We know that all dense materials emit electromagnetic radiation at all temperatures, in which radiation of several wavelengths is present. Which we get as spectrum.
Initially it was understood that this radiation is caused by oscillations of atoms and molecules, which is controlled by the interaction of atoms and molecules near them. But later it was discovered that even rarer gas emits radiation from the low pressure (where the gap between atoms are very large).
Hence the radiation emitted is not from the interaction between atoms and molecules, but from the single atom. It was proved in the early nineteenth century that there is a fixed spectrum of radiation emitted from each element, which greatly helped in understanding the internal structure of the atom. In 1885, John Jacob Balmar derived a simple empirical formula for the frequency of a group of lines emitted from hydrogen atom.
Ernest Rutherford, a former student of J J Thomson, conducted an experiment related to the scattering of alpha particles to find out the structure of an atom. After some time in 1911, Hans Geiger (1882-1945) and Ernst Marsden (1889-1970) did this experiment. The explanation of its results led to the nuclear model of the atom. But this experiment did not tell about the discrete pattern of the spectrum. A few more experiments later revealed this.
A TIMELINE OF ATOMIC MODELS
1. Atomic Model (1808) – John Dalton
- Matter is made of small indivisible atoms
- Atoms can’t be subdivided, created or destroyed
- Atoms of the same elements have the same property
- Atoms of the different elements have different properties
- Atoms of different elements can form compound
2. Plum Pudding Model (1904) – J.J.Thomson
(J.J Thomson was a physicist who is credited for discovering the electron. He used his research on cathode ray tube technology in this discovery)
- An atom is electrically neutral. It has no charge
- In an atom, magnitude of both +ve charges and -ve charges are equal.
- An atom is made out of a sphere of +ve charges with -vely charged electron embedded in it.
3. Nuclear Model (1911) – Ernest Rutherford
- Atom are mostly empty space.
- Most of the mass is concentrated in the center of atom. This tiny dense +vely charged core is called a nucleus.
- Electrons are revolve around the nucleus.
4. Planetary Model (1913) – Niels Bohr
(Niels Bohr agreed with the planetary model of the atom, but also knew that it had a few flaws. Using his knowledge of energy and quantum physics he was able to perfect Rutherford’s model. He was able to answer why the electrons did not collapse into the nucleus.)
- All electrons are revolve in orbit(energy level) around the nucleus.
- All orbits have specific size and energy.
- Radiation is when an electron moves from one level to another
5. Quantum Mechanical Model (1926) – Erwin Schrodinger
(Erwin Schrödinger was a revolutionary physicist who used Heisenberg’s uncertainty principle to come up with the atomic model that we still use today)
- Electrons do not move around the nucleus in orbits.
- electrons exist in specific energy level as a cloud.
- The electron cloud is the region of _ve charges, which surrounds the nucleus.
- Orbital is the region with a high probability of containing electrons.
| Introduction | Alpha Particle Scattering and Rutherford’s Nuclear Model of Atom |
| Atomic Spectra | Bohr Model of the Hydrogen Atom |
| The Line Spectra of the Hydrogen Atom | De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation |