Isotopes: UNIZOR.COM - Physics4Teens - Atoms - Nucleus and Electrons
Channel: Zor Shekhtman on UNIZOR Education
Date: 2023-02-12

UNIZOR.COM - Physics through a Looking Glass of Mathematician
Notes to a video lecture on http://www.unizor.com

Isotopes

Isotopes are atoms that have the same number of protons (atomic number Z) but different number of neutrons (N).
Since the number of electrons and their distribution among shells and subshells are the same for different isotopes of atoms with the same atomic number, these isotopes have practically the same chemical properties.

At the same time, since the number of neutrons of different isotopes of atoms with the same atomic number is different, certain physical properties of these atoms might be quite different.

Identification of different isotopes involves specifying their atomic mass.
For example, carbon has 6 protons and 6 electrons, but the number of neutrons can be 6, 7 or 8 in its nucleus with atomic masses of these isotopes, correspondingly,
A=Z+N=6+6=12,
A=Z+N=6+7=13 and
A=Z+N=6+8=14.
These isotopes are named, according to their atomic mass, carbon-12, carbon-13 and carbon-14.

Another example: uranium-235 with 92 protons, 92 electrons (atomic number is Z=92) and 143 neutrons (N=143), which has atomic mass A=235, and uranium-238 with the same number of protons, electrons and the same atomic number Z=92, but with 146 neutrons (N=146) and atomic mass A=238.

In most cases one particular isotope of an atom is sufficiently stable and can be found in nature, while other isotopes of the same atom might be more or less radioactive, that is their atoms break with time into other (smaller) atoms - a process called radioactive decay.

The role of neutrons in a nucleus is to stabilize it. Since protons are positively charged and repel each other, neutrons serve as a buffer between them and, therefore, their number is usually equal or greater than the number of protons.
Obviously, so-called strong forces inside a nucleus that bind together all particles (protons to protons, protons to neutrons, neutrons to neutrons) are the main forces that hold a nucleus as a whole.

Interestingly, most nuclei with even atomic number Z and even number of neutrons N demonstrate more stability than those with either Z or N or both are odd.


Radiocarbon Dating

As an example of the usage of isotopes, let's describe the determination of age of certain samples found by archeologists using radiocarbon dating based on isotope carbon-14.

The most stable and abundant isotope of carbon is the one with 6 protons and 6 neutrons in a nucleus - carbon-12.
This carbon isotope plays extremely important role in organisms, as carbon is one of the main building blocks of all living beings on Earth.

Let's examine where carbon-14 comes from.
High energy cosmic rays bombard the Earth and break atoms of the upper layer of atmosphere, releasing some elementary particles, electrons, protons and neutrons.

[See full description on UNIZOR.COM - Physics4Teens - Atoms - Nucleus and Electrons - Isotopes]