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Learning Outcomes
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Chapter Summary
  • Atoms are the tiny building blocks of all matter. Elements are substances that cannot be broken down into simpler substances.

  • Studies with radiation and cathode ray tubes indicated that atoms contained subatomic particles, one of which was the electron.

  • Experiments with radioactivity have shown that some atoms give off different types of radiation, called α rays, β rays,and γ rays. Alpha rays are composed of α particles. Beta rays are composed of β particles, which are actually electrons. Gamma rays are high-energy radiation.

  • Most of the mass of an atom resides in a tiny, dense region known as the nucleus. The nucleus contains positively charged particles called protons and electrically neutral particles called neutrons.

  • The atomic number, Z, is the number of protons in the nucleus of an atom. Atomic number determines the identity of the atom.

  • The mass number, A, is the sum of the protons and neutrons in the nucleus. Protons and neutrons are referred to collectively as nucleons.

  • Atoms with the same atomic number but different mass numbers are called isotopes.

  • Atomic mass is the mass of an atom in atomic mass units. The periodic table contains the average atomic mass (sometimes called the atomic weight) of each element.

  • The periodic table arranges the elements in rows (periods) and columns (groups). Elements in the same group exhibit similar properties.

  • Metals are good conductors of electricity; nonmetals are not good conductors of electricity. Metalloids have properties intermediate between those of metals and nonmetals.

  • Some of the groups have special names including alkali metals (Group 1A, except hydrogen), alkaline earth metals (Group 2A), chalcogens (Group 6A), halogens (Group 7A), noble gases (Group 8A), and transition metals (Group 1B and Groups 3B–8B).

  • A mole is the amount of a substance that contains 6.0221418 × 1023 [Avogadro's number (NA)] of elementary particles (such as atoms). Avogadro's constant, with units of reciprocal moles, is used to convert between moles and numbers of atoms.

  • Molar mass (M) is the mass of one mole of a substance, usually expressed in grams. The molar mass of an element in grams is numerically equal to the atomic mass of the element in amu.

  • Molar mass and Avogadro's constant can be used to interconvert among mass, moles, and number of atoms.

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Key Words

Alkali metal, 44

Alkaline earth metal, 44

Alpha particle (α), 36

Alpha rays (α), 36

Atom, 33

Atomicmass, 41

Atomic number (Z), 39

Average atomic masss, 41

Avogadro's number (NA), 45

Beta particle (β), 36

Beta rays (β), 36

Cathode ray tube, 34

Chalcogens, 44

Electron, 35

Element, 33

Gamma rays (γ), 36

Group, 44

Halogens, 44

Isotope, 39

Mass number (A), 39

Metal, 44

Metalloid, 44

Molar mass (M), 46

Mole, 45

Neutron, 38

Noble gases, 44

Nonmetal, 44

Nucleons, 39

Nucleus, 37

Period, 44

Periodic table, 43

Proton, 38

Radiation, 34

Radioactivity, 36

Transition elements, 44

Transition metal, 44

X rays, 36

Questions and Problems
Review Questions



Define the terms atom and element.



Use a familiar macroscopic example as an analogy to describe Dalton's atomic theory. Explain why a substance such as clay or oil is not useful for an analogy of this type.

Review Questions



Define the following terms: (a) α particle, (b) β particle, (c) γ ray, (d) X ray.



Name the types of radiation known to be emitted by radioactive elements.



Compare the properties of the following: α particles, cathode rays, protons, neutrons, and electrons.



Describe the contributions of the following scientists to our knowledge of atomic structure: J. J. Thomson, R. A. Millikan, Ernest Rutherford, and James Chadwick.



Describe the experimental basis for believing that the nucleus occupies a very small fraction of the volume of the atom.




The diameter of an argon atom is about 2 × 102 pm. If we could line up argon atoms side by side in contact with one another, how many atoms would it take to span a distance of 1 cm?



