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Chemistry Module 1

History of the Atom

Ancient Greek Philosophers      

    The ancient Greek philosophers posed the first recorded questions as to the nature of the physical world. There was firm belief that only a few things were the basis of all matter and phenomena that we see around us. There arose two schools of thought: the idea of the atom and the idea of the element.

    The most popular theory at the time was Aristotle's theory of the elements. He believed that one could divide up a piece if matter an infinite number of times, that is, one never came up with a piece of matter that could not be divided further. Aristotle's theory divided all mater in the four basic elements of earth, fire, water and air.

    The second theory was headed by a man named Democritus. he believed in the existence of an "elementary particle" which he called an atom. These atoms were indivisible, the smallest particle possible. He suggested that there many different types of unchangeable atoms, each with its own shape and size, in constant motion. Matter, therefore, was made up of a large number of different types of atoms. Because of Aristotle's popularity, his theory was recognized as the correct model and Democritus theory was long forgotten until the 1700's.

    In the seventeenth century, the concept of the atom was revived. Robert Boyle used the concept in his work on chemistry. Sir Isaac Newton had the theory in mind when formulating his theories in physics and optics. But it was not until the latter half of the eighteenth century that the atomic theory truly became a part of scientific thought.

     French chemist Antoine Lavoisier spent many years of the eighteenth century investigating why things burn. He discovered a number of pure chemical substances which could not be separated into other chemical substances, and he realized that burning was simply the process in which oxygen combined with those elements.

     Chemist John Dalton was the first to state several of the fundamental concepts of modern atomic theory, in the early 19th century. He stated that matter is made up of indivisible atoms; that all atoms of a specific element are identical; that different elements have different atoms (size, shape, etc); that atoms cannot be created or destroyed; and that a chemical compound is composed of molecules, which are made of a small, fixed number of atoms from each element in the compound.

     In 1827, British Botanist Thomas Brown noticed that when a pollen grain floating in a drop of water is examined using a microscope, it can be seen bouncing around in a random motion. This irregular motion is now called Brownian motion. In one of three papers published in the Annalen der Physik in 1905, Albert Einstein proved, as far as a theoretical paper could, the existence of atoms of finite size. He showed that Brownian motion, though random, obeys a statistical law, and that the pattern of behavior is exactly what should be expected if the pollen grain is being repeatedly bounced around by unseen particles. With all the evidence before them, modern scientists finally began to accept the atomic theory.

Organizing the Data

     More and more elements were being discovered, and chemists were searching for a way to organize them in predictable patterns. In 1869, Russian chemist Dimitri Ivanovich Mendeleev presented the first table of the elements. He had observed that many elements had similar properties, and that these similar properties recurred in a periodic fashion. Hence, the name 'periodic table'.

     Mendeleev's first table was based on atomic weights, rather than atomic numbers as the modern periodic table. This led to some discrepancies. For example, the mass of Tellurium was greater than Iodine by the best estimates of that time. But they didn't seem to fit the groupings he had devised. This is because although Tellurium has a greater atomic weight, it has a lower atomic number (i.e., number of protons). In 1871, Mendeleev presented his new, revised periodic table. Click here to view Mendeleev's Periodic Table.

     There were, obviously, gaps in the table. The attempt to fill in the gaps of the periods led to the discovery of many new elements. The 'natural elements', those occurring naturally in the world, fill in atomic numbers 1 (Hydrogen) through 92 (Uranium). The man-made elements, or 'trans-uranics' continue to be discovered. There are now known to be well over 110 elements, with the possibility of many more to come. The answer to the question "what makes the world" was no longer simple, and physicists began to look deeper.

Dmitri Mendeleev - Father of the Periodic Table

Dmitri Mendeleev, a Russian scientist born in Tobolsk, Siberia in 1834, is known as the father of the periodic table of the elements. The periodic table of the elements is an important tool used by students and chemists around the world to help them understand and simplify the often complex world of chemical reactions.

Not only did Mendeleev arrange the periodic table of the elements, he also wrote and published a 2 volume chemistry book entitled Principles of Chemistry as there was no thorough chemistry textbook at the time. Mendeleev dedicated his life to research and education. Mendeleev made it his special responsibility to educate people wherever he went. When he traveled, Mendeleev would ride in the third class section of the train just to share findings with peasants over a cup of tea. Mendeleev died on January 20, 1907 at the age of 73. 

The Big Task

Mendeleev set out to identify a pattern in the elements. Mendeleev looked at many pieces of evidence and made an important observation that some elements have similar chemical and physical properties. Mendeleev's hunch was that these similarities were the key to unlocking the hidden pattern of the elements. Mendeleev then embarked on the tedious task of organizing all known information for every element to help him decipher the pattern.

To begin his task, Mendeleev wrote facts about the elements on individual paper cards. On these cards, Mendeleev wrote information such as the elements' melting points, densities, colors, atomic masses (the average mass of one atom of that element), and bonding powers (the number of chemical bonds an element can form).

Once Mendeleev's cards were made, he tried arranging them in various ways. Finally, Mendeleev noticed that patterns appeared when the elements were arranged in order of increasing atomic mass. One of the trends that he noticed showed that the bonding power of the elements from lithium to fluorine change in an orderly way. For example, after fluorine, the next heaviest element Mendeleev knew was sodium, which has the same bonding power as lithium. Using this knowledge, Mendeleev placed the card for sodium below the card for lithium. This worked well -- as he laid out cards, each element had properties similar to the elements above and below it.

Mendeleev's table was not perfect, however. Arranging the elements by increasing atomic mass left three blank spaces in the table. Despite this development, however, Mendeleev boldly proposed that these blank spaces would be filled by elements that had not yet been discovered. Mendeleev was even able to use the patterns in his table to predict the properties of these undiscovered elements. This first periodic table of the elements was published in 1869. Click here to see Mendeleev's first periodic table of the elements.

The word "periodic" means that there is a repeating pattern -- that is, the properties of the elements repeat with each row -- or period -- of the table.

Amazingly, within 16 years of Mendeleev's first periodic table, chemists had discovered all three of the missing elements (scandium, gallium, and germanium), and their properties were very close to what Mendeleev had predicted.

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This page was last updated: 11/04/02