Overview of Atomic Physics in 20th century

Today we know quite a lot about the structure of atom -- we can use atoms in nuclear power plants for getting a supply of energy.

Let us see what were the broad developments during last 2 centuries which took us through to this stage.

Between 1770 to 1870 a lot of quantitative research was done by measuring, with help of accurate machines, the weight, volume, specific density, temp, pressure, vapour density etc.
Prominent scientists who experimented were Lavoisier, Charles, Boyle, Dalton, Gay-Lusaac, Avogadro, Victor Meyer, Mendeleev etc.

Lavoisier stated the first version of the law of conservation of mass, identified and named oxygen (1778) and hydrogen (1783) and later showed that they combined to form water. Charles and Boyle gave two gas laws about how the volume of gases changed with respect to temperature and pressure. Concept of vapour density was a method to tell how much a certain volume of gas was heavier than the same volume of hydrogen.

We have dalton's atomic theory to state 5 principles namely
1. All elements are made of a smallest unit of particles called ATOMS.
2. All atoms of a given element are identical but not the atoms of different elements - they differ in weights.
3. Atoms of one element can combine with a fixed number of atoms of other element to form chemical compounds; a given compound always has the same relative numbers of types of atoms.
4. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.
It was observed that whenever hydrogen and oxygen combined to form water, the weights of two gases consumed was always in the ratio of 1:8, the excess weights remaining unutilised. This is what led to the clause 3 above.

Avogadro could postulate that "equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties". Dalton and Gay Lussac also tried to prepare list of all chemical elements arranged according to weight.

Victor Meyer developed an apparatus for quick and accurate measurement of vapour density -- which further told about molecular and atomic weight. Many elements were measured this way and their molecular weights were recorded. Mendeleev once again arranged all elements in according to their atomic weight but in an imaginative manner, he made use of their chemical property of valency too. He found that elements showed a periodicity of 8 valencies, so he arranged them in a table of 8 columns and several rows and assigned one cell to one element. Whenever there was a mismatch between atomic weight and valency, he was bold enough to treat valency as the right cue and predict some other explanation for the mismatch. This was a very useful concept and with few corrections, his table has now become the standard "Periodic Table".

Now we need to look at a totally different aspect. In 1855, Geissler invented the mercury displacement pump and achieved a record vacuum of about 10 Pa. A number of new properties of gases became observable at this vacuum level, leading in turn, to 3 important developments.Scientists rushed to take a long glass tube fitted inside with two metal-plate electrodes, create high vacuum inside and study the effect of high voltage-high vacuum. This apparatus came to be known as Cathode-ray tube.

We find Cathode-ray tube studies taking us to 3 new branches. First, there was discovery of X-rays in 1895 - it led to a new branch in physics for studying molecular structures, it revolutionized medical investigations and it won the very first Nobel Prize of Physics for Rontgen in 1901. second was construction of mass-spectrograph to accurately measure the atomic weights and third was development of vacuum tube which marked the beginning of electronics.

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