Atomic Structure and Electron Configurations

Chemistry I Honors

J. Venables

Northwestern High School

Atomic Structure and the Periodic Table

• John Dalton’s Atomic Theory:– Each element is composed of atoms

– All atoms of an element are identical. The atoms of different elements are different.

– In chemical reactions, the atoms are not changed; they are rearranged.

– Compounds are formed when atoms of more than one element combine.

• Law of Definite Proportions –– A given compound always has the same

proportion (H2O, NaCl)

• Law of Conservation of Mass –– In a chemical reaction, mass is neither

created nor destroyed.

• Law of Multiple Proportions –– Different compounds can exist with different

ratios of the same elements (H2O, H2O2)

– (CO, CO2) (N2O, NO2)

Cathode Ray Tube –J. J. Thomson

Ernest Rutherford –Gold Foil Experiment

Isotopes, Atomic Numbers, and Mass Numbers

• Atomic number (Z) = number of protons in the nucleus.

• Mass number (A) = total number of nucleons in the

nucleus (i.e., protons and neutrons).• By convention, for element X, we write Z

AX.• Isotopes have the same Z but different A.• We find Z on the periodic table.

ExamplesComplete the following table:

Symbol Protons Neutrons Electrons

13H

12 13 12

35Cl

92 146 92

ExamplesComplete the following table:

Symbol Protons Neutrons Electrons

13H 1 2 1

1225Mg 12 13 12

35Cl 17 18 17

238U 92 146 92

The Atomic Mass Scale• 1H weighs 1.6735 x 10-24 g and 16O 2.6560 x 10-23

g.• We define: mass of 12C = exactly 12 amu.• Using atomic mass units:

1 amu = 1.66054 x 10-24 g1 g = 6.02214 x 1023 amu

Light and Waves• All waves have a characteristic wavelength, , and

amplitude, A.• The frequency, f, of a wave is the number of cycles

which pass a point in one second.• The speed of a wave, v, is given by its frequency

multiplied by its wavelength:

• For light, speed = c.

fc m∙s-1 Hz (s-1) m