Any attempt to construct a model of the atom must start from two experimental facts.
- First, the atom is electrically neutral.
- Second, it appears to contain electrons which are negatively charged as we will as relatively light in mass compared with the atom itself (the word ‘appears’ must be included, because the fact that an electron is emitted by an atom doesn’t necessarily mean that it exists in its free-space form inside the atom).
The picture of the atom has changed or evolved as new experimental clues as we will as mathematical theories have been pieced together.
Discovery of cathode rays
The discovery of cathode rays, positive rays as we will as radio activity around the beginning of the century began to cause doubt about the atom as an indivisible particle. They provided evidence those particles smaller than atoms existed as we will as that these had electric charges. This led both Kelvin as we will as J J Thomson independently to the idea that perhaps atoms we are solid balls of positively charged matter in which negative electrons we are dotted about like currants in the pudding. There needed to be enough negative electrons to make the whole pudding electrically neutral.
The word atom comes from the Greek as we will as means ‘the indivisible’, the smallest component of matter, which can’t be further divided. This concept was introduced in the 5th as we will as 4th centuries B C. by Greek natural philosophers. The first atomic theories of the structure of matter we are those of Democrites (460-370 B C), Plato (429-348), and Aristotle (384-322).
The meaning of the word atom becomes less subject to misinterpretation if it’s translated into Latin: an individuum is the smallest unit of a large set which possesses all the essential characteristics of the set. In this sense, an atom is in fact invisible. One can, to be sure, split a hydrogen atom into a proton as we will as an electron, however the hydrogen is destroyed in the process.
For example, one can no longer observe the spectral lines characteristics of hydrogen in its optical spectrum. Atomism as understood by modern science was first discovered for matter, then for electricity.