The Modern Concept of the Atom

I am going to discuss the modern concept of the atom. I am going to give a brief history of the atomic model including Thomson's atom, Rutherford's atom, Bohr's atom, and SchrÐ"¶dinger's atom. I am going to include a diagram for each of these models.

In 1897 physicist, J.J. Thomson, shortly after discovering electrons, proposed his Plum-Pudding model of the atom. He based this model on the two facts that he knew at the time: 1. atoms contain small negatively charged particles called electrons and 2. atoms behave as if they have no charge at all, meaning they are electrically neutral. Thomson assumed there must be something in an atom that was positively charged that would neutralize the electrons negative charge. His proposal was a model for the atom that included a cloud of positive electricity in which the negatively charged electrons were embedded. His model is known as the Plum-Pudding Model of the atom. However, after an experiment performed by physicist Ernest Rutherford in 1909, Thomson's Plum-Pudding Model would become outdated.

Ernest Rutherford studied alpha particles or small pieces of positively charged matter. In an experiment, he fired alpha particles at a piece of gold foil, with a zinc sulfide screen behind it that would glow revealing the path of the alpha particle. He expected the alpha particles to pass right through the gold atoms, because the electrons of the gold atoms were too small to cause resistance to the more massive alpha particles. What actually happened was some of the alpha particles did not pass right through; they deflected because they hit something. Rutherford then realized there must be something small but massive inside each gold atom that did offer resistance to the alpha particles. Rutherford concluded that atoms were mostly empty space with something inside of them that was small but massive with a positive charge. He called it the nucleus. His model consisted of a spherical atom with a tiny dense nucleus and the rest of the atom was composed of electrons moving around the nucleus in some way. He assumed they had to keep moving because if they stopped they would be sucked in to the dense

positive charged nucleus but how were they moving? This problem would lead to a revolution in physics.

Classical physics states that objects can have any amount of energy and that works well for large objects. However, the problem Rutherford was facing was how very small electrons moved. In 1913 physicist Niels Bohr proposed an answer to the electron movement problem. He stated that the electrons energies were quantized. This meant that electrons had particular discrete amounts of energy. Bohr stated that electrons moved around the nucleus in circular orbits. He proposed there was only a certain amount of orbits, or shells at fixed distances from the nucleus and he assigned each shell a principle quantum number that determined its size and energy. This was revolutionary because by imposing these restrictions he was stating that the electrons could only have particular energies. In addition, electrons in the shells closest to the nucleus had lower energy levels because