Atomic Structure: Gold Foil Experiment and Nuclear Model

Atomic Structure and the Gold Foil Experiment

Introduction

The study of atomic structure was revolutionized by Ernest Rutherford's Gold Foil Experiment, which provided evidence for the existence of a positively charged nucleus within the atom. This experiment, along with subsequent models, helped clarify the arrangement of subatomic particles.

Experimental Setup

Rutherford's experiment involved bombarding a thin sheet of gold foil with alpha particles emitted from a radioactive element. The behavior of these particles provided insight into atomic structure.

• Alpha Particle: A radioactive particle consisting of 2 protons and 2 neutrons (symbol: 4He or α).

• The radioactive element is usually iridium and is encased in a lead container with an opening.

• A screen coated with zinc sulfide is used to detect the scattered alpha particles.

Rutherford's Postulates

Based on the results of the experiment, Rutherford proposed several key ideas about atomic structure:

• The protons and neutrons are located in the nucleus, which is at the center of the atom.

• Although incredibly small, the nucleus comprises most of the atom's mass.

• Surrounding the dense, positively charged nucleus is a cloud of electrons.

Example Calculation: Number of Atoms in Gold Foil

If the gold foil used in Rutherford's experiment had a thickness of approximately $6.0 \times 10^{-3}$ mm, and a single gold atom has a diameter of $2.9 \times 10^{-8}$ cm, the number of atoms thick in the foil can be calculated as follows:

• Thickness of foil: $6.0 \times 10^{-3}$ mm = $6.0 \times 10^{-4}$ cm

• Diameter of one atom: $2.9 \times 10^{-8}$ cm

• Number of atoms thick: $\frac{6.0 \times 10^{-4}}{2.9 \times 10^{-8}} \approx 2.1 \times 10^{4}$ atoms

Nuclear Model of the Atom

Rutherford's findings led to the development of the Nuclear Model of the atom, which replaced the earlier Plum Pudding Model proposed by J.J. Thomson.

Plum Pudding Model	Nuclear Model
Electrons are embedded in a positively charged 'pudding' with no central nucleus.	Electrons orbit a small, dense, positively charged nucleus at the center of the atom.
Alpha particles pass through with little deflection.	Most alpha particles pass through, but some are deflected at large angles due to the nucleus.

Key Points from Rutherford's Experiment

• Electrons are negatively charged subatomic particles.

• Atoms are composed of protons, neutrons, and electrons.

• The nucleus contains protons and neutrons, while electrons occupy the surrounding space.

Summary Table: Atomic Structure

Subatomic Particle	Charge	Location
Proton	+1	Nucleus
Neutron	0	Nucleus
Electron	-1	Outside nucleus

Additional info: The Gold Foil Experiment is a foundational topic in GOB Chemistry, illustrating the transition from early atomic models to the modern understanding of atomic structure. It emphasizes the importance of experimental evidence in shaping scientific theories.