The Atomic Theory By Students, Class of 2000

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The Atomic Theory
By Students, Class of 2000

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The understanding of the nature of matter as we know it today was first presented by John Dalton. His postulates are the basis for all modern chemistry. Many similar facts have been proven long before Dalton’s time, and these very facts are the ones that led him to the formation of his theory. As proposed by Dalton, the atomic theory of matter consists of four postulates:

All matter is composed of very small particles called atoms.

All atoms of the same element are identical.

All atoms of different elements are not alike.

Atoms combine with each other to form compounds. A given compound has a number of different types of atoms. The elements that make up these compounds always combined in definite proportions by mass. In other words, all water molecules are made up of 2 part H and 1 part O. This is known as the Law of Definite Proportions.

** Dalton’s original theory was changed with the discovery of isotopes. An isotope contains the same number of protons but has a different number of neutrons. An English scientist named Mosely discovered that the number of protons in an elements determines the identity of the substance, and the number of neutrons determines the isotope of the element. Now Dalton’s theory states that all atoms of the same element have the same number of protons, but can contain a different number of neutrons.

The atomic theory of matter holds that atoms are fundamental units of all matter, and that they are conserved in chemical reactions. In other words, chemical reactions rearrange atoms to form compounds while the atoms themselves remain unchanged. Then only certain molecules can exist.

The Law of Conservation of Mass:

The total number of atoms that react in a chemical reaction does not change. The reaction occurs because the atoms rearrange themselves into new, different groups. Therefore, we say that the atoms are conserved in both their number and type. Matter can not be made nor destroyed. All matter is constant.

The Law of Conservation of Energy:

The same principle applies for energy as does for matter. In a chemical reaction, the amount of energy that is used in the initial reaction, remains the same once the process is complete. Energy is neither created, nor destroyed. Although sometimes energy is labeled as a product, this is only potential energy that is released.

The Law of Conservation of Mass-Energy:

The law of conservation of mass-energy states that the sum of mass and energy is always a constant. Mass and energy can be changed from one form to the other, but their sums are always constant. It can not increase or decrease.