As You Read

Explain how scientists discovered subatomic particles.
Explain how today's model of the atom developed.
Describe the structure of the nuclear atom.
Vocab: element, electron, proton, neutron, and electron cloud.
Think about the different models of the atom, draw them and compare between them.
Names: Dalton, Ernest Rutford, James Chadwik, J. J. Thomson, Niels Bohr

First Thoughts

Do you like mysteries? Are you curious? Don't be embarrassed! Humans have alway been curious. Someone always wants to know something that is not easy to detect or to see what can’t be seen. For example, people began wondering about matter more than 2,500 years ago. Some of the Greek philosophers thought that matter was composed of tiny particles. They reasoned that you could take a piece of matter, cut it in half, cut the half piece in half again, and continue to cut again and again. Eventually, you wouldn’t be able to cut any more. You would have only one particle left. They named these particles "atomos," a term that means "indivisible." Another way to imagine this is to picture a string of beads. If you keep dividing the string into pieces, you eventually come to one single bead.

Describing the Unseen

The Greek philosophers didn’t try to prove their theories by doing experiments as scientists now do. Their theories were the result of reasoning, debating, and discussion - not of evidence or proof. Today, scientists will not accept a theory that is not supported by experimental evidence. But even if the Greek philosophers had experimented, they could not have proven the existence of atoms. People had not yet discovered much about what is now called chemistry, the study of matter. The kind of equipment needed to study matter was a long way from being invented. Even as recently as 500 years ago, atoms were still a mystery.

Different Atomic Theories

Dalton's Atomic Theory

One of the first published theories of the atom was proposed by John Dalton, an English chemist born in 1766. His theory had five main points to it. 1.) Elements are made of tiny parts called "atoms." 2.) The atoms of a given element are different from those of any other element, and are distinguishable by their atomic weight. 3.) All atoms of a given element are identical. (Now known to be false - see the section on Isotopes [coming soon].) 4.) Atoms of one element can combine with atoms of another element to form chemical compounds. A given compound will always have the same relative numbers of different atoms. 5.) Atoms can neither be created nor divided by a chemical reaction, merely grouped together in different ways.

Discovering The Electron

The news of Crooke’s experiments excited the scientific community of the time. But many scientists were not convinced that the cathode rays were streams of particles. Was the greenish glow light, or was it a stream of charged particles? In 1897, J.J. Thomson, an English physicist, tried to clear up the confusion. He placed a magnet beside the tube from Crooke’s experiments. Light cannot be bent by a magnet, so the beam couldn’t be light. Therefore, Thomson concluded that the beam must be made up of charged particles of matter that came from the cathode.

Thomson then repeated the CRT experiment using different metals for the cathode and different gases in the tube. He found that the same charged particles were produced no matter what elements were used for the cathode or the gas in the tube. Thomson concluded that cathode rays are negatively charged particles of matter. How did Thomson know the particles were negatively charged? He knew that opposite charges attract each other. He observed that these particles were attracted to the positively charged anode, so he reasoned that the particles must be negatively charged.

These negatively charged particles are now called "electrons." Thomson also inferred that electrons are a part of every kind of atom because they are produced by every kind of cathode material. Perhaps the biggest surprise that came from Thomson’s experiments was the evidence that particles smaller than the atom do exist.

Thomson's Atomic Model

The Bohr model of the atom
Showing electron quantum jumping to ground state n=1

Some of the questions posed by scientists were answered in light of Thomson’s experiments. However, the answers inspired new questions. If atoms contain one or more negatively charged particles, then all matter, which is made of atoms, should be negatively charged as well. But all matter isn’t negatively charged. How can this be explained? Could it be that atoms also contain some positive charge? The negatively charged electrons and the unknown positive charge would then neutralize each other in the atom. Thomson came to this conclusion and included positive charge in his model of the atom.

Using his new findings, Thomson revised Dalton’s model of the atom. Instead of a solid ball that was the same throughout, Thomson pictured a sphere of positive charge. The negatively charged electrons were spread evenly among the positive charge. This is modeled by a ball of clay. The positive charge of the clay is equal to the negative charge of the electrons. Therefore, the atom is neutral. It was later discovered that not all atoms are neutral. The number of electrons within an element can vary. If there is more positive charge than negative electrons, the atom has an overall positive charge. If there are more negative electrons than positive charge, the atom has an overall negative charge. Kent-Meridian High School- Austin Shelfoe

Rutherford's Experiments

A model is not accepted in the scientific community until it has been tested and the tests support previous observations. In 1906, Ernest Rutherford and his coworkers began an experiment to find out if Thomson’s model of the atom was correct. They wanted to see what would happen when they fired fast-moving, positively charged bits of matter, called alpha particles, at a thin film of a metal such as gold. Alpha particles come from unstable atoms. Alpha particles are positively charged, and so they are repelled by particles of matter which also have a positive charge.

A source of alpha particles was aimed at a thin sheet of gold foil that was only 400 nm thick. The foil was surrounded by a fluorescent screen that gave a flash of light each time it was hit by a charged particle."I had observed the scattering of alpha particles, and... it was as if you had fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you... it was then that I had an idea of an atom with a minute massive centre, carrying a charge." Source: Rutherford et al., Project Physics Unit 5 Text, 1971

A New Model of the Atom

Most models of atomic structure are very difficult to comprehend, especially at introductory levels. Kreswell has suggested a more visually intuitive atomic model enabling a deeper understanding of how nature works at it most fundamental level.

See: A New Model of the Atom