Question #a13a9

When new scientific evidence indicates that part of a theory is incorrect, it is usually modified but not thrown out. For example, Dalton's atomic theory stated that atoms were indivisible. However, due to the continuing research by Ernest Rutherford, JJ Thomson, and James Chadwick, we now know that atoms are divisible, and consist of the subatomic particles protons, electrons, and neutrons. Research from particle accelerators indicates that the protons and neutrons are also divisible.

Another part of Dalton's atomic theory was that all atoms of an element are identical. However, JJ Thomson also discovered isotopes, showing that the atoms of an element were not all the same, but can differ in the number of neutrons.

The rest of Dalton's atomic theory still holds true:
1. All matter is made up of atoms.
2. The atoms of one element are different from the atoms of all other elements.
3. For each compound, different elements combine in a simple, fixed numerical ratio.

Dalton's atomic theory correctly explains the law of conservation of mass: if atoms of an element are indestructible, and in chemical reactions they are, then the same atom must be present after a chemical reaction as before, and the mass must be constant. Dalton’s atomic theory also explains the law of definite composition: if all the atoms of an element are alike in mass and if atoms unite in fixed numerical ratios, the percent composition of a compound must have a unique value without regards to the sample analyzed. Dalton's atomic theory also led to the creation of the law of multiple proportions.
http://chemwiki.ucdavis.edu/Physical_Chemistry/Atomic_Theory/Atomic_Theory