Atomic Structure

Unit: Atomic Structure

1. History of the Atom

1.1 Important Experiments Leading to Atomic Theory

2. Early Theorists

2.1 Democritus (400 B.C.)

• Greek philosopher; first to propose the existence of atoms.

• Believed matter is composed of tiny, indivisible particles called atoms.

• Lacked experimental evidence for his theories.

• Coined the term "atomos."

2.2 John Dalton (1766-1844)

• A meteorologist with experimental support for his atomic theory.

• Proposed four major postulates:

  • Postulate 1: All elements are made of indivisible particles called atoms.

  • Postulate 2: Atoms of the same element are identical; different elements have different atoms.

  • Postulate 3: Atoms of different elements combine in whole number ratios (e.g., Oxygen and Hydrogen combine in a 2:1 ratio to form water).

  • Postulate 4: During chemical reactions, atoms separate, join, or rearrange but do not change into other elements.

3. Key Experiments and Discoveries

3.1 J.J. Thompson (1859-1940)

• Conducted the Cathode Ray Tube Experiment; discovered the electron.

• Observed that particles in a gas were attracted to a positive plate, concluding they must carry a negative charge.

• Proposed the "Plum Pudding Model," where the atom is a solid positive sphere with electrons embedded in it.

3.2 Ernest Rutherford

• Conducted the Gold Foil Experiment to discover the nucleus.

• Bombarded gold foil with high-energy alpha particles.

• Conclusions from the Gold Foil Experiment:

  • Most alpha particles passed through, indicating atoms are mostly empty space.

  • Some particles were deflected at small angles, suggesting a dense center.

  • Very few particles were deflected at large angles, indicating a small, positively charged nucleus.

4. Discovery of Subatomic Particles

4.1 Major Discoveries

• Eugene Goldstein (1850-1930): Discovered the proton.

• James Chadwick (1891-1974): Discovered the neutron.

5. Revision of Dalton’s Postulates

• Some postulates have been proven incorrect:

  • Atoms are divisible into subatomic particles (protons, neutrons, electrons).

  • Not all atoms of the same element are identical due to the existence of isotopes.

6. Subatomic Particles Properties

6.1 Properties of Subatomic Particles

6.2 Location of Particles

• Protons: Located in the nucleus.

• Neutrons: Located in the nucleus.

• Electrons: Located in the electron cloud outside the nucleus within energy levels.

7. Neutral Atoms

• An atom is neutral when the number of protons equals the number of electrons (p+ = e-).