Atomic Structure: A Historical Perspective

John Dalton

• Mentioned as someone discussed in the previous lecture.

JJ Thompson

• Also mentioned as someone discussed in the previous lecture.
• Referred to as the "Plumming guy" or "Plum pudding guy."

Tokyo University Researcher (Santoro/On Toro)

• Followed JJ Thompson.

Ernest Rutherford

• Came after JJ Thompson.
• Conducted the gold foil experiment in $1908$. This was two years after the plum pudding model ($1906$).

Gold Foil Experiment Setup:

• Used an alpha particle gun.
• Shot alpha particles at a piece of gold foil.

Expected Result:

• If the atom was structured like plum pudding model, the alpha particles, being "little charges", would pass straight through the foil.

Actual Result:

• Most alpha particles did pass straight through.
• However, some alpha particles ricocheted back or were deflected at various angles.

Interpretation of Results:

• The ricocheting suggested a concentrated positive charge within the atom that repelled the alpha particles.
• This positive charge was named nucleus.

Rutherford's Contribution:

• Discovered that every atom has a nucleus.
• Corrected JJ Thompson's plum pudding model by stating that instead of small charged particles distributed throughout the atom, there is a single, concentrated positive charge in the center (nucleus).

Atomic Structure Revision:

• Initial model: Atom as indivisible.
• Thompson's model: Atom with distributed positive charge (plum pudding).
• Rutherford's model: Atom with a central nucleus.

Empty Space in Atom:

• Most alpha particles passed through the gold foil, implying that atoms are mostly empty space.
• $99.999\% +$ of an atom's volume is empty space.