History of Atomic Theory Flashcards
Which Greek philosopher discovered Atomic Theory?
Democritus (460-370 BCE)
What was John Dalton’s discovery?
How did he make this discovery?
- Supposed that all matter was made up of invisible atoms with distinct properties and created 3 laws on atomic theory
- Developed modern atomic theory
He conducted a gas experiment to make this discovery
What is the Law of Conservation of Mass? (1)
Who does it relate to?
Mass cannot be created nor destroyed
Relates to John Dalton
What is the Law of Constant Composition? (2)
Who does it relate to?
In any chemical compound, all samples (of that compound) will be made up of the same elements
Relates to John Dalton
What is the Law of Multiple Proportions? (3)
Who does it relate to?
Elements combine in simple, whole-number ratios to form chemical compounds
Relates to John Dalton
What was John Dalton’s experiment?
He investigated pressure/properties of gases, inferring that gases must exist of tiny particles in constant motion, with more than one element
What are the 5 points of Dalton’s Atomic Theory?
- All matter is made up of atoms
- Atoms of a given element has unique properties
- Atoms cannot be created or destroyed
- Atoms of different elements combine to form chemical compounds
- During chemical reactions, atoms are rearranged
What was J.J Thomson’s discovery?
Discovered atoms were made of smaller components
How did J.J Thompson make his discovery?
Through the “Cathode Ray Experiment”:
- Shot a cathode ray from negative cathode to positive anode
- Electrically charged plates were brought near the cathode ray path
- Positively charged plate attracted the cathode ray (negatively charged repelled)
- A magnet was brought to the ray, producing similar results
What did the Cathode Ray Experiment allow Thomson to hypothesize?
Hypothesized that the cathode ray was a stream of negatively charged particles (electrons)
What was J.J Thompson’s model of the atom?
The “Plum Pudding” model:
- Atom is a positively charged sphere
- Positively charged particles within the sphere
- Negatively charged electrons within the sphere
What was Ernest Rutherford’s discovery?
What is Rutherford also considered as?
The “Rutherford” or “Nuclear” Model:
- Discovered that atoms contained a dense, positively-charged, central region (the nucleus)
- Nucleus = 99.9% of an atom’s mass
- Diameter = 0.01% of an atom
The “Father of Modern Physics”
How did Rutherford make his discovery?
Through his “Gold Foil Experiment”:
- Directed a beam of positively charged particles towards gold foil (surrounded by a screen that flashed when hit)
- Particles shot by the beam got deflected back at the particle source due to the nucleus blocking it
- Concluded that the nucleus was larger than the electrons, causing blockage
What was Rutherford’s model of the atom?
The “Rutherford” or “Nuclear” Model:
- Positively charged, dense center (nucleus)
- Surrounded by negatively charged electrons
- Didn’t have fixed shells like Bohr’s, electron orbits overlapped
What was Niels Bohr’s discovery?
- Claimed that electrons orbit the nucleus of an atom: “Planetary” or “Bohr” model
- The greater the energy an electron has, the further it will be from the nucleus
- Electrons remain in fixed orbits
