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Questions on constituents of the atom
The proton, neutron and electron are set out by charge and mass in SI and in relative units, and specific charge is charge divided by mass. Nuclide notation carries the nucleon number A and the proton number Z, and isotopes of one element share Z while their A differs.
19 original questions · 54 marks · the constituents of the atom notes · Particles
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State the charge and the relative mass of the proton, the neutron and the electron.
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Proton: charge +1.60 × 10−19 C, relative mass 1. Neutron: charge 0, relative mass 1 (1). Electron: charge −1.60 × 10−19 C, relative mass about 1/1836 (1).Calculate the specific charge of a proton (e = 1.60 × 10−19 C, mp = 1.67 × 10−27 kg).
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Specific charge = Q/m = (1.60 × 10−19)/(1.67 × 10−27) (1)
= 9.58 × 107 C kg−1 (1)Explain what is meant by two isotopes of an element.
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Isotopes are atoms of the same element, so with the same number of protons and the same Z (1), that have different numbers of neutrons and so different mass numbers A (1).The nuclide of copper used in electrical wiring is written 6329Cu. State what the number 63 and the number 29 each represent.
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63 is the nucleon number A, the total number of protons and neutrons in the nucleus (1). 29 is the proton number Z, the number of protons (1).State the specific charge of the neutron. Explain your answer.
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Zero (1). Specific charge is charge divided by mass, and the neutron has mass but no charge, so Q/m = 0 (1).State one use of isotopic data.
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Carbon dating: the ratio of carbon-14 to carbon-12 in dead material falls with age, so measuring it dates archaeological finds (1). (Accept any valid use, e.g. identifying the origin of a sample from its isotopic signature.)Calculate the specific charge of an electron (me = 9.11 × 10−31 kg), and comment on how it compares with that of a proton.
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Specific charge = (1.60 × 10−19)/(9.11 × 10−31) (1)
= 1.76 × 1011 C kg−1 (1)
Far larger than the proton's, because the electron has the same magnitude of charge but a much smaller mass (1)A nucleus is written 146C. State the number of protons and neutrons it contains, and the number of electrons in the neutral atom.
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Protons: 6, since Z = 6 (1). Neutrons: A − Z = 14 − 6 = 8 (1). The neutral atom has 6 electrons (1).Calculate the specific charge of an alpha particle (a helium-4 nucleus: charge +2e, mass ≈ 4mp).
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Charge = 2e = 3.2 × 10−19 C (1)
Mass ≈ 4 × 1.67 × 10−27 = 6.68 × 10−27 kg (1)
Specific charge = 4.79 × 107 C kg−1 (1)An oxygen-16 atom loses two electrons to become an ion of charge +2e. Calculate the specific charge of this ion (mass ≈ 16mp).
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Charge = 2 × 1.60 × 10−19 = 3.2 × 10−19 C (1)
Mass ≈ 16 × 1.67 × 10−27 = 2.67 × 10−26 kg (1)
Specific charge = 1.20 × 107 C kg−1 (1)A bare nucleus produced in an ion source has a charge of +4.80 × 10−19 C and a mass of 1.17 × 10−26 kg. The candidate nuclides are helium-4 (Z = 2), lithium-7 (Z = 3), beryllium-9 (Z = 4) and boron-11 (Z = 5). Determine which nuclide it is (e = 1.60 × 10−19 C; nucleon mass = 1.67 × 10−27 kg).
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Number of protons = Q/e = (4.80 × 10−19)/(1.60 × 10−19) = 3 (1). Number of nucleons = m/mnucleon = (1.17 × 10−26)/(1.67 × 10−27) = 7 (1). Z = 3 and A = 7, so the nucleus is lithium-7 (1).In an electrostatic paint sprayer, a paint droplet of mass 2.4 × 10−15 kg acquires a specific charge of 2.0 × 10−4 C kg−1 by losing electrons. Calculate the charge on the droplet and hence the number of electrons removed (e = 1.60 × 10−19 C).
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Q = specific charge × m = 2.0 × 10−4 × 2.4 × 10−15 = 4.8 × 10−19 C (1). Number removed = Q/e = (4.8 × 10−19)/(1.60 × 10−19) (1) = 3 electrons (1).Chlorine has two stable isotopes, 3517Cl and 3717Cl. State the number of protons and the number of neutrons in each nucleus, and explain why the two isotopes have identical chemical properties.
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Both nuclei have 17 protons (1). Chlorine-35 has 35 − 17 = 18 neutrons and chlorine-37 has 20 (1). Chemical properties are set by the electrons, and both neutral atoms have the same number of electrons (17, equal to Z), so their chemistry is identical (1).A uranium nucleus is written 23892U. State the number of protons and neutrons, and calculate the specific charge of the nucleus (mass ≈ 238mp).
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Protons: 92 (1)
Neutrons: 238 − 92 = 146 (1)
Charge = 92e = 1.47 × 10−17 C (1)
Specific charge = (92 × 1.60 × 10−19)/(238 × 1.67 × 10−27) = 3.70 × 107 C kg−1 (1)Compare the specific charge of a single proton (a hydrogen-1 nucleus) with that of an alpha particle by calculating both, and state which is greater.
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Proton: (1.60 × 10−19)/(1.67 × 10−27) (1)
= 9.58 × 107 C kg−1 (1)
Alpha: 4.79 × 107 C kg−1 (1)
The proton's is greater, by about twice, because the alpha has twice the charge but four times the mass (1)Explain why the electron has by far the largest specific charge of the three sub-atomic particles.
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Specific charge is charge divided by mass (1). The electron carries the same magnitude of charge as the proton but has a mass roughly 1836 times smaller (1), so its charge-to-mass ratio is far larger than that of the proton or the uncharged neutron (1).A helium-3 nucleus contains 2 protons and 1 neutron. Show that the specific charge of a helium-3 nucleus is about 6 × 107 C kg−1. Go on to explain why this is greater than the specific charge of a helium-4 nucleus (e = 1.60 × 10−19 C; nucleon mass = 1.67 × 10−27 kg).
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Q = 2e = 3.20 × 10−19 C and m = 3 × 1.67 × 10−27 = 5.01 × 10−27 kg (1). Specific charge = Q/m = 6.39 × 107 C kg−1, which is about 6 × 107 C kg−1 (1). A helium-4 nucleus has the same charge of 2e but one more neutron, so a larger mass (1). The same charge divided by a larger mass gives a smaller specific charge, so helium-3's is the greater (1).A mass spectrometer identifies ions by their specific charge. A singly ionised neon atom is measured to have a specific charge of 4.36 × 106 C kg−1. Neon has two common isotopes, neon-20 and neon-22. Deduce which isotope the measured ion is. Treat the mass of an ion of nucleon number A as A × 1.67 × 10−27 kg; e = 1.60 × 10−19 C.
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A singly ionised atom carries charge Q = e (1). Neon-20: specific charge = e/(20 × 1.67 × 10−27) = 4.79 × 106 C kg−1; neon-22: e/(22 × 1.67 × 10−27) = 4.35 × 106 C kg−1 (1). The measured 4.36 × 106 C kg−1 matches the neon-22 value closely but is about 9% below the neon-20 value (1). The ion is neon-22 (1).Explain why no bare nucleus can have a specific charge greater than that of a single proton. Treat the proton and neutron masses as equal.
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For a nucleus, specific charge = Ze/(A × mnucleon), which is (Z/A) times the proton's specific charge e/mp (1). Every nucleus has A = Z + N with N ≥ 0, so Z/A ≤ 1 (1). The specific charge therefore cannot exceed e/mp, with equality only for hydrogen-1, which is a single proton (1).
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