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Atomic Theory – Supplemetal notes
THE HISTORY OF MATTER
2500 BC – Records like the Pharaoh’s scribe and ancient wall paintings outline activities of the Egyptian metal workers. Though metals such as
copper were known at the time, the Egyptians had no idea that these metals, in fact, elements. Metallurgy – the science behind the knowledge that
medals could be obtained from ore and purified became an important part of the culture of Egypt and Babylon. A close association between
priests, temples and gods was established. Chemical changes were thought to be due to some powerful magic.
600 BC – Ancient Greek philosopher began to wonder about the nature of matter. Very few experiments were performed. Their theories were
based on ideas alone.
400 BC –The Greek Philosopher “Democritus” taught his students that matter was made of tiny particles called ATOMS, which were thought to be
invisible, indivisible and indestructible. His theory, often called the “CLASSICAL ATOMIC THEORY” maintained that hard and compact substances
contained atoms which were interlocked in a very tight pattern. Liquids were thought to be made up of atoms held together much more loosely,
with a round shape which allowed them to be poured.
400 BC – The widely respected Greek Philosopher “Aristotle”- supported the “FOUR ELEMENT THEORY”. This theory stated that all matter was
made up of different combinations of four elements: Earth, Fire, Air and Water. The four element would dominate thinking in the Western world
for the next 2000 years.
300 BC – The practice of ALCHEMY was born. Alchemy was a reflection of the serious religious beliefs of the Middle Ages. It flourished in Chinese,
Hindu, Arabic, and European cultures. It was a study of the natural worlds in an attempt to blend the natural world with the spiritual world. The
ALCHEMISTS believed in “transmutation”. The ideas that matter could be changed from one form to another by altering the proportions of earth,
air, fire and water. The alchemists spent much of their time attempting to change cheap metal into gold.
NOTE: Elements known in ancient times were mainly metals and included Ag, As, Au, Bi, C, Cu, Fe, Hg, Pb, S, Sb, Sn
16th Century – European alchemists had separated into two groups. One continued to look at the spiritual side, the other focused on the discovery
of new compounds and their reactions. These alchemists invented equipment and many procedures still used to modern scientists. An example is
distillation.
1660 AD – Robert Boyle [1627-1691] introduced “THE SCIENTIDIC METHOD OF INVESTIGATION”. His studies were carefully controlled and his
findings were recorded in an organized way – what we refer to as “the 5 steps of the Scientific Method”.
1661 AD – Boyle published a book called “The Skeptical Chemist” which includes a definition of elements as “certain simple unmingled bodies”.
Boyle strongly believed the four element theory
1669 AD – German alchemist Hennig Brand isolated a mysterious substance from human urine. He named the substation “eternal fire” as it burst
into flames whenever exposed to air.
1671 AD – Boyle recognized Brand’s “eternal fire” as an element. It was named Phosphorus.
1700 AD – The PHILOGISTON THEORY. As the coal-powered industrial revolution began, scientists turned their attention explaining burning. They
thought an invisible fluid “phlogiston” was contained in fuels and escaped during burning. Eventually three chemists made a series of discoveries to
help explain the true nature of burning.
1766 AD – Henry Cavendish [1731-1810] mixed a metal with an acid and prepared a flammable gas that floated. He had discovered Hydrogen.
1774 AD – Joseph Priestly [1733-1810] prepared a gas by heating a red powder [HgO]. He called the gas “lively air” because it made candles burn
brightly and mice run more briskly. He had discovered oxygen.
Henry Cavendish [1731-1810] discovered that when he mixed the gases: hydrogen and oxygen gas it produced water.
Late 1700’s – Antoine Lavoisier [1731-1810] developed “THE LAW OF CONVERSATION OF MASS” which stated that during a chemical reaction, the
total mass of a substance remains unchanged. He also discovered that the process of burning used up one fifth of the air
1800 AD – Alessandro Volta [1745-1827] devised the world’s first battery. He used this continuous source of electric current to decompose water
into hydrogen and oxygen.
1807 AD – Sir Humphrey Davy [1766-1844] developed a method for extracting elements from compounds using electric current. He isolated more
metallic elements: Na, K, Ba, B, Ca, Sr, and Mg.
1808 AD – John Dalton [1766-1844] published this famous ATOMIC THEORY. It states…
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All matter is made up of atoms, which are particles too small to be seen
Each element has its own kind of atom. Atoms of the same elements have different masses
Compounds are created when atoms of different elements link to form compound atoms. In a compound, all the compound-atoms are
alike
Atoms cannot be created or destroyed
Dalton’s theory was readily accepted as it proved and predicted the Laws of Chemical Change: Conservation and Mass of Definite Proportions.
Dalton also contributed a table of atomic mass values calculated from his experiments. His work prompted chemists to compare the masses of the
elements, first with hydrogen, and later with carbon, to develop the relative atomic mass scale we still use today.
1830’s Michael Faraday showed that atoms could gain electric charges. This led to a modification of Dalton’s theory to include
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Matter must contain positive and negative charges
Opposite charges attract, and like charges repel
Atoms combine to form the particles of a compound because of the electrical attraction between charged atoms
1869 AD – Dmitri Mendeleev [1834-1907] presented a logical arrangement of the 60 odd elements known at this time. Mendeleev’s PERIODIC
TABLE arranged the elements in order of increasing atomic mass with groupings according to similarities in physical and chemical properties.
1870 AD – Sir William Crookes developed Cathode Rays
1894-1900 – William Ramsey using the predictive nature of Mendeleev’s periodic table discovered the non-reactive gases: Argon, Neon, krypton,
Xenon, and Radon. This is now the family we call the NOBLE GASES.
1870 AD – Prof. Wilhelm Roentgen – discovered x-rays.
1898 AD – Pierre and Marie Curie – discover the element radium.
1898 AD – J.J Thompson the negatively charged the electron. This led to a major change to the atomic theory. J.J Thompson proposed the “RAISONBUN’ MODEL. It included the following ideas:
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Atoms contain electrons
Electrons have a negative charge and a very small mass
The rest of the atom has a positive charge
Electrons are embedded randomly in the positive part of the atom
Electrons can be removed from or added to atoms to create charged atoms
1911 AD – Earnest Rutherford [1871-1937] discovered the presence of the atomic nucleus during his famous gold foil experiments. This led to
further changes in the atomic theory.
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Most of the atom is empty space
Most of the mass of the atoms is concentrated in a tiny , dense, positively charged central core – the nucleus
The nucleus contains positively charged particles called protons particles with no charge called neutrons.
1913 Ad Neil bohr proposes a theory “ Bohr Model” of the atom in which the electrons are restricted to specific shells around the nucleus. The
amount of energy an electron has is related to how far it is from the nucleus.
1920’S Further discoveries were made about the behaviour of electrons in atoms- there are a maximum number of electrons that can occupy one
shell. This lead to further modifications resulting in the atomic theory of matter That is currently used.
1964- Gell Mann was working on the classification of elementary particles. He hypothesized about the structure of proton and neutrons. In 1964
AD, QUARKS were discovered during an electron- proton scattering experiment. Six different types of quarks have been found: up, down, charm,
strange, top and bottom