- How and why the atomic model has changed over time.
- We revise or replace scientists theories by ones in the light or new evidence.
Early ideas about atoms:
The ancient Greeks were the first to have ideas about particles and atoms. however, it was not until the early 1800s that these ideas became linked to strong experimental evidence when John Dalton put forward his idea about atom. From his experiments, he suggested that substances were made up of atoms. His theory is given below.
Dalton’s atomic theory:
John Dalton developed an atomic theory in the 1800s. He did experiments, worked out some atomic weights and invented symbols for atoms and molecules. His most important conclusions are summarised below:
- All matter is made of atoms
- Atoms cannot be broken down into anything simpler
- All the atoms of a particular element are identical to each other and different from the atoms of other elements
- Atoms are rearranged in a chemical reaction
- Compounds are formed when two or more different kinds of atoms join together
Dalton’s theory was developed and changed as new evidence was discovered.
Evidence for electrons in atoms:
At the end of the 1800s, a scientist called J.J. Thomson discovered the electron. this is a tiny, negatively charged particle that was found to have a mass about 2000 time smaller than the lightest atom. Thomson was experimenting by applying high voltages to gases at low pressure as shown in figure below.
J.J. Thomson’s discovery of the electron:
- J.J. Thompson discovered the electron in 1897.
- This showed that the atom contained smaller pieces, whereas Dalton had thought that atoms could not be broken down into anything simpler.
Evidence for the nucleus:
The next break through in understanding the atom came about 10 years later. Geiger and Marsden were doing an experiment with radioactive particles. They were firing dense, positively charged particles. They expected the particles to pass straight through the gold atoms with their diffuse cloud of positive charge.
Their results were used to suggest a new model for the atom. Rutherford suggested that Thomson’s atomic model was not possible. the positive charge must be concentrated at a tiny spot in the center of the atom. Otherwise the large, positive particles fired at the foil could never be repelled back towards their source. It was proposed that the electrons must be orbiting around this nucleus, which contains very dense positively charged protons.
Rutherford’s nuclear atom:
- In 1911 Ernest Rutherford used experimental evidence to show that, an atom must contain a central nucleus.
- This was further evidence that an atom contained smaller pieces.
Evidence for electrons in shell (energy levels):
The next important development came in 1914, when Niels Bohr revised the atomic model again. He noticed that the light given out when atoms were heated only had specific amounts of energy. He suggested that the electrons must be orbiting the nucleus at set distances, in certain fixed energy levels. The energy must be given out when excited electrons fall from a high to a low energy level. Bohr matched his model to energy values observed.
Bohr’s electron orbits: