Dalton's Atomic Theory
- All matter is composed of ultimately small particles, called atoms.
- Atoms are permanent and indivisible - they can neither be created nor destroyed.
- Elements are characterized by their atoms. All atoms of a given element are identical in all respects. Atoms of different elements have different properties.
- Chemical change consists of a combination, separation, or rearrangement of atoms.
- Chemical compounds are composed of atoms of two or more elements in fixed ratios.
Particle |
Charge |
Relative Mass |
Mass |
| Electron (e-) |
-1 |
1/1840 |
9.11 x 10-28g |
| Proton (p or H+) |
+1 |
ª1 |
1.67 x 10-24g |
| Neutron (n) |
0 |
ª1 |
1.67 x 10-24g |
Some important terms
- Atomic number (Z) - the number of protons in the nucleus. This number is characteristic of a given element.
- Atomic mass number (A) - the sum of the protons and neutrons in a given atom (p + n).
- Atomic mass - the actual mass of an average atom in a sample. The characteristic atomic masses for Earth are shown on periodic tables.
- Atomic Mass Unit - the atomic mass unit = amu is a unit of mass for atoms. It is defined as 1/12 the mass of one atom of 12C, where the mass of 12C is defined as 12 exactly.
Last modified 11 September 2008