History & Development of the Atomic Model Dr. Knorr Honors & AP Chemistry

John Dalton 1803 Elements are made of tiny, indivisible particles called atoms. All atoms of a given element are identical. The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative atomic weights.

Dalton’s Atomic Model 1803 Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.

Sir Joseph John “J.J.” Thomson 1897 Discovered the electron through a series of cathode ray experiments Rays were deflected in an electric field and made of “corpuscles” Also discovered charge to mass ratio of electron

Thomson’s Setup 1897 Cathode ray tube and applied external magnetic field.

Thomson’s Setup 1897 Cathode ray tube and applied external electric field – Note opposite direction as magnetic field

Thomson’s Setup 1897 Cathode ray tube with both fields applied – Effects cancel each other.

Thomson’s Atomic Model 1897 Atoms are divisible, with corpuscles (aka electrons) being the building blocks Atoms are neutral because corpuscles are uniformly distributed in a sea of positive charge: “Plum pudding” (think of chocolate chip cookie dough)

These are some points to be made about the experiment: 1. The two plates were 16 mm across, "correct to about .01 mm." 2. The hole bored in the top plate was very small. 3. The space between the plates was illuminated with a powerful beam of light. 4. He sprayed oil ("the highest grade of clock oil") with an atomizer that made drops one ten-thousandth of an inch in diameter. 5. One drop of oil would make it through the hole. 6. The plates were charged with 5,000 volts. 7. It took a drop with no charge about 30 seconds to fall across the opening between the plates. 8. He exposed the droplet to radiation while it was falling, which stripped electrons off. 9. The droplet would slow in its fall. The drops were too small to see. What he saw was a shining point of light. 10. By adjusting the current, he could freeze the drop in place and hold it there for hours. He could also make the drop move up and down many times. 11. Since the rate of ascent (or descent) was critical, he has a highly accurate scale inscribed onto the telescope used for droplet observation and he used a highly accurate clock, "which read to 0.002 second."

Ernest Rutherford 1911 A very small, positively charged nucleus is orbited by electrons Most of the mass of an atom is contained in the nucleus

Rutherford’s Setup 1911

Rutherford’s Results Expected Actual Figure 2.13 (a) & (b)

Atomic Nucleus

Niels Bohr 1913 The electrons can only travel in special orbits: at a certain discrete set of distances from the nucleus with specific energies Each orbit has a fixed energy called an “energy level” (like rungs on a ladder)

Bohr’s Atomic Model 1913 Energy levels in an atom are not evenly spaced Orbitals that are farther from the nucleus have higher energy Change orbit by gaining (or losing) energy

Robert Andrews Millikan 1916 Determined charge and mass of a single electron through a series of oil drop experiments All electrons have the same charge

Millikan’s Setup 1916 He sprayed oil with an atomizer, which made very tiny drops. The hole bored in the top plate was very small, so that only one drop would make it through the hole. He exposed the droplet to radiation while it was falling, which stripped electrons off. By adjusting the current, he could freeze the drop in place and hold it there for hours.

Other Notable Discoveries 1886: Eugen Goldstein discovers protons. 1904: Hantaro Nagaoka suggests a planetary model of the atom: electrons orbit about a central nucleus. 1932: James Chadwick confirms the existence of the neutron.