The radius of an atom is on the order of 10,000 times as large as its nucleus. If the atom were enlarged such that the radius of its nucleus were 1.0 cm, what would be the radius of the atom in meters, and in miles (1 mi = 1609 m)?

Review Questions



Use the argon-40 isotope to define atomic number and mass number. Why does knowledge of the atomic number enable us to deduce the number of electrons present in an atom?



Why do all atoms of an element have the same atomic number, although they may have different mass numbers?



What do we call atoms of the same elements with different mass numbers?



Explain the meaning of each term in the symbol AZX.




What is the mass number of an iron atom that has 31 neutrons?



Calculate the number of neutrons of 243Pu.



For each of the following species, determine the number of protons and the number of neutrons in the nucleus: 63Li, 2813Al, 2913Al, 5023V, 7734Se, 19377Ir.



Indicate the number of protons, neutrons, and electrons in each of the following species: 178O, 2914Si, 5828Ni, 8939Y, 18073Ta, 20381Tl.



Write the appropriate symbol for each of the following isotopes: (a) Z = 11, A = 23; (b) Z = 28, A = 64; (c) Z = 50, A = 115; (d) Z = 20, A = 42.



Write the appropriate symbol for each of the following isotopes: (a) Z = 75, A = 187; (b) Z = 83, A = 209; (c) Z = 33, A = 75; (d) Z = 93, A = 236.



Determine the mass number of (a) a beryllium atom with 5 neutrons, (b) a sodium atom with 12 neutrons, (c) a selenium atom with 44 neutrons, and (d) a gold atom with 118 neutrons.



Determine the mass number of (a) a chlorine atom with 18 neutrons, (b) a phosphorus atom with 17 neutrons, (c) an antimony atom with 70 neutrons, and (d) a palladium atom with 59 neutrons.



The following radioactive isotopes are used in medicine for things such as imaging organs, studying blood circulation, and treating cancer. Give the number of neutrons present in each isotope: 198Au, 47Ca, 60Co, 18F, 125I, 131I, 42K, 43K, 24Na, 32P, 85Sr, 99Tc.

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Review Questions



What is the mass (in amu) of a carbon-12 atom? Why is the atomic mass of carbon listed as 12.01 amu in the table on the inside front cover of this book?



Explain clearly what is meant by the statement “The atomic mass of gold is 197.0 amu.”



What information would you need to calculate the average atomic mass of an element?




The atomic masses of 3517Cl (75.78 percent) and 3717Cl (24.22 percent) are 34.969 and 36.966 amu, respectively. Calculate the average atomic mass of chlorine. The percentages in parentheses denote the relative abundances.



The atomic masses of 204Pb (1.4 percent), 206Pb (24.1 percent),207Pb (22.1 percent), and 208Pb (52.4 percent) are 203.973020, 205.974440, 206.975872, and 207.976627 amu, respectively. Calculate the average atomic mass of lead. The percentages in parentheses denote the relative abundances.



The atomic masses of 203Tl and 205Tl are 202.972320 and 204.974401 amu, respectively. Calculate the natural abundances of these two isotopes. To seven significant figures, the average atomic mass of thallium is 204.3833 amu.


The atomic masses of 6Li and 7Li are 6.0151 amu and 7.0160 amu, respectively. Calculate the natural abundances of these two isotopes. The average atomic mass of Li is 6.941 amu.


The element rubidium has two naturally occurring isotopes. The atomic mass of 85Rb (72.17 percent abundant) is 84.911794 amu. Determine the atomic mass of 87Rb (27.83 percent abundant). The average atomic mass of Rb is 85.4678 amu.



There are three naturally occurring isotopes of the element magnesium. The atomic masses of 25Mg (10.00 percent abundant) and 26Mg (11.01 percent abundant) are 24.9858374 and 25.9825937 amu, respectively. What is the atomic mass of 24Mg (78.99 percent abundant) given the average atomic mass of Mg is 24.3050 amu?

Review Questions



What is the periodic table, and what is its significance in the study of chemistry?



