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General Chemistry

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Radioactivity

Question
5 out of 11
 

Substance X has a radioactive half-life of 12 years. How much time must have elapsed if only 9 grams is left from an original sample of 150 grams?

A 12 years
B 24 years
C 36 years
D 48 years
Ans. D This question tests your knowledge of the concept of half-life. Half-life is defined as the amount of time for a radioactive substance to decay to half its initial quantity. Since the half-life of substance X is 12 years, it takes twelve years for substance X to decay to half its initial quantity. We have 9 grams of substance X present. The amount of substance X that was originally present is 150 grams. So it must have undergone radioactive decay for at least four half-lives. Write the decay mode, starting from the original amount present.

Each arrow indicates one half-life, for a total of four half-lives. Since one half-life is 12 years, the total time elapsed is 12 x 4 = 48 years. The exact answer is a little above 48 years. But the approximation is correct since the choices are far apart and no values higher than 48 are given.

Radioactivity Flashcard List

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In nuclear reactions, significant changes occur in the composition of the nuclei of the atoms involved. These reactions usually release tremendous amounts of energy. One of the reasons for the nuclear changes can be attributed to the stability of the nucleus. The formation of nucleus from the subatomic particles - neutrons and protons, results in the release of energy. The mass of these individual particles in the nucleus is greater than that of the actual nucleus that is formed. This loss of mass is due to the change of mass into energy. The energy-mass relation can be represented in terms of the equation: E = mc2, where m represents the mass, and c represents the speed of light (3x108 m/s). If the nucleus of an atom is not stable, it can get transformed into another nucleus. A plot of the number of neutrons versus the number of protons is often used to assess the stability trends of elements. If the number of protons and neutrons are equal, the nucleus is considered to be reasonably stable. As the atomic number increases, the trend changes. Isotopes of elements having atomic numbers above ≈83 are unstable atoms. These unstable atoms can undergo disintegrations. The half-lives of some radioactive elements are shown in Table 1. All the following are true regarding radioactive rays, except:A α-particles are positively chargedB β-particles are negatively chargedC γ rays are electromagnetic rays and can be deflected by an electric fieldD There are radioactive emissions in which the mass number is not affected
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In nuclear reactions, significant changes occur in the composition of the nuclei of the atoms involved. These reactions usually release tremendous amounts of energy. One of the reasons for the nuclear changes can be attributed to the stability of the nucleus. The formation of nucleus from the subatomic particles - neutrons and protons, results in the release of energy. The mass of these individual particles in the nucleus is greater than that of the actual nucleus that is formed. This loss of mass is due to the change of mass into energy. The energy-mass relation can be represented in terms of the equation: E = mc2, where m represents the mass, and c represents the speed of light (3x108 m/s). If the nucleus of an atom is not stable, it can get transformed into another nucleus. A plot of the number of neutrons versus the number of protons is often used to assess the stability trends of elements. If the number of protons and neutrons are equal, the nucleus is considered to be reasonably stable. As the atomic number increases, the trend changes. Isotopes of elements having atomic numbers above ≈83 are unstable atoms. These unstable atoms can undergo disintegrations. The half-lives of some radioactive elements are shown in Table 1. Which of the following is true regarding radioactivity?A As the atomic number increases, eventually the neutron-proton ratio values become < 1B As the atomic number increases, eventually the neutron-proton ratio values become > 1C As the atomic number increases, eventually the proton-neutron ratio values become > 1D None of the above
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In nuclear reactions, significant changes occur in the composition of the nuclei of the atoms involved. These reactions usually release tremendous amounts of energy. One of the reasons for the nuclear changes can be attributed to the stability of the nucleus. The formation of nucleus from the subatomic particles - neutrons and protons, results in the release of energy. The mass of these individual particles in the nucleus is greater than that of the actual nucleus that is formed. This loss of mass is due to the change of mass into energy. The energy-mass relation can be represented in terms of the equation: E = mc2, where m represents the mass, and c represents the speed of light (3x108 m/s). If the nucleus of an atom is not stable, it can get transformed into another nucleus. A plot of the number of neutrons versus the number of protons is often used to assess the stability trends of elements. If the number of protons and neutrons are equal, the nucleus is considered to be reasonably stable. As the atomic number increases, the trend changes. Isotopes of elements having atomic numbers above ≈83 are unstable atoms. These unstable atoms can undergo disintegrations. The half-lives of some radioactive elements are shown in Table 1. The most probable set of particles that were given off during the series of nuclear changes from 232Th to 224Ra are:A Two alpha particles and one beta particleB One alpha particle and two beta particleC One alpha particle and three beta particlesD Two alpha particles and two beta particles