Unit 4

Ernest Rutherford

(1871-1937), British physicist, is considered the father of nuclear physics. Developed the planetary model of the atom (nuclear atom) after the "gold foil" experiments. Particles named by him include the alpha particle, beta particle and proton.

Sir James Chadwick

(1891-1974), English physicist, worked with Rutherford on radioactivity. Discovered protons. Chadwick's work showed that the nucleus also contained another subatomic particle, a neutral particle called the neutron. A neutron has a mass nearly equal to that of a proton, but it carries no electrical charge. The nucleus was found to be composed of two kinds of particles: o Some of these particles are called protons Charge = +1 Mass is about the same as a hydrogen atom o The other particle is called a neutron Has no charge Has a mass slightly more than a proton

Dalton's Atomic Theory

All matter is composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass, and chemical properties. Atoms cannot be created, divided into smaller particles, or destroyed. Different atoms combine in simple whole number ratios to form compounds. In a chemical reaction, atoms are separated, combined, or rearranged. Note: Some of Dalton's ideas had to be revised as additional information was learned: o Atoms are divisible into subatomic particles. o Also, atoms of an element may have slightly different masses (isotopes).

Aristotle

Aristotle was an influential Greek philosopher who lived between 384-322 B.C. He was considered one of the greatest thinkers of the world and his written works encompassed all major areas of thought: logic, science, metaphysics, ethics, and politics. Aristotle's Influence Many philosophers criticized Democritus's ideas because he could not explain what holds atoms together. Aristotle rejected Democritus's concept of the atom, as it did not agree with his own ideas on nature! Aristotle mistakenly believed matter was made up of only four elements: earth, water, air, and fire. Because of his influence, Aristotle's concept of matter went unchallenged for almost 2000 years!

Democritus

Greek Philosophers: Can matter be subdivided into fundamental particles? Democritus (460-370 B.C.): All matter can be divided into indivisible atomos. "Atomos" is Greek for indivisible Democritus believed that everything in the universe was composed of atoms, which were themselves microscopic and indestructible.

Robert Millikan

In 1909, American physicist ________ determined the charge and mass of an electron, using the "oil-drop method." Oil-Drop Experiment ____ determined the charge on one electron to be 1.60 x 10-19 Coulombs (defined as a charge of −1.) Knowing the charge-to-mass ratio of the electron, he calculated the mass of the electron to be 9.109 x 10-31 kg. 1/1840 mass of a hydrogen atom!

JJ Thomson

J. J. Thomson, a Professor of Physics at the University of Cambridge in England, conducted a series of cathode ray tube experiments in the late 1890s to determine the negatively charged particle's mass. He concluded that the mass of the charged particle was much less than that of a hydrogen atom, thelightest of atoms This meant that there were particles smaller than the atom! Thomson had identified the first subatomic particle - the electron.

John Dalton

The concept of the atom was revived in the 18th century, but no significant advances were to occur for 100 years. John Dalton (1766 - 1844), a teacher in England, revised Democritus's ideas based upon the results of scientific research he conducted.