How do people imagine what an atom looks like?

Good Morning,

Alchemists
The alchemists had no concept of atoms. They were generally trapped in Aristotle's worldview. And according to Aristotle, there is nothing indivisibly small (like an atom) in matter.

Before Aristotle, the Greek philosophers Leucippus and his student Democritus had assumed that all matter consisted of indivisible particles (atoms) of a primordial substance, each of which had a different shape. They assumed that things were the way they were depending on the number and composition of these differently shaped atoms.
But Aristotle contradicted this idea by pointing out that it may be that there is no known instrument capable of dividing matter further and further, but that one can always imagine that such an instrument must exist.
In doing so, he dismissed the notion of indivisibility and instead promoted the philosophy of the elements: fire, water, earth, and air. This idea remained the prevailing opinion until the 17th/18th century (modern era). Since the Age of Alchemy also lasted roughly until then, you can assume that the alchemists had no concept of atoms.

John Dalton
The old idea of ​​​​the indivisible small building blocks of matter (atoms) was then taken up again by the natural scientist John Dalton.
Around 1803, he put forward an atomic hypothesis (which he published in 1806) because, with the idea that there could be invisibly small, indivisible particles of matter, he was able to explain the three fundamental chemical laws known at that time (the "law of conservation of mass", the "law of constant proportions" and the "law of multiple proportions").
Dalton's key messages were:

• Every substance consists of tiny, massive and indivisible, spherical particles, the atoms.
• All atoms of a particular element have the same volume and mass.
• The atoms of different elements differ in their volume and mass.
• Atoms are indestructible. They can neither be destroyed nor created.
• In chemical reactions, the atoms of the starting materials are simply rearranged and bonded together in certain ratios.

Although almost all of these core statements have proven to be incorrect according to current knowledge, Dalton remains credited with having created the first truly usable atomic model with which certain facts could be well explained.

Depending on the context, Dalton's atomic model is still used today for certain interpretations (particle model).

Joseph Thomson
In 1903, approximately 100 years after the publication of Dalton's atomic hypothesis, Joseph Thomson published a slightly modified version of the atomic concept. This had become necessary because Thomson himself had discovered the electron six years earlier (1897), which could undoubtedly escape from the atoms of matter. To save Dalton's appropriate concept of atoms, Thomson claimed that atoms do contain electrons, but that these electrons are embedded in a positively charged primordial mass, just like raisins in cake batter ("raisin cake model").
This did raise the problem that atoms were obviously not indivisible (after all, there were obviously smaller particles in them, namely electrons), but Thomson solved this problem by (rightly) mentioning that the world of electrons was a completely different one and that, therefore, the atom could still be regarded as the smallest building block of matter (with the properties of matter).

Ernest Rutherford
The scattering experiments (1909) that Rutherford and his colleagues Marsden and Geiger carried out with radioactive material on various metal foils then fundamentally changed the idea of ​​​​the structure of atoms.
Before Rutherford's discovery in 1896, not much was known about radioactivity. Apparently, there were three forms: alpha , beta , and gamma radiation. And it was known that radioactive radiation could penetrate matter. Why, no one knew. It was also known that alpha radiation was positively charged particle radiation.
The situation before the scattering experiment was therefore as follows: If you shot a focused beam of alpha radiation at a very thin rolled out gold foil, then it was expected that the alpha rays would pass through the material (because radioactive radiation could do such a thing).
Later, however, Geiger reported to the astonished experiment leader Rutherford: "It turns out that alpha particles are sometimes deflected very strongly. About one in ten thousand is even reflected back, as if it had hit a solid obstacle."
Rutherford was so astonished that he remarked, "It was just as incredible as if they fired a 15-inch shell at a piece of tissue paper and it came back and hit you!"

This was inexplicable. Okay, it was known that radioactive radiation could somehow penetrate matter. But if that were the case, then all radiation should penetrate matter. Then there shouldn't be such a thing as deflected radiation particles. And certainly not reflected ones!
Another possibility would have been that the alpha particle beam would have shot a hole in the gold foil. But that wasn't the case either.

From the observations of the scattering experiment, Rutherford developed a completely new atomic model in 1911, namely the so-called “nucleus-shell model”.

