SAMPLE EXERCISE 2.1 Illustrating the Size of an Atom
The diameter of a U.S. penny is 19 mm. The diameter of a silver atom, by comparison, is only 2.88 Å. How
many silver atoms could be arranged side by side in a straight line across the diameter of a penny?
Solution The unknown is the number of silver (Ag) atoms. We use the relationship 1 Ag atom = 2.88 Å as a
conversion factor relating the number of atoms and distance. Thus, we can start with the diameter of the penny,
first converting this distance into angstroms and then using the diameter of the Ag atom to convert distance to
the number of Ag atoms:
That is, 66 million silver atoms could sit side by side across a penny!
PRACTICE EXERCISE
The diameter of a carbon atom is 1.54 Å. (a) Express this diameter in picometers. (b) How many carbon atoms
could be aligned side by side in a straight line across the width of a pencil line that is 0.20 mm wide?
Answers: (a) 154 pm, (b) 1.3 106 C atoms
, SAMPLE EXERCISE 2.2 Determining the Number of Subatomic Particles in Atoms
How many protons, neutrons, and electrons are in (a) an atom of 197Au (b) an atom of strontium-90?
Solution (a) The superscript 197 is the mass number, the sum of the number of protons plus the number of
neutrons. According to the list of elements given on the front inside cover, gold has an atomic number of 79.
Consequently, an atom of 197Au has 79 protons, 79 electrons, and 197 – 79 = 118 neutrons. (b) The atomic
number of strontium (listed on the front inside cover) is 38. Thus, all atoms of this element have 38 protons
and 38 electrons. The strontium-90 isotope has 90 – 38 = 52 neutrons.
PRACTICE EXERCISE
How many protons, neutrons, and electrons are in (a) a 138Ba atom, (b) an atom of phosphorus-31?
Answer: (a) 56 protons, 56 electrons, and 82 neutrons; (b) 15 protons, 15 electrons, and 16 neutrons.
The diameter of a U.S. penny is 19 mm. The diameter of a silver atom, by comparison, is only 2.88 Å. How
many silver atoms could be arranged side by side in a straight line across the diameter of a penny?
Solution The unknown is the number of silver (Ag) atoms. We use the relationship 1 Ag atom = 2.88 Å as a
conversion factor relating the number of atoms and distance. Thus, we can start with the diameter of the penny,
first converting this distance into angstroms and then using the diameter of the Ag atom to convert distance to
the number of Ag atoms:
That is, 66 million silver atoms could sit side by side across a penny!
PRACTICE EXERCISE
The diameter of a carbon atom is 1.54 Å. (a) Express this diameter in picometers. (b) How many carbon atoms
could be aligned side by side in a straight line across the width of a pencil line that is 0.20 mm wide?
Answers: (a) 154 pm, (b) 1.3 106 C atoms
, SAMPLE EXERCISE 2.2 Determining the Number of Subatomic Particles in Atoms
How many protons, neutrons, and electrons are in (a) an atom of 197Au (b) an atom of strontium-90?
Solution (a) The superscript 197 is the mass number, the sum of the number of protons plus the number of
neutrons. According to the list of elements given on the front inside cover, gold has an atomic number of 79.
Consequently, an atom of 197Au has 79 protons, 79 electrons, and 197 – 79 = 118 neutrons. (b) The atomic
number of strontium (listed on the front inside cover) is 38. Thus, all atoms of this element have 38 protons
and 38 electrons. The strontium-90 isotope has 90 – 38 = 52 neutrons.
PRACTICE EXERCISE
How many protons, neutrons, and electrons are in (a) a 138Ba atom, (b) an atom of phosphorus-31?
Answer: (a) 56 protons, 56 electrons, and 82 neutrons; (b) 15 protons, 15 electrons, and 16 neutrons.
