History Of the Atom

Question 1

TimeLine:

Answer 1

-Democritus (460 - 370BC)
-John Dalton (1803)
-Thomson (1897)
-Ernest Rutherford (1911)
-Niels Bhor (1913)
-James Chadwick (1932)

Question 2

Democritus proposed?

Answer 2

• Matter consisted of tiny indivisible particles with nothing between them.
• Called them “atomos”.

Question 3

Democritus Limitations?

Answer 3

• Lack of experimental evidence made it difficult to verify his theory.
• Later, sub-atomic particles were discovered, disproving his theory.

Question 4

John Dalton proposed?

Answer 4

• Elements are composed of extremely small particles called atoms.
• Atoms of same element are identical in mass, size, and chemical properties.
• Atoms cannot be destroyed or changed in a chemical reaction.
• Chemical reaction involves separation, combination or re-arrangement of atoms.
• Compounds are formed when atoms of more than one element combine in a specific ratio.

Question 5

John Daltons Limitations:

Answer 5

• Does not account for isotopes.

Question 6

JJ Thomson proposed?

Answer 6

• Discovered negative sub-atomic particles (electrons) through cathode ray experiment.
• These sub-atomic particles were 1/1000th of a hydrogen atom.
• Atom was a large positively charged sphere, with many small electrons imbedded inside. (plum pudding model)

Question 7

JJ Thomson Limitations?

Answer 7

• It was found that after Rutherfords gold foil experiment, the atom was more dense in the center containing most of its mass there rather than elsewhere…
• Also, the alpha particles shot by Rutherford were deflected by some degree, so it was later found that the electrons were not spread throughout the atom but rather set in orbitals around the nucleus.

Question 8

Cathode Ray Experiment:

Answer 8

Used a CRT… Cathode ray tube produced a cathode ray by a cathode (negative terminal)… These cathode rays was ejected into an evacuated tube, and would travel to the anode (postive terminal)… After experimenting, he found that these cathode rays were deflected by negatively charged plates, and attracted to positivel charged plates… He concluded that this ray was negatively charged… Also, it was found that it was 1/1000th the mass of a hydrogen atom… Hence, he concluded that it was a subatomic particle that was negatively charged…

Question 9

Ernest Rutherford Proposed?

Answer 9

• Atom consisted of a dense center called the nucleus that was mainly positive charge
• Discovered the proton by bombarding a nitrogen nuclei with alpha particle (4He2) which produced a +1 hydrogen atom.

Question 10

Gold foil experiment:

Answer 10

• shot alpha particles into a sheet of gold foil… Some were deflected and were detected by the surrounding sheet of zinc sulfur while others went straight through…

Question 10

Niels Bhor proposed?

Answer 10

• The planetary model
• Proposed that electrons orbit the nucleus in orbitals/energy shells at a fixed distance from the nucleus.
• The further away an orbital is from the nucleus, the higher the energy level for the orbital.
• If energy is added to an electron it can go to an “excited state”.
• Its ground state is its neutral or stable state.
• This transition from excited to ground causes a photon to be emitted.

Question 11

Niels bhor limitations:

Answer 11

• Electrons orbited in pairs of electrons rather than at random.
• They were packed closely in pairs and there were specific numbers of these pairs in each orbital/energy level.

Question 12

James Chadwick proposed:

Answer 12

• Experimental data showed that the protons made up only half the nucleus in relation to its mass
• Therefore, there must’ve been a neutral particle within the nucleus
• Discovered the neutron as a product of bombarding a beryllium atom with alpha particles.