1. Fundamental Subatomic Particles
• Electron (e-): Discovered by J.J. Thomson (1897). Negatively charged, negligible mass, orbits
the nucleus.
• Proton (p+): Discovered by Goldstein/Rutherford (1886/1919). Positively charged, found in
the nucleus.
• Neutron (n): Discovered by Chadwick (1932). Neutral, found in the nucleus with protons.
• Nucleons: Protons and neutrons together are called nucleons (nucleus).
2. Evolution of Atomic Models
• Dalton’s Theory: Assumed atoms are indivisible particles.
• Thomson’s Plum Pudding Model: Atom is a positive sphere with electrons embedded (like
seeds in a watermelon). Disproved by nucleus discovery.
• Rutherford’s Nuclear Model (Gold Foil Experiment):
o Most space is empty (most alpha(a)-particles passed through).
o Small, dense, positive center called nucleus (some deflected).
o Electrons orbit the nucleus.
o Drawback: Could not explain why electrons don't fall into the nucleus (atom
stability).
• Bohr’s Model: Electrons move in specific, fixed shells/energy levels (K, L, M, N) without
losing energy.
3. Key Atomic Terms
• Atomic Number (Z): Number of protons (Z=p=e in neutral atom). Defines the element.
• Mass Number (A): Total number of protons + neutrons(A=p+n)
• Valence Electrons: Electrons in the outermost shell.
• Valency: Combining capacity; electrons lost/gained to achieve a stable octet (8 in outer
shell).
• Electron (e-): Discovered by J.J. Thomson (1897). Negatively charged, negligible mass, orbits
the nucleus.
• Proton (p+): Discovered by Goldstein/Rutherford (1886/1919). Positively charged, found in
the nucleus.
• Neutron (n): Discovered by Chadwick (1932). Neutral, found in the nucleus with protons.
• Nucleons: Protons and neutrons together are called nucleons (nucleus).
2. Evolution of Atomic Models
• Dalton’s Theory: Assumed atoms are indivisible particles.
• Thomson’s Plum Pudding Model: Atom is a positive sphere with electrons embedded (like
seeds in a watermelon). Disproved by nucleus discovery.
• Rutherford’s Nuclear Model (Gold Foil Experiment):
o Most space is empty (most alpha(a)-particles passed through).
o Small, dense, positive center called nucleus (some deflected).
o Electrons orbit the nucleus.
o Drawback: Could not explain why electrons don't fall into the nucleus (atom
stability).
• Bohr’s Model: Electrons move in specific, fixed shells/energy levels (K, L, M, N) without
losing energy.
3. Key Atomic Terms
• Atomic Number (Z): Number of protons (Z=p=e in neutral atom). Defines the element.
• Mass Number (A): Total number of protons + neutrons(A=p+n)
• Valence Electrons: Electrons in the outermost shell.
• Valency: Combining capacity; electrons lost/gained to achieve a stable octet (8 in outer
shell).