Write the names and symbols for four elements in each of the following categories: (a) nonmetal, (b) metal, (c) metalloid.



Give two examples of each of the following: (a) alkali metals, (b) alkaline earth metals, (c) halogens, (d) noble gases, (e) chalcogens, (f) transition metals.



The explosion of an atomic bomb in the atmosphere releases many radioactive isotopes into the environment. One of the isotopes is 90Sr. Via a relatively short food chain, it can enter the human body. Considering the position of strontium in the periodic table, explain why it is particularly harmful to humans.




Elements whose names end with -ium are usually metals; sodium is one example. Identify a nonmetal whose name also ends with -ium.



Describe the changes in properties (from metals to nonmetals or from nonmetals to metals) as we move (a) down a periodic group and (b) across the periodic table from left to right.



Consult the WebElements Periodic Table of the Elements (http://www.webelements.com) to find (a) two metals less dense than water, (b) two metals more dense than mercury, (c) the densest known solid metallic element, and (d) the densest known solid nonmetallic element.



Group the following elements in pairs that you would expect to show similar properties: K, F, P, Na, Cl, and N.



Group the following elements in pairs that you would expect to show similar chemical properties: I, Ba, O, Br, S, and Ca.



Write the symbol for each of the following biologically important elements in the given periodic table: iron (present in hemoglobin for transporting oxygen), iodine (present in the thyroid gland), sodium (present in intracellular and extracellular fluids), phosphorus (present in bones and teeth), sulfur (present in proteins), and magnesium (present in chlorophyll).

Review Questions



Define the term mole. What is the unit for mole in calculations? What does the mole have in common with the dozen and the gross? What does Avogadro's number represent?



What is the molar mass of an element? What units are commonly used for molar mass?

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Earth's population is about 6.8 billion. Suppose that every person on Earth participates in a process of counting identical particles at the rate of two particles per second. How many years would it take to count 6.0 × 1023 particles? Assume that there are 365 days in a year.



The diameter of a human hair is 25.4 μm. If atoms with a diameter of 121 pm were aligned side by side across the hair, how many atoms would be required?



How many atoms are there in 1.60 moles of selenium (Se)?



How many moles of nickel (Ni) atoms are there in 5.00 × 109 (5 billion) Ni atoms?



How many moles of strontium (Sr) atoms are in 93.7 g of Sr?



How many grams of platinum (Pt) are there in 26.4 moles of Pt?



What is the mass in grams of a single atom of each of the following elements: (a) Os, (b) Kr?



What is the mass in grams of a single atom of each of the following elements: (a) Sb, (b) Pd?



What is the mass in grams of 2.00 × 1012 tin (Sn) atoms?



How many atoms are present in 4.09 g of scandium (Sc)?



Which of the following has more atoms: 4.56 g of helium atoms or 2.36 g of manganese atoms?



Which of the following has a greater mass: 173 atoms of gold or 7.5 × 10−22 mole of silver?

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A sample of uranium is found to be losing mass gradually. Explain what is happening to the sample.



The element francium (Fr) was the last element of the periodic table discovered in nature. Because of its high radioactivity, it is estimated that no more than 30 g of francium exists at any given time throughout the Earth's crust. Assuming a molar mass of 223 g/mol for francium, what is the approximate number of francium atoms in the Earth's crust?



One isotope of a metallic element has 35 neutrons in the nucleus. The neutral atom has 30 electrons. Write the symbol for this atom.



An isotope of a nonmetallic element has mass number 131. An atom of this isotope contains 54 electrons. Write the symbol for this atom.



Using the information provided in the table, write a symbol for each atom. If there is not enough information, state what additional information is needed to write a correct symbol for that atom.

  Atom of element
  A B C D
Number of electrons 6 11 29  
Number of protons 6   29 36
Number of neutrons 6 7   47



Using the information provided in the table, write a symbol for each atom. If there is not enough information, state what additional information is needed to write a correct symbol for that atom.

