CHAPTER 3:

Atoms: The Building Blocks of Matter

Section 2

• An atom is described as the smallest particle of an element that retains the chemical property of that element.

• One conclusion made by John Thomson that led to the development of current atomic theory was that electrons were found in all atoms.

• One conclusion made by Ernest Rutherford that led to the development of current atomic theory was most of the mass of an atom is found in the nucleus and that the nucleus occupies very little space within an atom.

Once conclusion made by Robert Millikan that led to the development of current atomic theory is that he confirmed the negative charge of the electron and suggested the approximate value of the electron’s mass.

• The cathode is connected to the negative terminal of the voltage source. (negative electrode)

• The Anode is connected to the positive terminal of the voltage source. (positive electrode)

• The cathode-ray tube is connected to a vacuum pump because the charges will only flow through gases at very low pressures. The tube containing gas at low pressures led scientists to conclude that the rays were composed of negative particles.

ParticleParticle Electric Electric ChargeCharge

Actual MassActual Mass

ElectronElectron -1-1 9.1 x 109.1 x 10-31-31

ProtonProton +1+1 1.673 x 10 1.673 x 10 -27-27

NeutronNeutron 00 1.675 x 101.675 x 10-27-27