Category Chemistry

The half-life of a radioactive substance is a measure of the rate at which the nuclei of its atoms are breaking up or decaying. It is the time it takes for half the atoms in a sample to decay. Thorium, for example, has a half-life of 24 days, while radium-221 has a half-life of only 30 seconds. Uranium has a half-life of 4.5 thousand million years. Of course, as each isotope of an element has a certain number of protons and neutrons in its nucleus, it changes as it decays, forming other elements. For example, plutonium-242 decays to become uranium-238, which in turn breaks down to become thorium-234.

The half-life of a radioactive substance is a characteristic constant. It measures the time it takes for a given amount of the substance to become reduced by half as a consequence of decay, and therefore, the emission of radiation.

Archeologists and geologists use half-life to date the age of organic objects in a process known as carbon dating. During beta decay, carbon 14 becomes nitrogen 14. At the time of death organisms stop producing carbon 14. Since half-life is a constant, the ratio of carbon 14 to nitrogen 14 provides a measurement of the age of a sample.

In the medical field, the radioactive isotope Cobalt 60 has been used for radiotherapy to shrink tumors that will later be surgically removed, or to destroy cancer cells in inoperable tumors. When it decays to stable nickel, it emits two relatively high-energy gamma rays. Today it is being replaced by electron beam radiation therapy systems.
The half-life of isotopes from some sample elements:

Oxygen 16 – infinite
uranium 238 – 4,460,000,000 years
uranium 235 – 713,000,000 years
carbon 14 – 5,730 years
cobalt 60 – 5.27 years
silver 94 – .42 seconds

In the illustration above, 50% of the original mother substance decays into a new daughter substance. After two half-lives, the mother substance will decay another 50%, leaving 25% mother and 75% daughter. A third half-life will leave 12.5% of the mother and 87.5% daughter. In reality, daughter substances can also decay, so the proportions of substance involved will vary.

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The isotope called carbon-14 has a half-life of 5730 years. All living things on our planet contain this form of carbon, but they stop taking it in when they die. Scientists can examine ancient substances to see how much the carbon in it has decayed. They can then give a fairly accurate date for when the substance was alive. This is particularly useful for archaeologists and historians, who can date objects they find, helping to build up a picture of the past.

Radiocarbon dating is a method of what is known as “Absolute Dating”. Despite the name, it does not give an absolute date of organic material – but an approximate age, usually within a range of a few years either way. The other method is “Relative Dating” which gives an order of events without giving an exact age: typically artifact typology or the study of the sequence of the evolution of fossils.

There are three carbon isotopes that occur as part of the Earth’s natural processes; these are carbon-12, carbon-13 and carbon-14. The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method. The other two isotopes in comparison are more common than carbon-14 in the atmosphere but increase with the burning of fossil fuels making them less reliable for study; carbon-14 also increases, but its relative rarity means its increase is negligible. The half-life of the 14C isotope is 5,730 years, adjusted from 5,568 years originally calculated in the 1940s; the upper limit of dating is in the region of 55-60,000 years, after which the amount of 14C is negligible. After this point, other Absolute Dating methods may be used.

Today, the amount of carbon dioxide humans are pumping into Earth’s atmosphere is threatening to skew the accuracy of this technique for future archaeologists looking at our own time. That’s because fossil fuels can shift the radiocarbon age of new organic materials today, making them hard to distinguish from ancient ones. Thankfully, research published yesterday in the journal Environmental Research Letters offers a way to save Libby’s work and revitalize this crucial dating technique: simply look at another isotope of carbon.

Carbon-12 is a stable isotope, meaning its amount in any material remains the same year-after-year, century-after-century. Libby’s groundbreaking radiocarbon dating technique instead looked at a much rarer isotope of carbon: Carbon-14. Unlike Carbon-12, this isotope of carbon is unstable, and its atoms decay into an isotope of nitrogen over a period of thousands of years. New Carbon-14 is produced at a steady rate in Earth’s upper atmosphere, however, as the Sun’s rays strike nitrogen atoms.

Radiocarbon dating exploits this contrast between a stable and unstable carbon isotope. During its lifetime, a plant is constantly taking in carbon from the atmosphere through photosynthesis. Animals, in turn, consume this carbon when they eat plants, and the carbon spreads through the food cycle. This carbon comprises a steady ratio of Carbon-12 and Carbon-14.

When these plants and animals die, they cease taking in carbon. From that point forward, the amount of Carbon-14 in materials left over from the plant or animal will decrease over time, while the amount of Carbon-12 will remain unchanged. To radiocarbon date an organic material, a scientist can measure the ratio of remaining Carbon-14 to the unchanged Carbon-12 to see how long it has been since the material’s source died. Advancing technology has allowed radiocarbon dating to become accurate to within just a few decades in many cases.

Carbon dating is a brilliant way for archaeologists to take advantage of the natural ways that atoms decay. Unfortunately, humans are on the verge of messing things up. The slow, steady process of Carbon-14 creation in the upper atmosphere has been dwarfed in the past centuries by humans spewing carbon from fossil fuels into the air. Since fossil fuels are millions of years old, they no longer contain any measurable amount of Carbon-14. Thus, as millions of tons of Carbon-12 are pushed into the atmosphere, the steady ratio of these two isotopes is being disrupted. In a study published last year, Imperial College London physicist Heather Graven pointed out how these extra carbon emissions will skew radiocarbon dating.

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There are two kinds of nuclear reaction, both of which give off huge amounts of energy. Nuclear fusion happens when two nuclei collide and combine to form one larger nucleus. This gives off enormous power. Nuclear fission happens when neutrons bombard the nucleus of an atom, causing the nucleus to split apart.

