Betsy has a Ph.D. in biomedical engineering from the University of Memphis, M.S. from the University of Virginia, and B.S. from Mississippi State University. She has over 10 years of experience developing STEM curriculum and teaching physics, engineering, and biology.
Nuclei and Isotopes
Imagine for a minute that you were really small, so small that you could see into a single atom. What would you find in there? For a long time, scientists thought that the atom was the smallest particle that existed, but they were wrong. Inside each atom are even smaller particles!
In the center of each atom is a structure called a nucleus that contains two types of particles, protons and neutrons, that are tightly bound to each other.
- Protons are always positively charged
- Neutrons are neutrally charged
- Together they are called nucleons, because they are always in the nucleus.
Outside the nucleus, there are tiny negatively charged particles called electrons.
Two elements are different from each other because they have a different numbers of protons. For example, an atom of hydrogen always has exactly 1 proton, while an atom of carbon always has exactly 6 protons.
However, atoms of the SAME element can have different numbers of neutrons. Sometimes a carbon atom has 6 protons but 8 neutrons! Either way, it's still carbon. Atoms of the same element that have different numbers of neutrons are called isotopes.
Although it seems like an atom could have any number of neutrons, that's not really what happens. Some isotopes are much more stable than others and, therefore, more likely to exist. Why does this happen?
A scientist named Maria Goeppert-Mayer wondered that too back in the 1940's, and she was determined to find out what was going on. What she discovered would totally change the way scientists understood atomic nuclei!
Who Was Maria Goeppert-Mayer?
Maria Goeppert (the name Mayer was added later) was born in Germany in 1906. Members of her family had been professors, scholars, and scientists for six generations before she was born, and her father always encouraged her to get an education and pursue her dreams.
She graduated from high school a year early and enrolled at the University of Gottingen. Although she was originally studying mathematics, she fell in love with physics and decided to pursue a Ph.D.
Goeppert got her Ph.D. in physics in 1930 and married an American chemist named Joseph Mayer the same year (adding the name Mayer). He was soon hired as a professor at Johns Hopkins, and Goeppert-Mayer followed him to America.
For many years, it was quite difficult for her to continue her own research. She was not allowed to work as a professor at the universities where her husband worked, but she managed to keep going anyway. She worked for free or for very little pay, and traveled back to Germany in the summers to continue her research at the University of Gottingen.
With the outbreak of World War II, things changed for Goeppert-Mayer. She was hired to work part time on the Manhattan Project, in which a group of scientists in America were working to build the world's first atomic weapon. After the war, she was able to continue working at the newly opened Argonne National Laboratory in Chicago. By that time, her husband was also working at the University of Chicago.
The Nuclear Shell Model
During her time in Chicago in the late 1940's, Goeppert-Mayer began to study isotopes and try to find out why some were more likely to occur than others. She found that there were certain special numbers of protons and neutrons that were always more stable. She took all this data and came up with a new model of the atomic nucleus that we call the nuclear shell model.
Before Goeppert-Mayer began studying atomic nuclei, scientists already knew that certain numbers of electrons were more stable also. This occurs because electrons are located in specific orbits, often called energy shells, that are associated with different amounts of energy.
When there are exactly enough electrons for an atom to have an outer energy shell that is completely filled, then the atom will be very stable and non-reactive. If the energy shell is not completely filled, then the atom will be more likely to react with other atoms to form compounds.
Goeppert-Mayer hypothesized that something similar might be happening inside the nucleus. She found that, just like electrons, nucleons like protons and neutrons could also be in different energy shells. When an energy shell was completely full, the nucleus would be very stable, but when it was not full, the atom would be more likely to undergo nuclear reactions or be radioactive.
This totally changed the way scientists thought about atomic nuclei, and brought Goeppert-Mayer personal success as well. In 1963, Goeppert-Mayer became only the second woman to ever receive the Nobel Prize in Physics.
She also was finally offered a full-time position as a professor of physics at the University of California, San Diego in 1960. However, her health began to decline soon after she arrived so she wasn't able to continue working for as long as she would have liked. She died in 1972 at the age of 66.
Today, she is remembered as one of the most important physicists of the twentieth century. Each year, the American Physical Society gives out an award in her honor to a promising young female physicist. The physics building at UCSD is still named after her, and a symposium for female scientists is held there every year.
Maria Goeppert-Mayer (1906-1972) was a German-American scientist who discovered that nucleons (protons and neutrons) existed in specific energy shells within the nucleus of an atom. This is very similar to how electrons are located in different energy shells outside the nucleus. Her model of the atomic nucleus is now called the nuclear shell model and it explains many interesting phenomena, including why some isotopes are more stable than others and why some atoms are more likely to be radioactive.
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