The goal of the Greek philosopers was to explain the natural world. In an effort to organize many of the phenomena they observed, philosophers believed that a single "primary matter" existed. It was of this primary matter, modified in various ways, that all other things were created. Democritus expanded the idea to state that matter was composed of small particles called "atoms" that could be divided no further. These atoms were all composed of the same primary matter with the only differences between them being their size, shape and weight. The differences in these characteristics explained the differences in the properties of the matter around us.

Unfortunately for Democritus, and mankind in general, his ideas were largely ignored for the next 2000 years.

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For nearly 2000 years science was unable to devise experiments capable of testing the theories first put forward by Democritus. During the 19th century, a vast amount of data on how substances react with each other was collected. From this data, some simple laws of chemical reactivity had been devised. Among these were the law of conservation of matter and law of mulitple proportions. While others had proposed very similar theories, John Dalton is usually credited with developing the first coherent atomic theory.

Dalton's theory can be summarized as follows:

1. Matter is composed of small particles called atoms.
2. All atoms of an element are identical, but are different from those of any other element.
3. During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged.
4. Atoms always combine in whole number multiples of each other. For example, 1:1, 1:2, 2:3 or 1:3.

In addition to helping to explain the laws of chemical reactivity, Dalton's theory also helped further the concept of atomic weights. By assuming that nature would be a simple as possible, Dalton assumed that the elements preferred to combine in one to one ratios, and was therefore able to tabulate a set of relative weights.

For example:

If nature is to be simple, the formula for water will be HO. Since it was known that water contained 8 grams of oxygen for every one gram of hydrogen, then oxygen atoms must be eight times more massive than hydrogen atoms. It was also known that, in the compound formed between hydrogen and sulfur, there were 16 grams of sulfur for every one gram of hydrogen. Sulfur atoms must therefore be 16 times more massive than hydrogen. From this data we could start to build a table.

We know now that nature is not always so simple, and that elements often combine in ratios other than one to one. As such, many of Dalton's weights were incorrect. This table did, however, represent a big step forward.

SOME OF DALTON'S SYMBOLS FOR THE ELEMENTS

Most of the elements listed correspond to the presently known elements, just not in the correct order.
	1 H 2 N 3 C 4 O 5 P 6 S 13 Fe 14 Zn 15 Cu 16 Pb 17 Ag 18 Pt 19 Au 20 Hg

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Work by Amedeo Avogadro, Stanilao Cannizarro and J.J. Berzulius in the first half of the 19th century led to the accurate determination of the atomic weights of the elements and chemical formulas of compounds made from them. These determinations paved the way for the first periodic table of the elements.

While writing a textbook for his chemistry students Dmitri Mendeleev attempted to classify the elements not by some "accidental, or instinctive reasons, but by some exact principle." He believed that this exact system should be numerical in nature to eliminate any margin of arbitrariness. The only unchanging numerical data available at this time was the atomic weight. By arranging the elements in order of increasing atomic weight he discovered that there existed a periodicity of the elemental properties. He used this periodicity to create a table in which that elements with similar properties were vertically aligned with each other.

In making such alignments Mendeleev was able to determine that several, as yet unidentified, elements should exist (the elements with masses 44, 68 and 72 are examples). He went on to make predictions about the properties of these missing elements which aided in their discovery. The discovery of scandium (44), gallium (68) and germanium (72) and examination of their properities (which were very similar to those predicted by Mendeleev) provided evidence for the validity of the periodic table.

THE FIRST PERIODIC TABLE (1869)

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The are at least two important points to notice:

1. The positive particles are bent toward the negative plate, the negative particles are bent toward the positive plate and the neutral particles are not bent in either direction.
2. The extent to which the path of a particle is bent as it passes through an electric field depends on its mass and its charge
   • the larger the charge on the particle, the further it is bent.
   • the larger the mass of the particle, the less it is bent.

While alpha particles were determined to have a larger charge than the beta particles (+2 vs. -1), they also have over 7000 times the mass of the beta particle. Therefore, their path is bent much less than that of the beta particle.

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At approximately the same time as radioactivity was being investigated, J.J. Thomson and others were performing experiments with cathode ray tubes. A cathode ray tube is an evacuated tube that contains a small amount of gas between two metallic plates. When a potential is placed between the cathode (the negatively charged plate) and the anode (the positively charged plate) a "ray" of electric current passes from one plate to the other. Thomson discovered that this ray was actually composed of particles.

When a second set of plates is placed around the tube, the ray is bent toward the positive plate indicating that the ray is composed of negatively charged particles. By varying the potential on the plates, Thomson was able to determine the mass to charge ratio of these particles.