According to this model, an atom is not a solid sphere (as in Dalton's or Thomson's atomic models). Rather, it consists of a tiny nucleus and a comparatively enormous atomic shell. The shell consists of nothing. Within the shell are negatively charged electrons.
The nucleus, however, is massive. It consists of positive charge and contains virtually all of the mass. The nucleus is about 10,000 times smaller than the shell.

Only with this new understanding of atomic structure was Rutherford able to explain not only why radioactive radiation can pass through matter at all, but also why some alpha rays are deflected and a few are even reflected.

Most alpha particles simply fly through the atomic layers because the atoms consist mostly of a shell of nothing.
However, when the positively charged alpha particles come close to the also positively charged atomic nucleus, the equally charged particles repel each other from the nuclei, so that these alpha particles are deflected to a greater or lesser extent.
However, if an alpha particle flies directly towards the nucleus of an atom, it actually bounces off it because the nucleus is not only positively charged but also massive.
Since the nucleus is about 10,000 times smaller than the shell, this only happens with about every 10,000th alpha particle.

You see, the core-shell model explains all observations surrounding the scattering experiment with radioactive radiation…

Incidentally, the question of why one can't simply dip one's finger into a tabletop when the atomic shell consists largely of nothing could be simply explained by the electrons in the shell. If all atoms have electrons in the shell made of nothing, then two atoms cannot penetrate each other because the like-charged electrons in the two atomic shells repel each other…

Niels Bohr
While the nucleus-shell model revealed the groundbreaking new idea that an atom is not a solid sphere, but consists mostly of nothing (and has only a positively charged massive nucleus), it said nothing further about the fine structure of the shell (or the nucleus).
Niels Bohr first did this with the shell two years later (1913). He claimed that the electrons in the shell weren't just randomly located, but rather orbited the atomic nucleus. This idea had the advantage of allowing Bohr to explain why the negatively charged electrons weren't attracted to the positively charged nucleus and ultimately crashed into it?! Using the circular orbit, Bohr was able to explain that the attractive force between the oppositely charged electrons and the nucleus was canceled out by the centrifugal force of the moving electrons.
This, however, presented a new problem. Physicists of that time were well aware that moving charge radiates energy. If that were the case, the electrons would also lose energy in their circular orbits. Then they would have to slow down, the centrifugal force would have to decrease, and the electrons would not be able to maintain their circular orbit. Following a spiral path, they would ultimately crash into the nucleus…

But Bohr overcame this problem by simply making two postulates (unproven claims):

1. The electrons can only move in very specific circular orbits (the so-called “allowed” orbits).
2. The electrons orbit on the permitted orbits without radiating energy.

When energy is supplied to electrons, this can only be achieved with very specific amounts of energy (quanta). When the energy level is reached, the electrons "jump" to the next permitted orbit. When they jump back to their original orbit shortly thereafter (quantum jump), they release the previously absorbed energy in the form of electromagnetic radiation.

The scientists of that time would surely have simply laughed at Niels Bohr for all these statements, if… yes, if he hadn't been able to precisely calculate the spectrum of hydrogen with the help of this model and even predicted spectral lines that were only actually discovered through the prediction. That was something! Some atomic physicist babbles about "permissible trajectories" along which charged electrons are supposedly allowed to "move radiation-free" (ie, utter nonsense), but with this nonsense, he can make calculations that are 100% consistent with reality!

This is how Bohr's atomic model was accepted, even though it was not supposed to be correct.

Nevertheless, the calculations were already incorrect for the next element (helium). The Bohr atomic model fails in general for all multi-electron systems (ie, for all other elements). It is only valid for hydrogen.

And yet, it is usually the atomic model that most people with a corresponding scientific education use to imagine the structure of an atom.

Later, Arnold Sommerfeld (1915/16) expanded Bohr's atomic model by allowing not only circular orbits but also elliptical orbits. This made the spectral line calculations of multi-electron systems somewhat more accurate, but they were never as precise as those for hydrogen.

So! Now you have a brief overview of the development of (classical) atomic models.

If you do it right, it's a very exciting and gripping topic.

Good luck with your presentation…

Greetings from the waterfront