400 B.C.E.(Democritus) 1804

(Dalton)1903

(Thomson)

1913 (Bohr)

1911 (Rutherford)

1932(Chadwick)

Quantum ModelA History of

Atomic Models

Elements of Dalton’s Atomic Theory

All matter is made up of tiny particles called atoms.

The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and

types of atoms.

Chemical reactions involve reorganization of the atoms -- changes in the way they are bound together. The atoms themselves are not changed

in a chemical reaction.

Conservation of Mass

Conservation of Mass

Conservation of Mass

Summary of Modern Atomic Theory

Atoms have internal structure of Protons, Neutrons, Electrons, ..........

Number of protons = Number of electrons

The mass of the atom is concentrated in the Nucleus (Neutrons + Protons)

The atom is mostly empty space.

Atoms of the known elements differ in the number of Protons, Electrons, and Neutrons

The number of protons distinguishes one element from another.

Definitions A. Atomic Number - The number of protons or positive charges in the nucleus

B. Mass Number - The number of protons plus the number of neutrons in the nucleus

C. Symbolic Representation

X = Symbol for element A = Mass Number Z = Atomic Number

Number of protons = ? Number of electrons = ? Number of neutrons = ?

D. Atomic Mass Unit (amu) - The mass of a proton (≈ the mass of a neutron)

1 amu = 1.67 x 10-24 gram

E. Isotopes - Atoms of the same element which differ in the number of neutrons in their nuclei. Consider carbon.

C C C C

F. Atomic Weight - The weighted average of the masses of the naturally occurring isotopes of an element, expressed in amu.

1. Example: Carbon

98.99 % C-12 1.11 % C-13 Weighted average = 12.01 amu trace % C-14

2. Example: Chlorine

75.5 % Cl-35 Weighted average = 35.45 amu 24.5 % Cl 37

G. Where do you find atomic number, mass number, and atomic weight on the periodic table???

11 12 13 146 6 6 6

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