In further experiments he varied what metal was used to make the electrodes and what gas was used to filled the tube. In each case, the properties of the ray particles were exactly the same. He concluded that the negatively charged particles were subatomic particles that were part of every atom. He further surmised that, since atoms were electrically neutral, the atom must also contain some positive charge. Based on these conclusions Thomson proposed that an atom was composed of a spherical ball of positive charge with "corpuscles" of negative charge imbedded in it. The corpuscles would later become known as electrons.

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Thomson had identified that the atom was composed of positive and negative charges and had proposed that the atom was a solid mass of these particles. If this model were true, any particles shot at the atom should be deflected by it. If the negative and positive charges were in some arrangement that left empty space in the atom, particles shot at the atom might be able to pass through them. In 1909, Rutherford set a fellow scientist, Hans Geiger, and a student, Ernest Marsden, to work on this problem. They devised a system that allowed alpha particles (the nuclei of helium atoms) to be shot at a very thin piece of gold foil and the trajectory of the particles monitored. They observed that while most of the particles passed through the foil with little or no deflection, some were deflected to a great degree.

Since the gold film was so thin, Rutherford proposed that all of the deflections observed were from single encounters of alpha particles with the atom. In order to deflect the relatively large and swiftly moving alpha particles to such a large extent, a large force was required. This force, he contended, could only be caused by a large concentration of positive charge within the atom. This large concentration of charge was located at the center of the atom and became known as the nucleus. The negative charge, in the form of electrons, was then distributed throughout the rest of the space occupied by the atom.

In order to account for the fact that many of the alpha particles passed through the gold film, Rutherford discounted Thompson's solid ball model of the atom, and believed that the central positive charge of the atom represented only a small fraction of the atom's size, and that the remainder was primarily empty space. He calculated that, while an individual atom was about 1x10-10 meters in diameter, the nuclear diameter was only about 1x10-14 meters.

While solving the problem of the observed alpha particle deflection, Rutherford's model created another. If the positive charge was located at the center of the atom, why were the negatively charged electrons not immediately drawn into it (opposite charges attract). Rutherford was not unaware of this problem but his model so adequately (and mathematically) explained the scattering results that it became widely accepted.

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The idea that an elementary unit of charge should exist seems to have originated with Benjamin Franklin around 1850. It was not until the work of Robert Millikin that the number value of this charge could be determined. It was known that X-rays could be used to impart a negative charge to an oil droplet in a chamber that contains it. Like all objects of mass, the influence of gravity causes the oil droplet to fall. Millikin placed charged plates at the top and bottom of his chamber. By varying the potential between the plates, he discovered that he was able to suspend the droplet in mid-air. The droplet remained suspended when the downward force of gravity was exactly balanced by the upward electrical force caused by the charged plates. Since both gravitational and electrical equations were known to determine these forces, Millikin was able to calculate the charge on each of the droplets he tested (Note 1). The calculated charges on the droplets all turned out to multiples of a single number. Millikin therefore reasoned the elementary charge, or the smallest of charge, must be equal to this value. By combining his new information with the mass to charge ratio for the electron determined by Thomson, the mass of an electron was calculated for the first time.

¹While Millikan did observe that the drops could be suspended motionless in the chamber, his calculations actually used the time it took for the charged particle to rise in the presence of an electric field and to fall in the absence of an applied electric field.

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In 1886, Eugen Goldstein pierced several holes in the cathode of a cathode ray tube and noticed a stream of particles in the space behind the cathode. He called these streams canal rays. Using powerful magnetic fields in 1898, Wilhelm Wien verified that these particles were positively charged and had masses comparable to a hydrogen atom. Additionally, he concluded that, as the particles were not all influenced by the magnetic field to the same extent, they must have different masses. These two results were in stark contrast to the normal cathode ray that was composed solely of electrons (small and uniform in mass).

Thomson devised a system very similar to the modern mass spectrometer to further study canal rays. Using much the same kind of thinking as Millikin did, Thomson was able to determine that all the particles had charges that were multiples of the same number. In this case, it was an elementary positive charge.

Thomson's work contained two other major points:

1. A different set of particles is created when different gases were placed in the cathode ray tube. This property could make the instrument suitable for chemical analysis.
2. Even highly purified samples of neon contained particles of two distinct masses (20 and 22). This suggested that all of the atoms of neon were not identical and that naturally occurring neon was actually composed of two chemically identical gases. These gases were identified as neon-20 and neon-22 (Note 1).

¹Isotopes of radioactive elements had been discovered in 1910 by Frederick Soddy, but this was the first experimental observation of naturally occuring isotopes.

