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2 Atoms, Molecules, and Ions Solutions to Exercises 2.18 (a) r = d/2; r = 2.7 10⁻⁸ 2 cm 1 10⁻⁸ 1 Å cm = 1.35 = 1.4 Å r = 2.7 10⁻⁸ 2 cm 1m = 1.35 X 10⁻¹⁰ = 1.4 10⁻¹⁰ m (b) Aligned Rh atoms have diameters touching. d = 2.7 10⁻⁸ cm = 2.7 10⁻¹⁰ m 6.0 µm 1 1 10⁻⁶ µm m 1 Rh 10⁻¹⁰ atom m = 2.2 10⁴ Rh atoms (c) V = 4/3 π r³; r = 1.35 X 10⁻¹⁰ = 1.4 10⁻¹⁰ m V = (4/3)[(π(1.35 m³ = 1.031 10⁻²⁹ = 1.0 10⁻²⁹ m³ 2.19 (a) proton, neutron, electron (b) proton = +1, neutron = 0, electron = -1 (c) The neutron is most massive. (The neutron and proton have very similar masses). (d) The electron least massive. 2.20 (a) The nucleus has most of the mass but occupies very little of the volume of an atom. (b) True (c) The number of electrons in a neutral atom is equal to the number of protons in the atom. (d) True 2.21 (a) Atomic number is the number of protons in the nucleus of an atom. Mass number is the total number of nuclear particles, protons plus neutrons, in an atom. (b) The mass number can vary without changing the identity of the atom, but the atomic number of every atom of a given element is the same. 2.22 (a) 31 16 X and 16 32 X are isotopes of the same element, because they have identical atomic numbers. (b) These are isotopes of the element sulfur, S, atomic number = 16. 2.23 p = protons, n = neutrons, e = electrons (a) has 18 p, 22 n, 18 e (b) has 30 p, 35 n, 30 e (c) has 31 p, 39 n, 31 e (d) has 35 p, 45 n, 35 e (e) has 74 p, 110 n, 74 e (f) has 95 p, 148 n, 95 e 2.24 (a) has 15 p, 17 n (b) has 24 p, 27 n (c) has 27 p, 33 n (d) has 43 p, 56 n (e) has 53 p, 78 n (f) has 81 p, 120 n 22