Chapter 2.3: The Observations that led to the Nuclear Atom Model

Flashcards from Chapter 2.3 of Chemistry: The Molecular Nature of Matter and Change.

Electric charge

While it was known this existed, scientists for many years did not understand their composition, only their effects

Cathode rays

Observed as part of an experiment to investigate current; with a light-sensitive phosphor coated end, a ray could be seen when the electric power source was turned on

• Gave way to naming of electrons, as it was recognized that these particles are found in all matter

Charge-mass ratio

Ratio found in 1897 by J. J. Thomson stating that the electron was about 2000 times lighter than hydrogen via a cathode ray tube, giving -1.76 × 10⁸ coulombs per gram

• Went against theorem that atoms could contain smaller particles

Charge

Discovered via Robert Millikan’s oil drop experiment, where ionized oil droplets falling in between two variably electrically-charged plates allowed him to measure the charge based on their rate of fall, giving -1.602 × 10^-19 coulombs

• Was used to solve for mass using the charge/mass ratio, giving 9.10 × 10^-28 grams

Plum-pudding model

Model proposed by J. J. Thomson for a spherical, positively-charged sphere of mater with electrons embedded within it

• Tested by Ernst Rutherford by firing alpha particles at a gold foil, expecting straight-pass through with this model

• Actually found that atoms deflected the particles, thus reasoning something small and dense in the atoms was the cause — the nucleus

Nucleus

Discovered by Ernst Rutherford after his gold-foil experiment, concluding that an atom was mostly space occupied by electrons and a tiny nucleus region with all the positive charge of the atom

• Also proposed existence of positively-charged protons

• Later modified in 1932 by James Chadwick, who discovered the neutral neutron here