Lecture 1 - Nucleus of atom and Geiger-Marsden experiment

ACKNOWLEDGEMENT:

Figure	Source of Figure
Figure 1	www.livescience.com
Figure 2	commons.wikimedia.org

Atomic Structure:

Atom is the smallest particle of an element. It consists of a centralized nucleus containing Proton(s) and Neutron(s), while the Electron(s) revolve around the nucleus.

Figure 1:

Nucleus:

Nucleus is the central part of the atom. It contains proton and neutrons. It is positively charged in nature because of the presence of positively charged protons, present in it. It is the most important part of an atom and gives the atom its characteristic properties.

Neutron:

Neutron is an electrically neutral particle inside the nucleus of an atom.

Proton:

Proton is a positively charged particle inside the nucleus of an atom. Proton and Neutron are relatively same in size.

Electron:

Electron is a negatively charged particle that revolves around the nucleus of an atom. It is the smallest particle with 1/1840 in size when compared to a proton or a neutron. That is 1840 electrons together have a mass of 1 proton or neutron.

Table 1 summarizes this:

Table 1:

	Proton	Neutron	Electron
Charge	Positive	Neutral	Negative
Mass	1	1	1/1840

Geiger Marsden Experiment – Proof of the atomic structure:

Geiger Marsden or Rutherford gold foil experiment conducted between 1908 to 1913, laid foundation to the structure of the atom mankind knows today.

Experimental Arrangement:

The apparatus consisted of a radioactive substance source of alpha particles arranged parallel to a gold foil. The gold foil was surrounded by a fluorescent screen.

Figure 2:

Positively charged alpha particles were bombarded perpendicularly on the gold foil and the fluorescent screen was observed.

Experimental Observation and Conclusion:

Some distinctive patterns were seen: 

Observation	Conclusion
Most of the alpha particles passed through the foil and caused fluorescence on the screen directly in front of the source.	Atom consist largely of empty space. Confirmation of the existence of electrons in large empty orbits, around the nucleus.
A small fraction of alpha particles were deflected at angles lesser than 90° and caused fluorescence on the screen at angles lesser than 90°.	Alpha particles passing near by the nucleus were deflected slightly. Alpha particles passing near by the nucleus were deflected largely. Alpha particles that collided with the nucleus itself, bounced back towards the source, causing fluorescence on the screen behind the source. Positive charge is concentrated near/in the center of the atom. Confirmation of the existence of protons together in the center of the atom (along with the neutrons that were discovered after 1932.)
A tiny fraction of alpha particles were deflected at angles greater than 90° and caused fluorescence on the screen at angles greater than 90°.
A tiny fraction of  alpha particle bounced back towards the source and caused fluorescence on the screen behind the source.