In fact, by scientific methodology, I meant more about uncertainty and significant figures. Whatever let’s start by the different atomic model through history.
1.1 : Atoms and their representations through history
A long time ago, in Ancient Greece, many philosophers were arguing about the constitution of matter. The most two important ones were of Aristotle (-384 to -322) and of Democritus (-460 to -370). Aristotle thought that matter was composed of four elements : fire, earth, water and air. Depending on the properties of an object, that object could be made of one or many of these elements. See this link for clarification :
https://en.wikipedia.org/wiki/Aristotle#Five_elements . On the other hand, Democritus thought that matter was composed of tiny and undividable particles called “atoms”. However, Aristotle’s theory, even though not true, was more popular than Democritus’, for there was no way any of these men could prove their theory.
A. Dalton’s atomic model
Before I start, I have to tell you about a very important, yet basic law in chemistry (Antoine de Lavoisier’s law of matter conservation) : matter cannot be created nor destroyed; it can only be transformed. For instance, if water isn’t present at the beginning of a chemical reaction, it can’t appear at the end of the reaction. Remember this law, whatever let’s continue. Chemistry was a very popular science in 18th century and many experiments and laws were established in that same century. That’s when, in 1808, an english scientific and professor named John Dalton (1766 to 1844) established a theory based on those laws and on Democritus’ idea (matter is composed of atoms). Here’s Dalton’s theory :
- Matter is composed of very small and chemically undividable particles called “atoms”.
- All atoms of a same element have the same properties such as mass, size and else.
- All atoms that are of different elements don’t have the same properties.
- Atoms can be bound together to form chemical compounds depending on certain proportions.
- Chemical reactions produce new compounds but cannot create or destroy elements.
John Dalton thought that atoms look like spheres that are undividable and solid.
B. Thomson’s atomic model
It is recommended that you watch this video from at least at 0:40 because the beginning is a spoiler for my text.
In 1897, Joseph John Thomson, an english physicist, was studying cathode rays and discovered another particle : the electron. He supposed that electrons were negatively charged particles that were part of the atom and that could easily be unbound from it to go on their separate ways, which meant that atoms couldn’t be undividable.
Here, you can see Thomson’s atomic model represented by a huge positively charged particle covered by tiny negatively charged ones.
Here is a funny way to represent this model. You can imagine as a muffin filled with raisins.
C. Rutherford’s atomic model
In the 1900s, scientists have discovered that in radioactive rays are distinguished by three kinds of rays (omg, hl3 confirmed) : alpha rays (positive), beta rays (negative) and gamma rays (electrically neutral). In 1911, Ernest Rutherford, a new-zealandish physicist, was trying to find the location of electrons inside the atom. Thus, he decided to shoot an alpha beam at a thin sheet of gold and thought that it would cross it and that only some rays would slightly be deviated. Nonetheless, some rays were entirely reflected and he concluded that, considering that alpha particles are positively charged and that they were repulsed, that the positive charge of the atom comes from one smaller particle : the proton. The proton is a tiny positively charged particle that constitutes the atom’s nucleus. Also, because the atom is electrically neutral, Rutherford supposed that there were as many electrons as protons.
Here is a simplified image of the experiment.
Here is an image illustrating Rutherford’s atomic model.
D. Rutherford and Bohr’s atomic model
E. Simplified atomic model
F. Lewis’ notation