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When an atom is bombarded with energetic electrons, a rearrangement of the core electrons can result in the emission of an x-ray. It had been observed that the frequency of the emitted x-rays was characteristic of the element bombarded. Since the core electrons are those closest to the nucleus the frequency of the emitted x-ray can be influenced by the charge and structure of the nucleus. As the nuclei of different elements have different properities, it stands to reason that the emitted x-ray should have different frequencies.

When Henry Moseley bombarded a number of elements and determined the frequency of the resulting x-rays he found that they fit the general form shown below, where Q is taken as the number of the element as it appeared in Mendeleev's periodic table of the elements.

This relationship led Moseley to believe that "there is in the atom a fundamental quantity, which increases by regular steps as we pass from one element to the next." As the atomic weight of the elements does not increase in regular steps he concluded that the "quantity can only be the charge on the central positive nucleus" of the Rutherford atom. This "Q" value would become known as the atomic number.

Notice that there are three small anomalies on the graph (black arrows), and four gaps (red arrows). The anomalies represented elements that Mendeleev had reversed in his table (argon and potassium, cobalt and nickel, tellurium and iodine). The gaps represented elements yet to be discovered. Halfnium (72) and rhenium (75) are naturally occurring and were discovered in 1923 and 1925, respectively. Technetium (43) and promethium (61) are both short-lived radioactive elements that must be created in nuclear reactors.

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Following his gold scattering work that led to the nuclear model of the atom, Rutherford pursued a number of experiments with lighter elements. While bombarding a sample of nitrogen gas with alpha particles, Rutherford detected a particle with properties identical to that of a hydrogen nucleus. After investigating all possible sources of hydrogen atom contamination, Rutherford concluded that the nitrogen atom disintegrated under the great force of the collision and the particle released was a constituent part of the nitrogen atom nucleus. In 1920, Rutherford reported these results and suggested that the hydrogen nucleus be called a "proton" (during the same talk he proposed the existence of a neutron, which was later identified by Chadwick).

AN INTERESTING FOOTNOTE

During this time period (the 1920's and early 30's) the center of this kind of work was in Germany. Later, in the 1940's it shifted to the United States. Had Rutherford pursued his proposal about the existance of the neutron at this time, the first atomic bomb would likely have been developed in Germany, rather than in the United States. That might have changed history just a bit.

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As of 1930, only two known elementary particles had been identified, the proton and the electron. Protons were known to have a mass of 1 and a charge of +1, while electrons had essentailly no mass and a charge of -1. Moseley had shown convincingly that the charge on the nucleus increases in steps of +1 as one traverses the periodic table. To account for this it was apparent that the nucleus of each atom contained a number of protons equal to its atomic number. In order to remain electrically neutral, it also contained an equivalent number of electrons.

The primary difficulty remaining was accounting for the extra mass found in the nucleus. The most prominant theory held that this mass was provided by extra proton/electron pairs in nucleus. Consider helium as an example. It is element #2 and therefore contained 2 protons and 2 electrons. However, the helium atom was known to have a mass of 4. To reach this mass would require an extra 2 protons. In order to remain electrically neutral, an extra two electrons must also be added.

Unfortunately, experimental studies involving the spins (angular momentum) of atoms, protons and electrons showed that the proton/electron theory was invalid. The problem of the extra nuclear mass was solved in 1932 when James Chadwick identified the neutron. While studying the radiation resulting from the bombardment of beryllium with alpha particles, Chadwick noted a particle with approximately the same mass as a proton being released. He determined that, as the particle was not bent by electrical fields and was highly penetrating, it was electrically neutral.

It was originally proposed that a neutron was simply a proton and electron in very close proximity to each other. However, Heisenberg's uncertainty principle definitively killed this idea, and the neutron became accepted as the third elementary particle.

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This all brings us to the modern view of the nuclear atom.

• Atoms are composed of three elementary particles: the electron, the proton and the neutron.
• Most of the mass of the atom is concentrated in the nucleus of the atom. The protons and neutrons reside in the nucleus while the electrons exist outside of the nucleus.
• In neutral atoms, the number of protons is equal to the number of electrons.
• The type of element each atom is determined by the number of protons it has.
• The number of protons in an element is equal to the atomic number - the number on which the periodic table is based.
• While every atom of an element has the same number of protons, they do not all have the same mass. Atoms of the same element with different masses are called isotopes.
• The sum of the number of protons and neutrons in a particular atom is called the mass number. The mass number is different for different isotopes of the same element.

Scientists use a shorthand method of representing different atoms:

Example:

The three isotopes of carbon are carbon-12, carbon-13 and carbon-14. Write the symbol for each and then determine the number of protons, electrons and neutrons in each.

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Information on this page take from http://acpcommunity.acp.edu/Facultystaff/genchem/GC1/lecture/assign/atomic/hisatom.htm