Developing the Atomic Theory 1 Lesson 5 October 1st, 2010

Evolving Models of the Atom An atom is the smallest part of an element that has all of the element’s properties. Different kinds of atoms give elements different properties.

Evolving Models of the Atom Atomic theory is the study of the nature of atoms and how they combine to form all types of matter. Atomic theory helps us to understand why there are different kinds of atoms. It explains how atoms combine to form over 100 known elements and all other forms of matter, including compounds and mixtures.

Evolving Models of the Atom Scientists have been wondering what matter is made of for over 2500 years.

Evolving Models of the Atom Empedocles theorised that all substances were made from the combination of four fundamental elements: earth, air, fire, water.

Today, we still use the term element, though in a different way Today, we still use the term element, though in a different way. For example, we still believe that most substances are built up from simpler ones.

About 440 B.C.E., the Greek philosopher Democritus hypothesized that breaking down rock into powder and then breaking that into smaller and smaller pieces unit a single invisible particle was reached. He called this particle an Atom which means uncutable or invisible in Greek

Neither ancient scientist did any experimentation when they developed these models.

Atomic Theory Takes Shape Atomic theory is rooted in the idea that an understanding of atoms and their structure can help us predict many of the properties of matter.

John Dalton (1766–1844), a scientist and teacher in England, reconsidered the ancient idea that each different kind of element is composed of a different kind of atom. Dalton thought that the atom would be like a featureless sphere.

John Dalton (1766–1844), He used the following theory to explain the nature of matter: All matter is made of small, indivisible particles called atoms. All the atoms of an element are identical in properties such as size and mass.

John Dalton (1766–1844), Atoms of different elements have different properties. Atoms of different elements can combine in specific ways to form new substances. Atoms cannot be created, destroyed, or subdivided in a chemical change.

Dalton also devised a series of element symbols to represent the atoms of different elements. These symbols are shown in his Table of Elements from 1808. The small round symbols were meant to resemble atoms.

Atoms Are Composed of Smaller Particles J. J. Thomson (1856–1940), an English physicist, researched the idea that atoms might be made from a combination of particles. He experimented with electric currents in glass tubes called cathode ray tubes.

J. J. Thomson (1856–1940), He found that the atom is not the smallest particle. There were particles within the atom. He theorized that an atom was a positively charged sphere with negative charges embedded in it.

J. J. Thomson (1856–1940), It is also known as the plum pudding model In 1897 he proposed a revolutionary new model for atoms. It is known as the raisin bun model. The dough would be the positively charged sphere and the raisins would be the negative charges.

The negative charges are known as electrons. This would leave the atom with a neutral charge as the positive charges would be balanced by the negative charges. The negative charges are known as electrons. Electron – Subatomic particle with a negative charge