Question #1, Sarah Lloyd, 2nd Period.

Important People in the Development of the Atom

Democritus - A Greek philosopher who came up with the atomic theory of the universe:

1. Everything is composed of atoms, which are physically indivisible.
2. Between atoms lies empty space.
3. Atoms are indestructible.
4. Always in motion.
5. Infinite number number of atoms which differ in shape and size.

Democritus reasoned that the solidness of the material that contains atoms corresponded to the shape of the atoms involved. His atomic theory inspired others to learn about the principles of the conservation of energy and of matter. Democritus used pictures and images of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts.

Lavoisier - a French chemist who was the founder of modern chemistry. Lavoisier clarified the concept of an element as a simple substance that can't be broken down. He made the theory that the formation of chemical compounds is from elements.

Dalton - British chemist who developed the atomic theory.

1. The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative atomic weights.
2. All atoms of an element are identical.
3. Atoms of one element can combine with atoms of other elements to form chemical compounds.
4. Atoms cannot be created, divided into small pieces, nor destroyed in the chemical process.
5. Elements are made of tiny particles called atoms.

Dalton's atomic theory has survived because the conviction that atoms can't be divided, created, or destroyed is inconsistent with existence of nuclear fusion and fission but nuclear reactions are not chemical. His most important contribution to science was the theory that matter is composed of atoms of differing weights and combine in simple ratios by weight.

The Curies - Pierre Curie and Marie Curie stated that radioactive materials cause atoms to break down spontaneously which releases radiation in energy form and subatomic particle form. Their experiments used techniques for isolating radioactive isotopes which led to the discovery of two new elements, polonium and radium. Marie Curie began the first studies of conducting the treatment of cancers using radiative isotopes.

JJ Thompson - discovered the electron. He discovered electrons by his explorations on the properties of cathode rays. He found that the rays could be deflected by an electric field. He compared the deflection of a beam of cathode rays by electric and magnetic fields and was able to measure the particle's mass. This showed that cathode rays were matter, but he found that the particles were lighter and then concluded that the rays were composed of light nevatively charged particles which were electrons.

Ernest Rutherford - discovered that positively charged nucleus was orbited by electrons. He found this out by his Rutherford model. Rutherford also found the existence of neutrons, which compensated for the repelling effect of the positive charges of protons by causing a nuclear force. This kept the nuclei from breaking apart.

Niels Bohr - found that atoms have a positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus, like the solar system, but with electrostatic forces that provided attraction. The Rutherford model helped him and he also made the Bohr model. The Bohr model is a complex idea of valence shell atoms.

Chadwick - discovered that neutron because of the lack of its electric charge. He learned that neutrons do not need to overcome any Coulomb barrier and penetrated and split the nuclei of elements. His discovery helped make it possible to make uranium in the laboratory.

Each scientist provided more and more insight on how the atom is made up and how it works in our everyday lives. As time went on, the scientist had more technology to test all theories and proved each one. Bohr and Rutherford helped each other by making models of the atom which eventually led to the quantum model.