Atomic Structure: From Dalton to Rutherford

From Dalton to the Electron

The simple model set by Dalton needed updating. In 1894, Crookes discovered that cathode rays were produced just by applying a very high voltage between two electrodes. He observed that he could get a shadow, so his conclusion was that "some particles exit from the metal plate placed in the cathode." As those particles were attracted by the positive electrode, the so-called cathode rays had to have a negative charge. Thomson (1897) interpreted the experiment as showing that cathode rays were a stream of electrons. Atoms, as introduced by Dalton, were not indivisible. Since electrons were issued from a metal, atoms were made of electrons (negative charge) and something else with a positive charge because matter is electrically neutral.

The Plum Pudding Model and Rutherford's Experiments

By the end of the 19th century, the plum pudding model of the atom was set by Thomson. It described the atom as a positively charged mass where electrons could be extracted. In 1910, Millikan measured the charge of electrons (1.6 x 10^-19 C). Rutherford investigated the microscopic nature of matter by bombarding a film of gold with alpha particles, since radioactivity had been worked out one decade before. Certain substances produce invisible reactions. Rutherford's experiment showed that a very dense metal could be punched through by little, heavy particles, although a few rebounded as if colliding with something massive while the rest of the plate seemed to be empty.

The Nuclear Model of the Atom

Rutherford modified Thomson's model and adopted the idea of Nagaoka: atoms were like a little planetary system where electrons turn around a little nucleus, which has almost the total mass of the atom. A few years later, Rutherford discovered that the nucleus was not a simple ball but was made of particles having a positive charge.

Pure Substances and Mixtures

Pure Substances

A pure substance contains only one type of molecule (the smallest part of a substance). Molecules cannot be broken by physical means (only chemical reactions can). If the molecule is made of different types of atoms, we have a compound.

Mixtures

A mixture contains several types of molecules (each one having different chemical properties). We can separate components of a mixture by physical means.

Types of Mixtures:

• Homogeneous: We cannot distinguish the elements.
• Heterogeneous: We can see its components.

Solutions

Homogeneous mixtures are all called solutions:

• Solvent: The majority of the solution.
• Solute: The minority component.

Solid-in-solid solutions are called alloys.

Adding more solute when it is saturated makes a precipitate. This process depends on temperature.