Discovery of the Atom

The Philosophers No scientific tools thus scientific exploration was limited to the power of the mind and intellectual thought. Scholarly thinkers were known as philosophers who considered the mysteries of life. Many concluded that matter was composed of the four elements, earth, water, air and fire.

Democritus First person to propose the idea that matter was not infinitely divisible and was instead made up of tiny particles called atomos. Matter is composed of empty space through which atoms move Atoms are solid, homogeneous, indestructible and indivisible. Different kinds of atoms have different sizes and shapes. Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves. Could not prove or disprove his theories when challenged by Aristotle. Thus the existence of atoms was dismissed.

John Dalton A 19th century schoolteacher who brought about the beginning of the development of modern atomic theory. Revised Democritus’ ideas based on his scientific research.

John Dalton All matter is composed of extremely small particles called atoms. All atoms of a given element are identical, same size, mass, & chemical properties. Atoms cannot be created, divided, or destroyed. Different atoms combine in simple whole number ratios to form compounds. In chemical reactions, atoms are separated, combined or rearranged.

His theories explain the laws of conservation of mass, that atoms are merely rearranged, not created or destroyed. He was wrong about atoms being indivisible, and that all atoms of a given element having identical properties. (Atoms of an element may have slightly different masses).

William Crookes The cathode ray (Crookes’ tube) – tube filled with low pressure gas where a thin beam of electricity travels through. Caused small flashing lights – led to the invention of the television.

JJ Thomson Magnetic field shows distortion of the beam of negatively charged beam. Proved that subatomic particles exist

Electron: Negatively charged particles that are part of all matter. He was unable to directly measure the weight of the electron.

If you were to decide what the inside of an atom looks like given what Thomson discovered what model would you come up with?

Plum Pudding

Proposed “Plum pudding” model with matter containing evenly distributed positive and negative charges throughout.

Robert Millikan Successfully measured the weight of the electron = 9.11 x 10-28 or 1/1840 of the mass of a hydrogen atom.

Ernest Rutherford Directed alpha particles (radioactive substance) through a thin layer of gold foil. Particles were redirected proving the existence of a central nucleus.

Gold Foil Experiment

Nucleus: Tiny dense region located at the center of an atom. The typical atom’s diameter, which is defined by the volume of space through which the electrons move. If an atom had the diameter of two football fields, the nucleus would be the size of a nickel.

Image taken from: www.wired.com/.../news/2009/02/dayintech_0227 James Chadwick (1891 – 1974) Realized that the atomic mass of most elements was double the number of protons  discovery of the neutron in 1932 Worked on the Manhattan Project Worked with Ernest Rutherford Won a Nobel Prize Image taken from: www.wired.com/.../news/2009/02/dayintech_0227

What is in the Nucleus? Proton: Subatomic particle carrying a charge equal to but opposite of an electron. (+1) Neutron: Nearly equal mass to a proton but has no charge.

Properties of subatomic particles Symbol Location Electrical charge Relative mass Actual mass (grams) Electron e- In the space surrounding the nucleus -1 1/1840 9.11x10-28 Proton p+ In the nucleus +1 1 1.673x10-24 Neutron n0 1.675x10-24