Problem 25
(a) Define atomic number and mass number. (b) Which of these can vary without changing the identity of the element?
Problem 26
(a) Which two of the following are isotopes of the same element: \(106 \mathrm{X},{ }_{46}^{107} \mathrm{X},{ }_{4}^{107} \mathrm{X} ?(\mathbf{b})\) What is the identity of the element whose isotopes you have selected?
Problem 28
Each of the following isotopes is used in medicine. Indicate the number of protons and neutrons in each isotope: (a) samarium-153, \((\mathbf{b})\) lutetium-177, \((\mathbf{c})\) bismuth- \(213,\) (d) molybdenum-99, \((\mathbf{e})\) lead-212, \((\mathbf{f})\) caesium- \(131 .\)
Problem 29
Fill in the gaps in the following table, assuming each column represents a neutral atom. $$ \begin{array}{l|c|c|c|c|c} \hline \text { Symbol } & 159 & & & & \\ \text { Protons } & & 29 & & & 37 \\ \text { Neutrons } & & 34 & 53 & & \\ \text { Electrons } & & & 42 & 34 & \\ \text { Mass no. } & & & & 79 & 85 \\ \hline \end{array} $$
Problem 30
Fill in the gaps in the following table, assuming each column represents a neutral atom. $$ \begin{array}{l|l|l|l|l|l} \hline \text { Symbol } & { }^{89} \mathrm{Y} & & & & \\ \text { Protons } & & 78 & & & 89 \\ \text { Neutrons } & & & 123 & & \\ \text { Electrons } & & & 81 & 50 & \\ \text { Mass no. } & & 195 & & 119 & 227 \\ \hline \end{array} $$
Problem 32
One way in which Earth's evolution as a planet can be understood is by measuring the amounts of certain isotopes in rocks. One quantity recently measured is the ratio of \({ }^{129} \mathrm{Xe}\) to \({ }^{130} \mathrm{Xe}\) in some minerals. In what way do these two isotopes differ from one another? In what respects are they the same?
Problem 33
(a) What isotope is used as the standard in establishing the atomic mass scale? (b) The atomic weight of boron is reported as 10.81 , yet no atom of boron has the mass of \(10.81 \mathrm{u}\). Explain.
Problem 34
(a) What is the mass in \(\mathrm{u}\) of a carbon- 12 atom? (b) Why is the atomic weight of carbon reported as 12.011 in the table of elements and the periodic table in the front inside cover of this text?
Problem 35
Iron has three major isotopes: \({ }^{54} \mathrm{Fe}\) (atomic mass = \(53.9396 \mathrm{u}\); abundance \(5.85 \%),{ }^{56} \mathrm{Fe}\) (atomic mass = 55.9349u; abundance 91.75\%), and \({ }^{57} \mathrm{Fe}\) (atomic mass \(=56.9354 \mathrm{u} ;\) abundance \(2.12 \%) .\) Calculate the atomic weight (average atomic mass) of iron.
Problem 36
Bromine has two naturally occurring isotopes, bromine- 79 (atomic mass \(=78.9183 \mathrm{u} ; \quad\) abundance \(=50.69 \%)\) a \(\mathrm{nd}\) bromine- \(81 \quad\) (atomic mass \(=80.9163\) u; abundance = \(49.31 \%)\). Calculate the atomic weight of bromine.