  Atom of element
  A B C D
Number of electrons 10   21 50
Number of protons   75 21 50
Number of neutrons 12 110 21  



Which of the following symbols provides more information about the atom: 23Na or 11Na? Explain.



Discuss the significance of assigning an atomic mass of exactly 12 amu to the carbon-12 isotope.



List the elements that exist as gases at room temperature. (Hint: Most of these elements can be found in Groups 5A, 6A, 7A, and 8A.)



For the noble gases (the Group 8A elements) 42He, 2010Ne, 4018Ar, 8436Kr, 13254Xe (a) determine the number of protons and neutrons in the nucleus of each atom, and (b) determine the ratio of neutrons to protons in the nucleus of each atom. Describe any general trend you discover in the way this ratio changes with increasing atomic number.



The carat is the unit of mass used by jewelers. One carat is exactly 200 mg. How many carbon atoms are present in a 2.5-carat diamond?


In the geologic record of Earth, the disappearance of the dinosaurs roughly 65 million years ago is marked by a thin line known as the K-T boundary. Because the K-T boundary contains an unusually high concentration of the element iridium, which is ordinarily quite rare in Earth's crust, geologists have deduced that the mass extinction was caused by the catastrophic impact of an asteroid. The two naturally occurring isotopes of iridium are 191Ir and 193Ir, with atomic masses of 190.960584 and 192.962917 amu, respectively; and abundances of 37.3 percent and 62.7 percent, respectively. Determine the average atomic mass of iridium and compare your result with the atomic mass given in the periodic table.



One atom of a particular element with only one naturally occurring isotope has a mass of 3.002 × 10−22 g. What element is this?



Identify each of the following elements: (a) a halogen containing 53 electrons, (b) a radioactive noble gas with 86 protons, (c) a Group 6A element with 34 electrons, (d) an alkali metal that contains 11 electrons, (e) a Group 4A element that contains 82 electrons.



Show the locations of (a) alkali metals, (b) alkaline earth metals, (c) the halogens, and (d) the noble gases in the given outline of a periodic table. Also draw dividing lines between metals and metalloids and between metalloids and nonmetals.



While most isotopes of light elements such as oxygen and phosphorus contain relatively equal amounts of protons and neutrons in the nucleus, recent results indicate that a new class of isotopes called neutron-rich isotopes can be prepared. These neutron-rich isotopes push the limits of nuclear stability as the large numbers of neutrons approach the “neutron drip line.” Neutron-rich isotopes may play a critical role in the nuclear processes of stars. Determine the number of neutrons in the following neutron-rich isotopes: (a) 40Mg, (b) 44Si, (c) 48Ca, (d) 43Al.



Fill in the blanks in the table:

Protons 14    
Neutrons 15   117
Electrons     79



Fill in the blanks in the table:

Protons     62
Neutrons   108 88
Electrons   73  



(a) Describe Rutherford's experiment and how the results revealed the nuclear structure of the atom. (b) Consider the 23Na atom. Given that the radius and mass of the nucleus are 3.04 × 10−15 m and 3.82 × 10−23 g, respectively, calculate the density of the nucleus in g/cm3. The radius of a 23Na atom is 186 pm. Calculate the density of the space occupied by the electrons outside the nucleus in the sodium atom. Do your results support Rutherford's model of an atom? (The volume of a sphere of radius r is .)



A cube made of platinum (Pt) has an edge length of 1.0 cm. (a) Calculate the number of Pt atoms in the cube. (b) Atoms are spherical in shape. Therefore, the Pt atoms in the cube cannot fill all the available space. If only 74 percent of the space inside the cube is taken up by Pt atoms, calculate the radius in picometers of a Pt atom. The density of Pt is 21.45 g/cm3, and the mass of a single Pt atom is 3.240 × 10−22 g. (The volume of a sphere of radius r is .)



