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By using his gold foil experiment, he demonstrated that the atom has a tiny and heavy nucleus. He designed this experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure.

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After using a cathode ray tube for his experiment, he proposed that atoms consist of negatively  charged particles called electrons. The electrons were embedded in a positively-charged sphere. His model for the electrons famously became known as the "Plum Pudding Model".

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Democritus stated that, “the universe is composed of two elements: the atoms and the void in which they exist and move.” According to him atoms were minuscule quantities of matter.

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Curie discovered the radioactive elements radium and polonium. By finding that radioactive atoms decompose into smaller particles, it also explained that atoms are made up of small particles.

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He discovered that the nucleus contains neutrons and theorized that neutrons have no electrical charge. Neutrons have a mass equal to the mass of a proton. He also determined that the unit of measurement for subatomic particles is the atomic mass unit (amu).

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Bohr believed Rutherford's prediction was correct, but it wasn't complete. He proposed electrons could only move between energy levels, rather then being able to move everywhere.
As the radius of the orbit decreases, the orbit becomes closer to the nucleus and the energy lowers.

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Dalton was the first scientist to put down the basic ideas for the modern atom. The main points are:
- Elements are made of extremely small particles called atoms.
- Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties.
- Atoms cannot be subdivided, created or destroyed.
- Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
- In chemical reactions, atoms are combined, separated or rearranged.