Dalton, Thomson and Crookes MODELS OF THE ATOM Dalton, Thomson and Crookes

John Dalton (1766-1844) Dalton was the first to define an element Element = a pure substance made up of one type of particle or atom. Each element has its own distinct properties and cannot be broken down into simpler substances by means of chemical change Dalton’s vision of the atom is also called the “Billiard Ball” model because he thought that the atom was a solid sphere of matter

Dalton’s Atomic Theory all matter is made up of small particles called ATOMS atoms cannot be created, destroyed or divided into smaller particles all atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements compounds are created when atoms of different elements link together in definite proportions

William Crookes (1832-1919) Using a gas discharge tube, he noticed that a positively charged pinwheel (“cathode”) would rotate when an electric charge was passed through the tube Crookes believed that this electric charge (called cathode rays) must have mass as well as motion

J.J. Thomson (1856-1940) Thomson took Crookes’ work and created a new experiment he designed an experiment to investigate how these charged corpuscles would move in an electric field found that the corpuscles were attracted towards positively charged plates - therefore the corpuscles must be negatively charged called them ELECTRONS

Protons Thomson conducted similar experiments, this time with anode rays and negatively charged plates anode rays were attracted to the negatively charged plates - therefore they must contain positively charged particles Thomson called these particles PROTONS

Thomson’s inferences... All atoms contain both protons and electrons all protons are identical, all electrons are identical. Electrons differ from protons electrons have a negative charge, protons have a positive charge an electron has the same amount of charge as a proton, even though the charges are opposite in kind. A proton has much more mass than an electron

Thomson’s model of the atom Thomson pictured the atom as a combination of protons and electrons he thought that the electrons were embedded in a sphere or positively charged protons looks like a blueberry muffin called the “Blueberry muffin” model