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Development of the Model of the Atom
As technology improved and we discovered more information about particles, the model of what we think an atom looks like has changed significantly over time.
JOHN DALTON
In the early 19th century, JOHN DALTON proposed that atoms were solid SPHERES, and different spheres represented different ELEMENTS.
JJ THOMSON
By 1897, JJ. THOMSON had discovered ELECTRONS through experiments, leading to the "PLUM PUDDING MODEL", which showed the atom as a SPHERE of POSITIVE charge with ELECTRONS EMBEDDED within it.
ERNEST RUTHERFORD
- ERNEST RUTHERFORD and his student Ernest Marsden conducted the ALPHA PARTICLE SCATTERING experiment, which disproved the Plum Pudding model.
- During this experiment, they fired ALPHA PARTICLES at a GOLD FOIL.
- The gold foil was so thin, that they could assume that it was ONE ATOM THICK.
- TWO main observations were made during this experiment which led to TWO discoveries about the atom.
- They observed that most alpha particles passed straight through the gold foil, but some were deflected more than expected. Suggesting a small, dense, positively charged nucleus at the centre of the atom.
- The Plum Pudding model does NOT explain the results because it shows the whole atom as a ball of positive charge with NO EMPTY SPACE.
- Rutherford's findings led to the conclusion that the atom consists mostly of empty space, with a dense nucleus surrounded by a cloud of electrons.
NIELS BOHR
- NIELS BOHR refined the nuclear model by introducing the idea that ELECTRONS ORBIT the nucleus in fixed SHELLS.
- Bohr's model suggested that electrons could only occupy certain orbits or ENERGY LEVELS, at FIXED DISTANCES from the nucleus.
- His theoretical calculations agreed with his experimental observations which confirmed his model.
- Bohr carried out further experiments which revealed the nucleus could be SUBDIVIDED into a whole number of smaller particles, each particle having the same amount of POSITIVE charge. These were called PROTONS.
JAMES CHADWICK
- 20 years later, JAMES CHADWICK later discovered NEUTRONS, neutral particles in the nucleus, solidifying the nuclear model close to what is accepted today.
Development of the Model of the Atom
As technology improved and we discovered more information about particles, the model of what we think an atom looks like has changed significantly over time.
JOHN DALTON
In the early 19th century, JOHN DALTON proposed that atoms were solid SPHERES, and different spheres represented different ELEMENTS.
JJ THOMSON
By 1897, JJ. THOMSON had discovered ELECTRONS through experiments, leading to the "PLUM PUDDING MODEL", which showed the atom as a SPHERE of POSITIVE charge with ELECTRONS EMBEDDED within it.
ERNEST RUTHERFORD
- ERNEST RUTHERFORD and his student Ernest Marsden conducted the ALPHA PARTICLE SCATTERING experiment, which disproved the Plum Pudding model.
- During this experiment, they fired ALPHA PARTICLES at a GOLD FOIL.
- The gold foil was so thin, that they could assume that it was ONE ATOM THICK.
- TWO main observations were made during this experiment which led to TWO discoveries about the atom.
- They observed that most alpha particles passed straight through the gold foil, but some were deflected more than expected. Suggesting a small, dense, positively charged nucleus at the centre of the atom.
- The Plum Pudding model does NOT explain the results because it shows the whole atom as a ball of positive charge with NO EMPTY SPACE.
- Rutherford's findings led to the conclusion that the atom consists mostly of empty space, with a dense nucleus surrounded by a cloud of electrons.
NIELS BOHR
- NIELS BOHR refined the nuclear model by introducing the idea that ELECTRONS ORBIT the nucleus in fixed SHELLS.
- Bohr's model suggested that electrons could only occupy certain orbits or ENERGY LEVELS, at FIXED DISTANCES from the nucleus.
- His theoretical calculations agreed with his experimental observations which confirmed his model.
- Bohr carried out further experiments which revealed the nucleus could be SUBDIVIDED into a whole number of smaller particles, each particle having the same amount of POSITIVE charge. These were called PROTONS.
JAMES CHADWICK
- 20 years later, JAMES CHADWICK later discovered NEUTRONS, neutral particles in the nucleus, solidifying the nuclear model close to what is accepted today.
