Isotopes are variants of elements with different atomic weights due to different numbers of neutrons. They have similar chemical properties but differ in physical characteristics. Unstable isotopes decay into other elements and emit radiation. The strong nuclear force binds protons and neutrons in the nucleus, but too many neutrons can make an isotope unstable. Radioactive decay occurs through alpha and beta decay. Half-life is the time it takes for half of a sample of an isotope to decay. Isotopes have various uses, including in nuclear energy, radiometric dating, and medicine.
An isotope is a variant of an element that has a different atomic weight from other variants. Except for the most common form of hydrogen, which has only one proton, every atomic nucleus in normal matter is made up of both protons and neutrons. Isotopes of a given element have the same number of protons, but different numbers of neutrons. They have essentially the same chemical properties, but differ slightly in their physical characteristics, such as melting point and boiling point. Some isotopes are unstable and tend to decay into other elements, emitting subatomic particles or radiation; these are radioactive and are known as radioisotopes.
When scientists refer to a particular isotope of an element, the mass number, or the number of protons plus the number of neutrons, appears in the upper left, next to the element’s symbol. For example, the form of hydrogen that has one proton and one neutron is written as 2H. Similarly, 235U and 238U are two different isotopes of uranium. These are also commonly written as uranium-235 and uranium-238.
The atomic nucleus
Neutrons are electrically neutral, but protons have a positive electric charge. Because like charges repel each other, a nucleus containing more than one proton needs something to keep these particles from separating. That something is called a strong nuclear force, sometimes referred to simply as a strong force. It is much stronger than the electromagnetic force responsible for repelling protons, but unlike this force, it has a very short range. The strong force binds protons and neutrons together in the nucleus, but the electromagnetic force wants to push the protons apart.
Stable and unstable nuclei
In lighter elements, the strong force is able to hold the nucleus together as long as there are enough neutrons to dilute the electromagnetic force. Typically, in these elements, the number of protons and neutrons is about the same. In heavier elements, there must be an excess of neutrons to provide stability. Beyond a certain point, however, there is no configuration that provides a stable core. None of the elements heavier than lead have stable isotopes.
Too many neutrons can also make an isotope unstable. For example, the most common form of hydrogen has one proton and no neutrons, but there are two other forms, with one and two neutrons, called deuterium and tritium, respectively. Tritium is unstable because it has too many neutrons.
When an unstable, or radioactive, nucleus decays, it turns into a nucleus of another element. There are two mechanisms by which this can happen. Alpha decay occurs when the strong force fails to hold all the protons in a nucleus together. Instead of ejecting a proton, however, an alpha particle made up of two protons and two neutrons is ejected. Protons and neutrons are tightly bound together and the alpha particle is a stable configuration.
Beta decay occurs when a nucleus has too many neutrons. One of the neutrons transforms into a proton, which remains in the nucleus, and an electron, which is expelled. In tritium, for example, one of its two neutrons will eventually turn into a proton and an electron. This gives a nucleus with two protons and one neutron, which is a form of helium, known as 3He or helium-3. This isotope is stable, despite the excess of protons, because the nucleus is small enough for the strong force to hold it together.
Half lives
There is a fundamental uncertainty about the time it takes for a single unstable nucleus to decay; however, for a given isotope, the rate of decay is predictable. It is possible to give a very precise value for the amount of time it takes for half of a sample of a particular isotope to decay into another element. This value is known as the half-life and can range from a small fraction of a second to billions of years. The most common form of the element bismuth has a half-life a billion times the estimated age of the universe. It was once thought to be the heaviest stable element, but was shown to be mildly radioactive in 2003.
Property
In addition to the radioactivity issue, different isotopes of an element exhibit different physical properties. Heavier forms, with more neutrons, typically have higher melting and boiling points, due to the fact that more energy is required to get their atoms and molecules moving fast enough to cause a change of state. For example, “heavy water,” a form of water in which normal hydrogen is replaced by the heavier deuterium, freezes at 38.9°F (3.82°C) and boils at 214.5°F (101.4°C), compared to 32 °CF (0°C) and 212°F (100°C), respectively, for plain water. Chemical reactions can proceed slightly slower for heavier isotopes for the same reason.
it is used
Probably the most famous isotope is 235U, due to its use in nuclear energy and weaponry. Its instability is such that it can undergo a nuclear chain reaction, releasing enormous amounts of energy. “Enriched” uranium is uranium with a higher concentration of this isotope, while “depleted” uranium has a much lower concentration.
Radiometric dating uses the proportions of different isotopes to estimate the age of samples, such as biological materials or rocks. Radiocarbon dating, for example, uses the radioactive isotope 14C, or carbon-14, to date carbon-containing materials of organic origin. The age and geological history of the Earth are known largely through comparisons of the proportions of various isotopes in rock samples.
In biology and medicine, small amounts of mildly radioactive isotopes can be used as atomic markers to track the movement of various substances, such as drugs, through the body. The more strongly radioactive isotopes can be used as a source of radiation to destroy tumors and cancerous growths. Helium-3, thought to exist in large quantities on the moon, is among the most promising long-term fuels for fusion reactors. However, using it effectively will first require mastering other forms of blending.
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