Beginnings of chemistry

1803 - John Dalton - Atomic Theory

Matter is made up of indivisible atoms. All atoms of an element are identical. Atoms are neither created nor destroyed. Atoms of different elements have different weights and chemical properties. Atoms of different elements combine in simple whole numbers to form compounds 1897 - J.J. Thompson Found that cathode rays could be deflected by an electric field Showed that cathode "rays" were actually particles Found the charge to mass ratio of the particles to be approximately 108 Coulomb (C) per gram. Same charge to mass ratio regardless of metal used for cathode/anode or gas used to fill the tube. Conclusion: Particles were a universal component of matter. Electron - (originally called corpuscles by Thompson) particles given off by the cathode; fundamental unit of negative electricity Raisin Pudding Model - Matter is electrically neutral and electrons are much lighter than atoms. Conclusion: There must be positively charged particles which also must carry the mass of the atom. Dalton's model is now incorrect because atoms are divisible. Raisin Pudding Model

Studied absorption of radioactivity.

• Alpha radiation - positive charge - absorbed by a few hundredths of a cm or metal foil

• Beta radiation - negative charge - could pass through 100x as much foil before it was absorbed

• Gamma rays - no charge - could penetrate several cm of lead

  1932 - James Chadwick

  Proved that neutrons, neutral particles in the nucleus that made up approximately half the mass of an atom, did exist. Summary of Subatomic Particles Particle Symbol Charge Mass Electron e- -1 0.0005486 amu Proton p+ +1 1.007276 amu Neutron n 0 1.008665 amu

  Atomic Rules The number of protons in the nucleus of an atom is equal to the atomic number (Z). In a neutral atom, the number of electrons is equal to the number of protons. The mass number (M) of an atom is equal to the sum of the number of protons and neutrons in the nucleus. The number of neutrons is equal to the difference between the mass number (M) and the atomic number (Z).

  Atomic number: protons (and electrons if neutral) Mass number: protons + neutrons (neutrons = mass number - atomic number)

It matters that we know the structure of the atom because understanding the characteristics of atoms gives us information of how they are classified with other elements. Another reason why we should know what they are is because we are made up of them.

The atomic theory is still considered a theory because the atom is indivisible under a nuclear change. There is a theory for the atom called the Modern Atomic Theory which really only changes the Dalton's Atomic Theory from all matter is composed of very small indivisible particles called atoms to all matter is composed of tiny particles called atoms and each atom is made up of smaller subatomic particles called protons, neutrons, and electrons.