Nucleus

Robert Deaton Spring 2022)

Discovery of the Nucleus

If we hearken back to the days of the turn of the 20th century (a truly revolutionary time in physics history), we have to consider the plum-pudding model of an atom put forth by J. J. Thompson just a few years ago in 1904. Thomson had discovered the electron in 1897, and to account for the neutral charge of regular matter and the negative charge of electrons, he proposed that atoms were made of some positively charged *stuff* (pudding) with negatively charged electrons embedded inside (plums). Over the next several years, many experiments headed by Ernest Rutherford and Hans Geiger showed that alpha particles emitted by radioactive elements move more slowly through solid matter than through air (testing with aluminium foil and gold leaf).

A Mathematical Model

Some helpful numbers, definitions, and mathematical concepts to know: protons: carry positive charge around nuclei, symbol: p

neutrons: no electric charge, relatively same mass as proton, symbol: n

electron: negatively charged, symbol: e

atomic number= number of protons, helps us define the chemical properties of an element so we can arrange it on the periodic table

mass number= number of protons + neutrons, this number can change slightly if the number of neutrons changes --> making an isotope

isotope: types of atoms that have the same number of protons and position in periodic table but a different number of neutrons

mass of a proton= 1.673x10^-27 kg, charge of proton: 1.6x10^-19 coulomb

mass of an electron= 9.109x10-31 kg, charge of electron = - 1.6x10^-19 coulomb

A computational model

Computer Modeling of the Rutherford Experiment -see page 414 of Matters and Interactions textbook

Examples

Question: what did Rutherford think he was going to see in his experiment based off the older plum pudding model?

answer: he expected to see alpha particles deflected only slightly through interactions with low-mass electrons and low-density positively charged atoms, but in reality they saw that an alpha particle sometimes bounced straight backward, leading him to believe the greater part of the mass of an atom was in a center that carried a charge, aka a nucleus

History

The nucleus was discovered in 1911, because Rutherford wanted to test Thomson's "plum pudding model" of the atom. In the plum pudding model, Thomson suggested that an atom had negative electrons scattered all around a positive charge, or group of them. Ernest Rutherford did an experiment that involved the deflection of alpha particles directed at a thin sheet of metal foil. He thought if this "Plum" model was correct then the positively charged alpha particles would pass through the foil with very little trouble, as the foil should act as electrically neutral. To his surprise, many of the particles were deflected at very crazy and bizarre angles. Because the mass of an alpha particle is about 8000 times that of an electron, it became apparent that a very strong force must be present if it could deflect the massive and fast moving alpha particles. This basically meant the whole plum pudding model was a giant lie and that the deflections of the alpha particles could only be explained if the positive and negative charges were separated from each other and that the mass of the atom was a concentrated point of positive charge. This concentrated point was identifies, and viola- the nucleus was born.

Connectedness

How is the idea of a nucleus connected to my major? As a neuroscience major, the concepts of atoms are essential to chemistry and physics and therefore our understanding of the brain. For example, understanding what an atom and atomic nucleus are helps us define certain elements on the periodic table such as sodium, calcium, and potassium, which are essential for the functioning of our brain to be able to send action potentials. Is there an interesting industrial application? Nuclear physics helps to make important medical devices such as MRIs among others, and nuclear physicists can help create nuclear weapons.