All matter is made up of tiny indivisible particles called atoms. *

2)     An element is made up of atoms that are all identical.*

3)     Different matter consists of different kinds of atoms.

4)     Elements combine in simple whole number ratios to form compounds (Law of Definite Proportions).When compounds are broken down, the combined elements are recovered.

JJ Thomson: Connecting Electricity and Matter

Matter and electricity were originally thought of as being two separate materials. The first connection between the two was made by JJ Thomson in the 1880s. Thomson studied cathode rays made from a cathode ray tube. The cathode rays originated from a hot plate exposed to a negative charge. These rays were actually extremely small particles. These particles were deflected by a magnetic field in such a way as to confirm that they were negatively charged. These particles were named electrons. Not long after, canal rays were discovered. These were deflected in the exact opposite direction as cathode rays and were identified as positive particles.

JJ Thomson’s model of the atom was called the plum pudding model. He proposed that electrons were embedded in a “pudding” of protons.

Rutherford’s Experiment: The Nuclear Atom

Thomson’s discovery of electrons and protons added much new detail to the model of the atom. Ernest Rutherford wanted to know more about the internal structure of the atom. In about 1900, he devised an experiment that he hope would give him more detail about the internal structure of the atom. The results of his experiment were very important in developing the modern day model of the atom. Rutherford’s experiment seemed easy enough. Shoot positively charged alpha particle “bullets” at a very thin foil of gold. The expectation was that they would all blast through the thin foil much like a speeding bullet passing through a tissue paper. While 99.9 % of the alpha particles passed through, a very small percentage were either deflected or reflected back.

If the gold atoms were like Thomson’s model, the alpha particles would have blasted through without any deflection or reflection. The small amount of deflection and reflection suggested that the alpha particles were on occasion, slamming into a very dense positive charge. This led Rutherford to two main conclusions about the atom:

1)     The atom is mostly empty space. (The majority of alpha particles passed through unhindered.)

2)     The atom has a very small, dense, positively charged nucleus surrounded by electrons in a planetary orbit. The nucleus contained the protons. (The deflection was caused by the repulsion between the positive alpha particle and positive nucleus.)

 Rutherford’s calculations determined that the nucleus contains 99.99 % of the mass of the atom but is only1 trillionth the size of the entire atom. The above drawing is not to scale. If it was, the nucleus would be so small we would not see it.

Neutrons and Isotopes

A third subatomic particle, called the neutron, was discovered just a few years later. Up to this point our model of the atom is a nucleus containing positively charged protons and neutral neutrons. Nearly all of the mass resides in the nucleus. Negatively charged electrons surround the nucleus.

The idea that all atoms of an element are the same had to be modified somewhat with the discovery of different varieties of atoms of the same element. It turns out that all atoms of a particular element have the same number of protons and electrons but can have different numbers of neutrons. The result is some are heavier and some are lighter. Some examples:

Carbon comes in three isotopes:

• Carbon – 12 (6 protons, 6 electrons and 6 neutrons)
• Carbon – 13 (6 protons, 6 electrons and 7 neutrons)
• Carbon – 14 (6 protons, 6 electrons and 8 neutrons)

The average atomic mass depends on the mass of each isotope and its relative abundance. Carbon’s average atomic mass is 12.011, not 13, due to the higher abundance of the C – 12 isotope.

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