In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle (such as a proton, neutron, or high energy electron) from outside the atom, collide to produce one or more nuclides that are different from the nuclide(s) that began the process (parent nuclei). Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction.

In principle, a reaction can involve more than two particles colliding, but because the probability of three or more nuclei to meet at the same time at the same place is much less than for two nuclei, such an event is exceptionally rare for an example very close to a three-body nuclear reaction). The term “nuclear reaction” may refer either to a change in a nuclide induced by collision with another particle, or to a spontaneous change of a nuclide without collision.

Natural nuclear reactions occur in the interaction between cosmic rays and matter, and nuclear reactions can be employed artificially to obtain nuclear energy, at an adjustable rate, on demand. Perhaps the most notable nuclear reactions are the nuclear chain reactions in fissionable materials that produce induced nuclear fission, and the various nuclear fusion reactions of light elements that power the energy production of the Sun and stars.

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Most elements do not change unless a force is applied to them that causes them to join with another element. They are said to be stable. But some elements are not stable. Their nuclei are constantly breaking down, or decaying, as they shed particles in an attempt to become stable. This is radioactivity, and the particles that are given off are known as radiation. Three types of particles are known to be emitted: alpha, beta and gamma rays.

As its name implies, radioactivity is the act of emitting radiation spontaneously. This is done by an atomic nucleus that, for some reason, is unstable; it “wants” to give up some energy in order to shift to a more stable configuration. During the first half of the twentieth century, much of modern physics was devoted to exploring why this happens, with the result that nuclear decay was fairly well understood by 1960. Too many neutrons in a nucleus lead it to emit a negative beta particle, which changes one of the neutrons into a proton. Too many protons in a nucleus lead it to emit a positron (positively charged electron), changing a proton into a neutron. Too much energy leads a nucleus to emit a gamma ray, which discards great energy without changing any of the particles in the nucleus. Too much mass leads a nucleus to emit an alpha particle, discarding four heavy particles (two protons and two neutrons).

Radioactivity is a physical, not a biological, phenomenon. Simply stated, the radioactivity of a sample can be measured by counting how many atoms are spontaneously decaying each second. This can be done with instruments designed to detect the particular type of radiation emitted with each “decay” or disintegration. The actual number of disintegrations per second may be quite large. Scientists have agreed upon common units to use as a form of shorthand. Thus, a curie (abbreviated “Ci” and named after Pierre and Marie Curie, the discoverers of radium (87) is simply a shorthand way of writing “37,000,000,000 disintegrations per second,” the rate of disintegration occurring in 1 gram of radium. The more modern International System of Measurements (SI) unit for the same type of measurement is the Becquerel (abbreviated “Bq” and named after Henri Becquerel, the discoverer of radioactivity), which is simply a shorthand for “1 disintegration per second.”

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A mass spectrometer is a machine that can measure the mass of atoms and so identify them. Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio of ions. The results are typically presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

A mass spectrum is a plot of the ion signal as a function of the mass-to-charge ratio. These spectra are used to determine the elemental or isotopic signature of a sample, the masses of particles and of molecules, and to elucidate the chemical identity or structure of molecules and other chemical compounds.

In a typical MS procedure, a sample, which may be solid, liquid, or gaseous, is ionized, for example by bombarding it with electrons. This may cause some of the sample’s molecules to break into charged fragments or simply become charged without fragmenting. These ions are then separated according to their mass-to-charge ratio, for example by accelerating them and subjecting them to an electric or magnetic field: ions of the same mass-to-charge ratio will undergo the same amount of deflection. The ions are detected by a mechanism capable of detecting charged particles, such as an electron multiplier. Results are displayed as spectra of the signal intensity of detected ions as a function of the mass-to-charge ratio. The atoms or molecules in the sample can be identified by correlating known masses (e.g. an entire molecule) to the identified masses or through a characteristic fragmentation pattern.

The atomic number of an element is the number of protons it contains. For example, hydrogen has one proton, so its atomic number is one. Tin has an atomic number of 50 because it has 50 protons in its nucleus.

Atomic number of a chemical element in the periodic system, whereby the elements are arranged in order of increasing number of protons in the nucleus. Accordingly, the number of protons, which is always equal to the number of electrons in the neutral atom, is also the atomic number. An atom of iron has 26 protons in its nucleus; therefore the atomic number of iron is 26. In the symbol representing a particular nuclear or atomic species, the atomic number may be indicated as a left subscript. An atom or a nucleus of iron (chemical symbol Fe), for example, may be written 26Fe.

The atomic number of a chemical element is the number of protons in the nucleus of an atom of the element. It is the charge number of the nucleus since neutrons carry no net electrical charge. The atomic number determines the identity of an element and many of its chemical properties. The modern periodic table is ordered by increasing atomic number.

The atomic number of hydrogen is 1; the atomic number of carbon is 6, and the atomic number of silver is 47: any atom with 47 protons is an atom of silver. Varying the number of neutrons in an element changes its isotopes while changing the numbers of electrons makes it an ion.

The atomic number is also known as the proton number. It may be represented by the capital letter Z. The use of capital letter Z comes from the German word Atomzahl, which means “atomic number.” Before the year 1915, the word Zahl (number) was used to describe an element’s position on the periodic table.

The reason the atomic number determines the chemical property of an element is that the number of protons also determines the number of electrons in an electrically neutral atom. This, in turn, defines the electron configuration of the atom and the nature of its outermost or valence shell. The behavior of the valence shell determines how readily an atom will form chemical bonds and participate in chemical reactions.

At the time of this writing, elements with atomic numbers 1 through 118 have been identified. Scientists typically talk about discovering new elements with higher atomic numbers. Some researchers believe there may be an “island of stability,” where the configuration of protons and neutrons of super heavy atoms will be less susceptible to the quick radioactive decay seen in known heavy elements.