History of the Atom 600 BC – Thales noticed that hair, feathers and other light weight objects were sometimes attracted to other objects as though they were magnetized. He experimented with static electricity, and this began the first recorded study of the nature of matter in ancient Greece.

History of the Atom Greece 460-370 BC – Democritus Where did he live? What did he say about atoms? Term “atom” = indivisible Small Particles differ in size, shape and never stop moving.

Early Greek Theories Democritus 400 B.C. - Democritus thought matter could not be divided indefinitely. This led to the idea of atoms in a void. fire air water earth Aristotle 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. Aristotle was wrong. However, his theory persisted for 2000 years.

History of the Atom 1766-1844 – John Dalton Where did he live? England Model was called the Billiard Ball

John Dalton 1800 -Dalton proposed a modern atomic model based on experimentation. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions. His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

Problems with Dalton’s theory All atoms of a given element are identical was proven wrong because of the existence of Neutrons. Atoms cannot be created, divided into smaller particles or destroyed was proven wrong because of the existence of Protons, Neutrons, and Electrons. Atom- The smallest particle of an element and still retains the properties of the element.

History of the Atom 1856-1940 – J.J. Thomson found that hitting matter with a large jolt of electricity would cause negative particles to be given off. He realized that matter is usually neutral (has no charge) so he theorized that there must be a positive part to matter also.

Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

- + Thomson’s Experiment Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment Voltage source By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field he found that the moving pieces were negative

History of the Atom 1897 – J.J. Thomson He called his negative particles “corpuscles” and said that atom was like “negative raisin in a positive bun”. Later the name electron was given to the negative particles of the atom. He Discovered the electron

Some Modern Cathode Ray Tubes

History of the Atom (1871-1937) – Ernest Rutherford Describe his experiment.

Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

What he expected

Because

He thought the mass was evenly distributed in the atom

Since he thought the mass was evenly distributed in the atom

What he got

How he explained it Atom is mostly empty. + Atom is mostly empty. Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough.

+

Mass of the Electron 1909 – Robert Millikan determines the mass of the electron. The oil drop apparatus Mass of the electron is 9.109 x 10-31 kg

History of the Atom 1911 – Ernest Rutherford What did he theorize? Atoms are mostly empty space with dense centers he called the nucleus Why is his work important? Discovered the nucleus and protons

History of the Atom Electrons move around the nucleus in a fixed orbit 1912 – Niels Bohr experimented with the hydrogen atom when he proposed some rules about the way electrons orbit the nucleus. What were his rules about electrons? Electrons move around the nucleus in a fixed orbit

Bohr Diagrams 2 p+ 2 n0 He 3 p+ 4 n0 Li

History of the Atom 1920 – Many scientists working together changed Bohr’s energy level model to the charged cloud model. In this model the electrons form a negative cloud around the positive nucleus. The location of any electrons at any given time can be predicted using quantum numbers.

History of the Atom (1891-1974) James Chadwick discovers evidence of another particle located in the nucleus. It has the same exact mass as a proton, but has no charge. Because it is neutral it is called a Neutron.

History of the Atom 1932 – Carl Anderson found evidence to back up the idea of anti-matter by discovering the positron. When and electron and a positron collide they disappear, and in their place are two photons of energy which speed away. Since this time, matter-antimatter methods of producing energy have been experimented with and used in science fiction.

History of the Atom 1933 – Enrico Fermi detects particles so small they can pass through most matter and leave no evidence of their visit. These particles are called neutrinos.

History of the Atom 1962 – Murry Gell – Mann while experimenting with bombarding atoms with extremely high energy, finds that atom breaks apart into mysterious new particles called “quarks”. He finds evidence of three different “flavors” of quarks that are called up, down, and strange. By putting these three quarks together in different arrangements scientists have been able to explain the over 100 different particles found in atom smashing labs and predict the discovery of new particles

History of the Atom 1987 – One of the newest theories, for which that is no evidence at this time, is called the superstring theory. It says the smallest particles of matter are actually tiny vibrating strings. They re believed to be over 100 billion billion times smaller than a proton. No one has ever directly seen an atom, so of course a proton has never been seen, much less one of the strings!

History of the Atom 2,000 and beyond – What are some of the new theories that are proposed about the structure of atoms? Atomic Age