In the facing page of the periodic table at the front of this book is an alphabetical list of elements with atomic masses given to the number of known significant figures. Compare the number of significant figures given for aluminum, bismuth, lead, and molybdenum. Look these elements up on webelements.com and explain why their atomic masses are reported to such different numbers of significant figures.

Answers to In-Chapter Materials



(a) p = 5, n = 5, e = 5. (b) p = 18, n = 18, e = 18. (c) p = 38, n = 47, e = 38. (d) p = 6, n = 5, e = 6.



(a) 94Be, (b) 5123V, (c) 12454Xe, (d) 6931Ga.



63.55 amu.



99.64% 14N, 0.36% 15N.



(a) 4.40 × 1024 atoms K, (b) 1.48 × 102 mol K.



(a) 6.32 × 1017 atoms He, (b) 3.87 × 10−3 mol He.



(a) 0.3066 mol Ar, (b) 1.72 × 10−3 mol Au, (c) 0.298 mol Hg.



(a) 1.10 × 102 g Ca, (b) 0.30 g He, (c) 4.125 × 10−3 g K.



(a) 3.225 × 1023 atoms Au, (b) 5.374 × 10−10 g Ca.



(a) 811.6 g Ca, (b) 1.439 × 1031 atoms Au.










20% 10B, 80% 11B.



121.76 amu.



4A, 5A, 6A.






8.68 × 10−2 mol As.



2.143 × 1018 atoms Kr.



7.46 × 1023 atoms Mg.



4.518 × 103 g Hg.

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Interconversion Among Mass, Moles, and Numbers of Atoms

Many problems require you to convert a mass to a number of moles or to a number of atoms. Conversely, you may need to convert from a number of moles or a number of atoms to a mass. The conversion factors necessary for these operations are molar mass (in grams per mole) and Avogadro's constant, NA (in number of atoms per mole). It's easy to make the mistake of multiplying when you should divide, or vice versa. Track your units carefully to make sure that you have not made this common error. Individual flowcharts for each operation are shown in the examples below.

One of the most common tasks you will encounter is the conversion of mass to moles:

How many moles of helium does a 71.3-g sample of helium contain?

From the periodic table, we get the molar mass of helium: 4.003 g/mol.

You may also be asked to convert from the mass of a sample to the number of atoms it contains.

How many helium atoms does a 197-g sample of helium contain?

Again, we use the molar mass of helium, 4.003 g/amu. Avogadro's constant is 6.0221418 × 1023 atoms/mol. (Because the original mass has only three significant figures, we can use 6.022 × 1023 atoms/mol in this calculation.)

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What number of moles of helium contains 8.80 × 1027 helium atoms?

In this case also, with only three significant figures in the number of atoms, we can use 6.022 × 1023 atoms/mol for Avogadro's number.

What is the mass of 8.80 × 1027 helium atoms?

Again, we use the molar mass of helium.

Key Skills Problems



Convert 0.189 g Cu to moles.

(a) 12.0 mol (b) 0.19 mol (c) 2.97 × 10−3 mol (d) 1.14 × 10−25 mol (e) 1.14 × 1023 mol



How many Zn atoms are in 545 g Zn?

(a) 8.33 atoms (b) 8 atoms (c) 1.38 × 10−23 atoms (d) 5.02 × 1024 atoms (e) 3.28 × 1026 atoms



One container holds a mixture of metals containing 72.09 g of sodium and 6.99 g of strontium. Another container holds 47.87 g of titanium. What mass of calcium would have to be added to the second container for it to hold the same number of atoms as the first container?

(a) 40.08 g (b) 113.6 g (c) 2.834 g (d) 3.215 g (e) 88.80 g



Arrange the following in order of increasing mass:

1.85 × 1024 Fe atoms 7.68 × 1023 Ba atoms
i ii
9.68 × 1023 Pd atoms 1.22 × 1024 Kr atoms
iii iv



i < iii < iv < ii



ii < i < iv < ii



i < iv < iii < ii



iv < iii < i < ii



ii < iv < iii < i