1. Democritus of Abdera - 4th century B.C, 1st suggested atom "atomus".

2. Indivisible, indestructible, fundamental units of matter.

3.Lacked experimental support.

4. 2200 years later John Dalton (1766-1844). Performed experiments and arrived at "Atomic Theory of Matter".

All elements composed of submicroscopic indivisible particles called atoms.

Atoms of the same element are identical. Atoms of one element are different from atoms of another.

Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios.

Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of an element don't change into atoms of another.

6. Can view atoms now with scanning-tunneling microscope.

4.2 Electrons, Protons, and Neutrons.

1. Revision to Dalton's hypothesis: atoms can be broken down into subatomic particles - electrons (-), protons (+), neutrons (0).

2. Sir J.J. Thomson discovered electrons in 1897. Used gas tube (page 86). Beam traveled from cathode (-) to anode (+): called cathode ray.

3. Devised experiment showing deflection of electron beam from negatively charged plates and attraction to positive. (page 87).

4. Millikan improved Thomson's calculations. Electron is one unit of electrical charge (-) and 1/840 mass of hydrogen atom.

5. 1886 E. Goldstein used cathode ray tube with holes in cathode. (page 88). Rays traveling in opposite direction are positive - protons (+).

6. 1932 Sir James Chadwick confirmed neutron (0).

4.3 The Structure of the Nuclear Atom.

1. 1911, Ernest Rutheford directed alpha particles (2 protons+) (page 90).

Towards gold foil with fluorescent screen around it.

2. Most went through in straight line. Some deflected at large angles. Some came straight back.

3. Rutheford concluded that the mass of the atom is concentrated in center - nucleus, which is protons and neutrons.

4. Surrounding is mostly "empty space".

5. Nucleus has positive charge. Electrons outside.

4.4 Atomic Number

1. The atomic number of an element is the number of protons in the nucleus of the atom of that element.

2. The number of protons in an atom determines the element: H 1, O 8, C 6, etc.

3. Elements are arranged in the periodic table in order by atomic number. Number increases across rows. Since "free" atoms are neutral, the number of protons equals the number of electrons.

4.5. Mass Number

1. The mass number is the total number of protons and neutrons in the nucleus.

2. Use atomic mass to calcualte number of protons and neutrons. See periodic table, page 354.

4.6 Isotopes of elements

1. Nuclei of the same element have the same number of protons, but neutrons may vary. C-12,14, Ne-20, 21,22.

2. Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

3. Despite different atomic masses, chemical nature of atoms are the same, due to number of protons.

4. 3 isotopes of hdrogen. H-1 (hydrogen), H-2 (deuterium), H-3 (tritium).

4.7 Atomic Mass

1. Masses of atoms are extreemly small: Fl 3.155 x 10 ^-23 g, etc. Too small and inconvient.

2. Instead we use relative masses based on C-12. Carbon assigned a ass of 12.00000 amu. An atomic mass unit is defined as one-twelfth the mass of a carbon - 12 atom.

3. Since protons = 1 amu and neutrons - 1 amu, atomic mass should be whole number.

4. Instead we see decimals. This is due to the fact that in nature elements occur as a mixture of two or more isotopes.

5. The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.