History of the Atom

Tale as old as time...

For as long as we can remember, people have wondered "who are we?" and "why are we here?" Part of the answer requires know what we are made of and consequently asking "what is the universe made of?"

Atomic Theory

Democritus (400 BC)

The first record of the word atom is by the Greek philosopher Democritus. His thought experiment goes something like this: If you cut an object in half you will get two equal masses. If we then cut the half in half we get a 4 smaller pieces. Now, can we keep cutting matter infinitely into infinitely small pieces?

No, we can't.

Democritus believed that there must be some basic unit of matter that was not separable. He called it atomos, "a-" meaning not, and "-tomos" meaning cuttable.

John Dalton (1803)

Democritus's idea of the atom was pretty much forgotten until it was brought back by English Chemist John Dalton. Dalton believed that different chemicals were made of different types of atoms, and that they had different properties like size and mass, etc.

JJ Thomson (1904)

Thomson was the first to discover that atoms were made of even smaller parts. He conducted experiments using a cathode ray tube. Base on the direction and distance the rays were deflected by charged plates, he determined that the particles in the cathode ray were smaller than atoms and that they had a negative charge.

Thomson's Atom

More Pudding Please...

Thomson's model of the atom had small negative particles he called "corpuscles" (electrons) scattered in a sphere of positive matter. He likened the atom to a plum pudding. Yum.

Ernest Rutherford (1911)

Rutherford is credited with the discovery of the nucleus using his famous gold foil experiment.

Rutherford's Atomic Model

Rutherford predicted that atoms were made of mostly empty space. to prove this he projected a beam of positively charged α (alpha) particles at a thin gold foil expecting it to pass through. He could detected the particles using a fluorescent screen that would spark when the particles hit. He was surprised when occasionally the α particles would be deflected in a random direction.

What was happening?

Rutherford's model of the atom is largely empty space except for a dense core where most of the mass is located. This core is the nucleus.

Enter the Strange Quantum World

Neils Bohr (1913)

Objects in space can typically exist on a continuous spectrum and can be anywhere between two points. For example satellites that orbit the earth can be 1,000 miles away or 1,001 miles away or even 1,000.1 miles away. Where a satellite is positioned in orbit is determined by it's velocity, i.e. the amount of kinetic energy it has.

Imagine scientists surprise when they noticed that electrons can only absorb certain amounts of energy. Scientists call the discrete amounts of energy a quanta.

Electrons can only exist in certain zones around the nucleus, called orbitals.

Electrons can only absorb certain discrete amounts of energy. When the electron absorbs enough energy it jumps to the next energy level. The graph would look like a step function.

Bohr model of the atom

Electrons can only exist in specific orbits around the atom. Bohr was able to determine the orbits for the element Hydrogen, but he was not able to create a general theory that could be applied to all elements.

Erwin Schrodinger (1926)

Schrodinger succeeded where Bohr failed to create a mathematical model that could be applied to all elements. This mathematical model shows the probability of finding an electron at any given time and maps what are known as electron clouds. The theory is called the quantum mechanical model of the atom.

Electrons do not travel in straight orbits like the planets do. Because of the speed at which electrons travel they exhibit properties of both particles and waves. Electrons can interfere with one another and create areas where they cannot exist, called nodes.

James Chadwick (1932)

In 1932, Chadwick bombarded beryllium with α particles and produced a new particle that was similar in mass to a proton, but had no charge. He discovered the neutron.

Summary Timeline

Page updated

Google Sites

Report abuse