Laws and theories on the nature of matter
Law of conservation of mass: the mass of the reactants must equal the mass of the products.
Law of constant composition aka law of definite proportions: In a pure substance there is a definite, fixed proportion/ratio of elements. Example: Calcium Sulfide will always be 44.4% Ca, 55.6% S.
Atomic Theory by John Dalton in 1805 states that:
- Things are made of small entities called atoms.
- Atoms of a given type are identical to each other but significantly different from atoms of another type.
- Atoms are indestructible.
- Chemical reactions are a matter of re-associations of atoms.
Gold Foil Experiment of 1911 under the supervision of Rutherford helped create the understanding that a positive charge existed in an area now known as the nucleus, most of the mass of an atom is concentrated there and that an atom is mostly empty space.
Nuclear model of atom: The nucleus is 10^-5 of the diameter or in other words, an atom is 100,000 times the size of its nucleus. Also, 99.5% of the mass is contained in the nucleus.
| mass | atomic mass unit | charge | |
|---|---|---|---|
| Proton | 1.67262 x 10^-24 | 1.0073 | +1 |
| Neutron | 1.67496 x 10^-24 | 1.0087 (HEAVIEST) | 0 |
| Electron | 0.00091 x 10^-24 | 0.00055 | -1 |
Numbers or names used to describe atoms
Atomic number is the number of protons in the nucleus of an atom. This number determines the element type of the atom. For instance, all neon atoms have exactly ten protons. If an atom has ten protons, then it must be neon. If an atom is neon, then it must have ten protons.
Mass number is the sum of protons and neutrons in an atom.
Neutron number is the number of neutrons in the nucleus of an atom. Remember that neutrons have no electric charge, so they do not affect the chemistry of an element. However, they do affect the nuclear properties of the element.
Isotope: Atoms of a given type with different number of neutrons.
 This is how we write fluorine-19. The mass number is the superscript while the atomic number is the subscript, followed by its symbol.
Atomic mass unit (amu): The units that the mass of an atom is measured in. An atom’s mass can be found by summing the number of protons and neutrons. Protons and neutrons have an approximate mass of 1 amu, and electrons have a negligible mass. The atomic mass measures the average mass of all atoms for an element.
