It was known that electrons are outside the nucleus but how the electrons were arranged in an atom was still a mystery. The atom is 10,000 times as large as the nucleus and is mostly empty space. All of the positive charge and essentially all of the mass of the atom are in its nucleus. Rutherford’s model of the atom is therefore, called the nuclear modelof the atom. Rutherford, using Coulomb’s law and Newton’s laws found that the results could be explained only if all the positive charge of the atom were concentrated in a tiny, central core, now called the nucleus. In fact, some alpha were bounced almost straight backward by the foil. While the great majority of the alpha particles did pass straight through the foil with no deflection, to everyone’s surprise, some alpha particles were deflected.

This was because the alpha particles were known to be very dense and, due to the “plum pudding” model of the atom where the mass of the atom was spread out evenly over the volume of the atom, the atoms of gold were thought to have very low density. It was thought that all of the alpha particles would pass straight through the gold foil with no deflection. The alpha particles were fired at a very thin sheet of gold foil. Rutherford used these alpha particles and the zinc sulfide coated screen in his gold foil experiment. These alpha particles would be detected by a zinc sulfide coated screen (scintillation counter) that emitted a small flash of light every time an alpha particle hit it. In 1896, Henri Becquerel discovered that uranium compounds emitted penetrating rays, some of which were massive, high speed, positively charged particles which were later named alpha particles (α). The mass of the atom divided by the volume of the atom and assuming an even distribution of mass resulted in a low density for the atom. In the Thomson model of the atom, the density of the atom was necessarily small. The next big step in the development of the model of the atom occurred in 1911 with Rutherford’s gold foil experiment. This was the so-called “plum-pudding” model with the positive charge playing the role of the pudding and the electrons playing the role of the plums. Thomson in 1903, the atom was represented as a positively charged sphere with the negatively charged electrons distributed around the exterior. Now that it was known that the atom had component parts, a new model was needed. His periodic table remains a major idea in chemistry today. In 1869, Dmitri Mendeleev ascertained that there were groups of elements which had the same (or very similar) chemical properties, the same valence, and similar physical properties and showed that the chemical and physical properties of the elements were periodically repeated. Dalton’s theory underwent a number of modifications in the next 150 years but many of its ideas are still part of the modern atomic theory.

It did convince a number of chemists right away but several decades were required for all opposition to cease. Dalton’s theory proposed a number of basic ideas: 1) All matter is composed of indivisible, tiny particles called atoms, 2) atoms can neither be created nor destroyed, 3) all atoms of the same element are identical (have the same mass), 4) elements differ from one another because they have different types of atoms (different mass), 5) compounds are composed of combinations of different elements because the atoms of different elements are bonded to one another, and 6) chemical reactions occur when atoms in compounds are rearranged.ĭalton’s theory did not convince everyone immediately. Dalton’s theory went beyond just a philosophical statement that there are atoms. Dalton’s theory was different from previous discussions of atoms because it had the weight of careful chemical measurements behind it.

John Dalton developed his atomic theory in 1803.

Niels bohr atomic model