Atomic TheoryStory of the Atom

Our Story Begins….Many years ago

400 B.C.

In a land far away

Two men had different points of view.

Democritus Aristotle

Democritus (460 – 370 B.C)

Each type of matter has a smallest piece beyond which it cannot be divided.Called this smallest piece of matter “atomos” which means “indivisible”Atoms were small, hard particles that were all made of the same material but were different shapes and sizes.

Atoms were infinite in number, always moving and capable of joining together.

Aristotle (384 – 382 B.C.)

All matter made of 4 elements: earth, air, fire and water4 qualities of elements were coldness, moistness, dryness, and hotness2 forces: conflict and harmony5th element called aither made up the heavens

And the winner was…Not science…Aristotle’s views won the day and the concept of the atom went away for nearly 2000 years.Although they were great observers and thinkers (natural philosophers), the Greeks did not perform experiments. They arrived at their conclusions by way of logical thinking.

Time passes….The early 1700’s brought about an

explosion of scientific investigations prompted, in part, by the increased use of the chemical balance to take before and after measurements of

chemical reactions.

Antoine Lavoisier (1743 – 1794)

First person to extensively use the chemical balance in studying chemical reactionsProposed the law of conservation of mass

Matter is neither created nor destroyed in a chemical reaction.The mass of the reactants equals the mass of the products.

Thought in terms of elements but not in terms of atoms.

Joseph Proust (1754 – 1826)

Proposed the Law of Constant Composition (1799) now known as the Law of Definite Proportions.

In samples of any chemical compound, the masses of the elements are always in the same proportion.

John Dalton (1766 – 1844)

Proposed the Law of Multiple Proportions (1802)

Whenever two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.

Return of the Atom

After an absence of about 2000 years, the atom returns

as an explanation for the discoveries being made by

scientists.

Dalton’s Atomic Theory (1808)

All matter is composed of a very large number of very small particles called atoms.For a given element, all atoms are identical in all respects. In particular, all atoms of the same element have the same constant mass, while atoms of different elements have different masses.The atoms are the units of chemical changes. Chemical reactions involve the combination, separation or rearrangement of atoms but atoms are neither created, destroyed, divided into parts or converted into atoms of any other kind.Atoms combine to form molecules in fixed ratios of small whole numbers.

Dalton’s Spherical ModelAlthough Dalton brought back the concept of the atom, his model remained incomplete.Dalton thought of atoms as solid balls of matter that could not be broken down into any smaller parts.

Journey into the AtomOnce the idea that matter is made up of atoms was accepted in the

scientific world, the hunt was on to find out more about these tiny

particles of which all substances are composed. It was up to the world of physics to discover that atoms are,

indeed, made up of smaller particles.

J. J. Thomson (1856 – 1940)

Performed cathode ray experimentsDiscovered the electron (1897)Developed the plum pudding model

Atoms consist of a positively charged matrix with negatively charged electrons embedded in it.The total positive charge equals the total negative charge giving the atom an overall neutral charge.

Determined the charge to mass ratio of the electron

Robert Millikan (1868 – 1953)

Performed the Oil Drop Experiments (1909)Discovered the charge on the electron

1.60 x 10-19 coulombs

Ernest Rutherford (1871 – 1937)

Gold foil experiments (1911)Discovered the nucleusDeveloped the nuclear model of the atom

Atom is mostly empty spaceSmall, dense, positively charged nucleus

Niels Bohr (1885 – 1962)Refined Rutherford’s model to include the arrangement of electrons around the nucleus.Bohr Model (1913)

Electrons orbit the nucleus without losing energy.Electrons could only move in fixed orbits of specific energy.Electrons of low energy would orbit closer to the nucleus while electrons with high energy would orbit further from the nucleus.

Worked with the emission spectrum of the hydrogen atom.

Louis de Broglie (1892 – 1987)

Proposed the wave-particle duality of the electron (1923)Explained why the Bohr model was successful with hydrogen’s single electron atoms but did not hold true for more complex atoms.

Werner Heisenberg (1901 – 1976)

Heisenberg’s Uncertainty Principle (1927)

It is impossible to know both the velocity and the position of a particle (electron) at the same time.

Erwin Schrodinger (1887 – 1961)

Quantum Mechanical Model (1926)Developed a set of mathematical equations known as the wave function that treat the electron as a wave moving in orbitals where there is a high probability of finding electrons.

James Chadwick (1891 – 1974)

Discovered the neutron in 1932His discovery was directly influential in the discovery of atomic fission and development of the atom bomb.

The Story isn’t Finished