The European Scholars

The Alchemists A.D. 500-1600

Theory:

Alchemists were part philosophers, part mystics, part magicians and part chemists. In their efforts to produce gold, they carried out some of the most earliest chemical experiments performed in Europe. They thought metals could 'grow' and 'ripen' like plants. When a metal ripened, the result would be gold. To help with their research, the Alchemists invented many laboratory tools: beakers, filters, stirring rods and distilling equipment. They also discovered many elements such as arsenic, phosphorus and zinc. However, they made little effort to create a model that would increase their understanding of the nature of matter.

John Dalton 1766-1844

Theory:

All matter is made up of tiny, invisible particles called atoms. Each element has it's own kind of atom. Atoms of the same element are identical. Atoms of different atoms differ from each other. Atoms cannot be created or destroyed, but they can be joined (bonded) or seperated (unbonded).

Evidence:

In 1661, Robert Boyle concluded that elements were 'simple unmingled bodies'.

By the 1770's, Lavoisier began measuring masses of materials involved in chemical change.

In 1799, Proust observed that water always contains 8g of oxygen for each 1g of hydrogen.

Model:

J.J. Thomas 1856-1940

Theory:

Atoms contain smaller subatomic particles called electrons. Electrons have a small mass and a negative charge. The rest of the atom has a positive charge caused by particles called protons. The electrons are embedded in the protons so that the resulting atoms are neutral or uncharged.

Evidence:

Thomas' experiments showed that beams of lightwieght particles (electrons) were given off by the atoms of a negatively charged negative plate. In inferred that atoms must contain positive particles-protons.

A proton was later found to have 2000 times as much mass as a electron.

Ernest Rutherford 1871-1937

Theory:

The positively charged part of the atom is concentrated in a dense central core called the nucleus. Most of the atom's mass is also concentrated in the nucleus. Most of the atom's volume is made of of empty space through which electrons move rapidly.

Evidence:

Rutherford bombarded thin metal sheets with energy rays and particles from radioactive elements.

He inferred that the existance of the nucleus from the pattern of the particles as they bounced off the metal or passed through it.

Model:

Niels Bohr 1885-1962

Theory:

The electrons surrounding the nucleus do not rush around randomly. Rather, there is a regular posistion to their pattern and motion.

Evidence:

Energy (as heat or electricity) was used to excite elements and make them glow. This light was passed through a spectroscope. These experiments showed light given off by each element consists of a particular set of wavelengths with a unique pattern.

James Chadwick 1891-1974

Theory:

Most atoms contain a third kind of subatomic particle in the nucleus called a neutron. The neutron has no charge. It's mass is about the same as a proton.

Evidence:

Chadwick bombarded the element beryllium with fastmoving helium neuclei. A beam of particles was given off. The beam was not affected by magnets or by charged objects. So the beam must have been made by uncharged particles.