Tuesday, January 5, 2021

Isotopes And Half-lives

 

    


    In our earlier discussion, we know that everything in the universe is composed of atoms as proposed by Democritus centuries ago. Further research showed that the atom is not as unbreakable as initially thought of,  but rather composed of still smaller particles---protons and electrons. The early model of the atom consisted of a positively charged protons inside a nucleus surrounded by a cloud of negatively charged electrons. They reasoned correctly that for these atoms to continue to exist, the positive nucleus must have the same amount of electrical charge as the cloud of negative electrons orbiting around it, otherwise these atoms disintegrated long time ago. 

     Different configurations of these particles differentiate one element from another. They assigned these elements symbols and atomic numbers. The atomic symbol is a one- or two- alphabetic characters and the number corresponds to the number of protons which is also the same as the number of electrons. Scientists started constructing the Periodic Table, a 2-dimensional array consisting of cubicles where to place the atomic symbols relative to each other.


    The smallest and the lightest element is the hydrogen (symbol: H). It has an atomic number 1 because it has only one proton and one electron orbiting around it. The electron is so tiny that its mass is almost negligible and the mass of the atom is mostly due to the nucleus. The heaviest naturally occurring element is uranium (symbol: U). It is assigned atomic number 92 because it has 92 protons in its nucleus and 92 electrons around it.

     The next lightest element is helium (symbol: He) with 2 protons in its nucleus and 2 electrons orbiting around it. In the periodic table, they assigned helium an atomic number 2. Because it has 2 protons, they assumed that helium atom is twice as heavy as hydrogen. But when they compared the two elements, they found out that helium is 4 times heavier than hydrogen! It means that helium has 4 proton-like particles inside the nucleus but only two have positive charges to balance the two negatively charged electrons! That is how neutrons were discovered. The neutron is a particle as heavy as the proton but without electrical charge. The scientists assign a new number to describe an element. They call it atomic mass unit (amu). Hydrogen has an atomic number of 1 and an amu of 1. Helium has atomic number 2 and an amu of 4 (2 protons plus two neutrons). The natural uranium has an atomic number of 92 and an amu of 238 because it has 92 protons and 146 neutrons.

     The universe is indeed a dynamic place. Billions of stars are continuously burning themselves, their nuclear furnaces spewing out charged and uncharged particles and rays of energies in all directions. In this pretty busy place, an atom or a group of atoms may catch extra neutrons or lose some.  So it is not unusual that atoms of the same element may not have the same neutrons in their nuclei. Atoms of the same element but don’t have the same number of neutrons are called isotopes. Every element found in the periodic table has isotopes. Isotopes of an element are described by the atomic symbol and the amu which is slightly higher or lower than the amu of the natural element. For example, natural carbon atom has 6 protons and 6 neutrons and is usually referred to as C-12. But some carbon atoms hold 2 more neutrons and it is referred to as C-14. Another good example is hydrogen. Some hydrogen atoms contain an exra neutron and are designated 2H while some contain 2 neutrons and are designated 3H. Hydrogen isotopes are given special names. Thus 2H is called deuterium and 3H is called tritium.

     Most isotopes are unstable. As such, they try to eject excess particles and radiate energies until they return to a stable state. The  process of ejecting those excess particles and energies is called radioactivity.

    The transformation from being an unstable isotope to a stable substance  is called a decay. The time it takes for a radioactive element to decay to half of its original amount is called half-life. The half life of a radioactive isotope is constant regardless of what the original amount was and is a distinct characteristic of that particular isotope. For example, Iodine 131 has a half-life of 8 days. If at the start you have 2 grams, 8 days later, what is left is 1 gram, another 8 days, one-half gram is left, another 8 days later, one-fourth gram is left, and so on… Radium 226, on the other hand has a half-life of 1,600  years!

     Radioisotopes have many uses in our modern world. The half-life of an isotope in a substance can be used to measure long periods of time that help our scientists determine events that happened thousands or even millions of years ago.

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