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Your guide: Miss SanaCounts atoms the way you count rupees: carefully.
All atoms of an element have the same atomic number, but their mass numbers can differ. Atoms of the same element with different numbers of neutrons are called isotopes.
Carbon has three isotopes, 12C, 13C and 14C, with 6, 7 and 8 neutrons. Hydrogen has three: hydrogen 1H, deuterium 2H and tritium 3H. 1H is the only atom with no neutron. 2H has twice the mass of 1H, and 3H three times.
Chemical properties depend on electrons, and isotopes have the same electrons. So isotopes of an element behave almost the same chemically, while their physical properties, such as mass, can differ.
Some isotopes have unstable nuclei that give out extra energy as radiation. This is radioactivity. Tritium is radioactive; the other two hydrogen isotopes are stable. In radioactive decay the element changes, for example uranium-238 becomes thorium-234.
Medical imaging: technetium-99m is injected in small amounts, and a special camera follows it through organs such as the brain and lungs. Isotopes also diagnose and treat cancer and thyroid disorders.
Radiocarbon dating measures how much carbon-14 is left in wood or bone. The older the sample, the less carbon-14 it has, so we can work out when the plant or animal died.
Industry: testing the strength of metals and concrete, generating nuclear power and finding oil fields. Radiation, for example from radium-226, can also knock an electron out of an atom: Na → Na+ + e− gives a cation.
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All atoms of an element have the same atomic number, but their mass numbers can differ. Atoms of the same element with different numbers of neutrons are called isotopes.
Carbon has three isotopes, 12C, 13C and 14C, with 6, 7 and 8 neutrons. Hydrogen has three: hydrogen 1H, deuterium 2H and tritium 3H. 1H is the only atom with no neutron. 2H has twice the mass of 1H, and 3H three times.
Chemical properties depend on electrons, and isotopes have the same electrons. So isotopes of an element behave almost the same chemically, while their physical properties, such as mass, can differ.
Some isotopes have unstable nuclei that give out extra energy as radiation. This is radioactivity. Tritium is radioactive; the other two hydrogen isotopes are stable. In radioactive decay the element changes, for example uranium-238 becomes thorium-234.
Medical imaging: technetium-99m is injected in small amounts, and a special camera follows it through organs such as the brain and lungs. Isotopes also diagnose and treat cancer and thyroid disorders.
Radiocarbon dating measures how much carbon-14 is left in wood or bone. The older the sample, the less carbon-14 it has, so we can work out when the plant or animal died.
Industry: testing the strength of metals and concrete, generating nuclear power and finding oil fields. Radiation, for example from radium-226, can also knock an electron out of an atom: Na → Na+ + e− gives a cation.
Isotopes share the proton number, the electrons and the chemistry. They differ in neutrons, mass number, mass and stability.
Q1. Find the protons, neutrons and electrons in each isotope of carbon: 126C, 136C and 146C.
6 protons and 6 electrons in each; 6, 7 and 8 neutrons. This matches the diagrams on p. 24.
Q2. Which isotope of hydrogen is radioactive, and how does radiocarbon dating use a radioactive isotope?
Tritium is radioactive. Radiocarbon dating uses the amount of carbon-14 left to find when the organism died.
✗ Isotopes have different numbers of protons.
✓ Different protons would make a different element. Isotopes differ only in neutrons.
✗ Heavier isotopes react differently because they are heavier.
✓ Chemistry depends on electrons, which are the same, so the chemical behaviour is almost the same. Only physical properties differ.
✗ Every isotope gives out radiation.
✓ Only isotopes with unstable nuclei are radioactive. Of hydrogen's three, only tritium is.
1. Which particle is present in different number in the isotopes?
(b) Exercise MCQ (iv) (p. 30): isotopes have the same protons and electrons but different neutrons.
2. How is radiocarbon dating useful for archaeologists?
(a) Exercise MCQ (vii) (p. 30); p. 26: the carbon-14 left tells when the organism died.
3. Why have isotopes not been shown in the periodic table?
(c) Exercise MCQ (iii) (p. 30): the table is ordered by atomic number, which isotopes share.
4. Which hydrogen isotope is radioactive?
(d) p. 25: tritium is radioactive; the other two are stable.
5. Isotopes of an element show the same chemical properties because they have the same:
(b) p. 24: chemical properties are determined by the number of electrons.
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