By Don Lincoln, Ph.D., University of Notre Dame
Since the atom was discovered, many theories tried to depict what an atom is like. They have likened it to a plum pudding, a small ball, and even a tiny solar system. Perhaps, it is also imagined as a core with a cloud of small and light particles surrounding it.
Atoms build everything in this world. Since the concept was revealed, scientists have tried to depict what an atom is like and what is inside it. The hypothetical image of the atom began to change and become more real when the particles inside it were discovered.
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The Plum Pudding Model
The first image of the atom was called the plum pudding model. In 1897, a British physicist called J. J. Thomson discovered a sub-particle for the atom and called it a corpuscle. Today, this particle is called an electron. He believed that the mass of this particle was about 1/2000 that of a hydrogen atom, carrying a negative electric charge. However, he knew that the atom has no electric charge. Thus, he concluded that there were some positively charged particles as well.
Thomson then decided that the atom was like a pudding with a positive charge, and the electrons were hard particles with the negative charge, scattered in the pudding. Hence, the atom in this model looked like chocolate chip dough. This model did not last that long.
Ernest Rutherford’s Experiment
In 1911, Ernest Rutherford and two of his fellow researchers conducted an experiment that proved the plum pudding model wrong. They shot alpha radiations at a thin gold foil to see how the particles were scattered. Despite their expectation, not all the alpha particles passed through the pudding, and some even bounced back. It was as strange as shooting a heavy cannonball to a tissue paper and watching it bounce back!
Rutherford then decided that the positive charge must be concentrated at the center of the atom, and not scattered all over the place. He also concluded that the electrons must be spinning around the cores, at considerable distances. Thus, the foundation of the modern atom models was formed.
This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on Wondrium.
The Bohr Model
Based on Rutherford’s conclusions, Danish physicist Niels Bohr introduced the Bohr model. The atom in this model is considered as a solar system. Long before Rutherford, Japanese physicist Hantaro Nagaoka likened atoms to Saturn. He believed the electrons act like the rings. However, Rutherford’s idea and the Bohr model were both original, as it is very unlikely that either of them had read Nagaoka’s writings. In the model, much of the atom is empty space.
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The Empty Space in the Atom
Almost all the atom is empty space. Rutherford showed that the atom nucleus is about 1/10,000th the size of the atom. Thus, a nucleus is 10-14m in diameter. Considering both the nucleus and the atom spherical, one can calculate the volume through 4/3 times pi times the radius cubed. Thus, the nucleus takes up one trillionth of the entire volume of the atom!
If we imagine the nucleus as a marble with a one-centimeter radius, the atom radius has to be around 100 meters. A football field with the marble in the middle is the imaginable size for the atom. The electrons orbit the marble at specific distances within the stadium.
Electrons are about 1/2000 the size of a proton. Hence, if the nucleus is the marble, the electron radius would be almost equal to the cross-section of a human hair. This means even in the scale of the football field, electrons are almost impossible to spot.
What is the Atom Like?
The atom looks like a tiny solar system with an extremely dense and heavy nucleus in the center, while electrons are orbiting it. The Bohr model was close to reality, but it is the simplest way to imagine an atom.
Considering a carbon atom as a football field and the nucleus as the marble, one can imagine six electrons would be zooming around in the stadium. Thus, most of the stadium, i.e., the atom, is empty space. The image becomes even more peculiar when one realizes that electrons are simultaneously everywhere.
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Electrons in the Atom
Electrons are not tiny balls swirling in the atom in their specific circular orbits. As far as the laws of quantum mechanics allow, electrons are simultaneously everywhere in the atom. This does not disturb the empty space, as each electron has its ‘spherical cloud’ and the distance from the nucleus. The electron orbits are called S, P, D, F, and G based on their complexity.
Conclusively, the atom is almost like Bohr said, but with much more complexity and considerable empty space.
Common Questions about What an Atom Is Like
Atoms build everything in our world. Previously, it was assumed that atoms are like small balls, but today the definitions differ. The atom is made up of three main particles, i.e., protons, neutrons, and electrons.
All materials are made of atoms. Thus, atoms are like the building blocks of everything and can combine to make new substances. Hydrogen (H) is one of the most common atoms.
An atom looks like a very small solar system, with the heavy nucleus in the center and the electrons orbiting it. However, the electrons are in layers and can be simultaneously everywhere that quantum allows.
Atoms are like extremely small bricks building any material. An atom is too small to be seen by the human eye.