Ernest Rutherford used about ________ thick gold foil for alpha scattering experiments.
- (a)1000 atoms
- (b)700 atoms
- (c)900 atoms
- (d)500 atoms
Answer
Why
Correct — A. Rutherford wanted the thinnest target he could get, so that an alpha particle met as few atoms as possible on its way through. The foil he used was about 1000 atoms thick.
Gold was chosen because it is the most malleable metal and can be beaten that thin without tearing.
The NCERT Class 9 chapter on the structure of the atom carries this figure in exactly these words → option (a).
Why the others are wrong
- (b)700 atoms — The account fixes one thickness only, about 1000 atoms. 700 has nothing behind it. The other number worth carrying from this experiment is different: roughly 1 in 12,000 alpha particles rebounded.
- (c)900 atoms — 900 is a near miss dressed as precision. The foil is described as about 1000 atoms thick, and the word about is doing real work — this is a round figure, not a measured one.
- (d)500 atoms — 500 halves the stated thickness. The foil had to be thin enough that most alpha particles passed straight through undeflected, and the thickness recorded for it is about 1000 atoms.
Concept
Rutherford fired alpha particles at a thin gold foil and recorded where they struck a fluorescent screen.
Most passed straight through. A small fraction were deflected through small angles, and a very small number bounced almost straight back — a result he compared to a shell rebounding from a sheet of tissue paper.
From that he concluded the atom is mostly empty space, with the positive charge and nearly all the mass packed into a tiny central nucleus and electrons moving around it.
The foil had to be about 1000 atoms thick precisely so that a single encounter, not a pile-up of many, produced each deflection.
The exam is quoting a textbook sentence, not a laboratory measurement. Learn it as written — about 1000 atoms thick — because the three wrong options are simply other round numbers.
Key facts
- Rutherford's gold foil was about 1000 atoms thick.
- Gold was used because it is the most malleable metal and can be beaten into an extremely thin sheet.
- About 1 in 12,000 alpha particles rebounded from the foil.
- The experiment established that an atom has a small, dense, positively charged nucleus and is largely empty space.
Study next
Common traps
- Confusing the foil thickness, about 1000 atoms, with the rebound ratio of about 1 in 12,000.
- Crediting the deflection to electrons when it is the nucleus that turns an alpha particle back.
Atomic-structure history is asked as recall of a named worker tied to a named result. 23 Sep 2024, 16:00, GA Q.11 asks who reconciled Dalton's atomic hypothesis with Gay-Lussac's results on combining volumes, and keys Avogadro.
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