The Theory of an Atom

Who was John Dalton?

John Dalton, stated the law of multiple proportions in 1803. He had observed this law at work in compounds of carbon and hydrogen, and in the oxides of nitrogen. The law of multiple proportions states that atoms of different elements always combine in whole-number ratios. For example: H₂O (atoms of hydrogen always combine in a fixed ratio with oxygen, 2:1).

Dalton's Atomic Theory?

• All matter is made of atoms. Atoms are indivisible and indestructible.
• All atoms of a given element are identical in mass and properties.
• Compounds are formed by a combination of two or more different kinds of atoms.
• A chemical reaction is a rearrangement of atoms.

What is a gas discharge tube?

The first evidence of the existence of electrons came from the invention of the gas discharge tube. The gas discharge tube is basically a glass tube of gas at a low pressure with electrodes places at opposite end of the tube. An electric current is passed through the tube and the gasses inside the tube illuminate. As electricity passes through the tube a ray is formed.

Who was J.J. Thompson?

J.J. Thompson used the gas discharge tube to obtain evidence of the existence of the electron and determined that it had a negative charge. Thompson placed magnetic and electric plates near the discharge tube and observed how the ray inside the gas electron tube would bend. From his experiments he was able to determine that the ray was composed of small particles called electron and that they had a negative charge and a mass 1/2000 of a hydrogen atom, proton, or neutron.

Who was Rutherford?

In 1909 Rutherford did a famous experiment that proved that atoms are not solid spheres but mostly consist of space. In his experiment he used alpha particles (positively charged helium nuclei). He used radioactive elements such as uranium to emit beams of alpha particles through a sheet of thin gold foil. The results of the experiment proved that most of the alpha particles passed straight through the gold foil and only a small amount had been deflected. This lead Thompson to conclude that the atom consists mostly of space and that the atom contained a small dense, positively charge nucleus as some of the particles were deflected at a 90 degree angle.

Who was Neils Bohr?

In 1913 Bohr hypothesized that electrons in the atom have certain energy levels and orbit around the nucleus. Each of these orbits is a fixed distance from the nucleus of the atom. The lowest energy level is called the K shell or energy level 1. Successively higher levels are labeled L, M, N, and so on. Only two electrons can occupy the first shell, the second can hold up to 8 electrons, the third can hold 18, and the forth electron shell can hold up to 32 electrons.