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General, Organic, and Biochemistry, 2e (Frost)

Chapter 2 Atoms and Radioactivity

2.1 Multiple-Choice

1) Two atoms must represent the same element if they both have the same:

A) number of electron shells

B) atomic number

C) number of neutrons

D) atomic mass

Answer: B

Diff: 1

Section: 2-1

2) The smallest particle of an element that can be identified as that element is:

A) a proton

B) a neutron

C) a molecule

D) an atom

Answer: D

Diff: 1

Section: 2-1

3) Which one of the following carries no electrical charge?

A) An electron

B) A proton

C) A neutron

D) A cation

Answer: C

Diff: 1

Section: 2-1

4) The neutral atom always contains:

A) the same number of protons and neutrons

B) the same number of neutrons and electrons

C) the same number of protons and electrons

D) the same number of protons, neutrons and electrons

Answer: C

Diff: 1

Section: 2-1

5) Which particle(s) are in the nucleus?

A) protons only

B) neutrons only

C) protons and neutrons

D) protons and electrons

Answer: C

2

Diff: 1

Section: 2-1

6) Almost all of the mass of an atom exists in its:

A) first energy level

B) outermost energy level

C) nucleus

D) electrons

Answer: C

Diff: 1

Section: 2-1

7) Which subatomic particle contributes least to the mass of the atom?

A) Proton

B) Neutron

C) Electron

D) All of these contribute equally.

Answer: C

Diff: 1

Section: 2-1

8) Mercury-202 has how many neutrons in its nucleus?

A) 22

B) 102

C) 122

D) 202

Answer: C

Diff: 1

Section: 2-2

9) Cobalt is element 27. Cobalt-60 is used in the medical treatment of cancer. How many neutrons and

protons are contained in the nucleus of this isotope?

A) 27 neutrons, 33 protons

B) 33 neutrons, 27 protons

C) 27 neutrons, 27 protons

D) 33 neutrons, 33 protons

Answer: B

Diff: 1

Section: 2-2

10) Adding one proton to the nucleus of an atom:

A) increases the atom ic number and the mass number by one unit

B) increases its atomic mass by one unit, but does not change its atomic number

C) increases its atomic number by one unit but does not change its atomic mass

D) does not change either its atomic number or its atomic mass

Answer: A

Diff: 1

Section: 2-2

3

4

11) Adding one neutron to the nucleus of an atom:

A) increases the atom ic number and the mass number by one unit

B) increases its atomic mass by one unit, but does not change its atomic number

C) increases its atomic number by one unit but does not change its atomic mass

D) does not change either its atomic number or its atomic mass

Answer: B

Diff: 1

Section: 2-2

12) The quantity (mass number – atomic number) provides:

A) the number of neutrons in a nucleus

B) the number of electrons in the atom

C) the number of protons in a nucleus

D) the combined number of all the neutrons and all the protons in a nucleus

Answer: A

Diff: 1

Section: 2-2

13) Isotopes have the:

A) same number of protons but different number of electrons

B) same number of protons but different number of neutrons

C) same number of neutrons but different number of electrons

D) all of the above

Answer: B

Diff: 1

Section: 2-2

14) How many neutrons are in a neutral atom of Ar-40?

A) 18

B) 22

C) 40

D) 58

Answer: B

Diff: 1

Section: 2-2

15) How many neutrons are in a neutral atom of Fe-56?

A) 26

B) 30

C) 56

D) 82

Answer: B

Diff: 1

Section: 2-2

5

16) A hypothetical element contains three isotopes of mass 16.0 amu, 17.0 amu, and 18.0 amu with

relative abundances of 20.0%, 50.0% and 30.0%, respectively. The average atomic mass is:

A) 16.9 amu

B) 17.1 amu

C) 17.3 amu

D) 17.5 amu

Answer: B

Diff: 2

Section: 2-3

17) Which of the following represents a pair of isotopes?

A) C, N

B) H, H

C) S, S-2

D) O2, O3

Answer: B

Diff: 1

Section: 2-3

18) The element rhenium (Re) exists as two stable isotopes and 18 unstable isotopes. Rhenium-185

(185Re) has in its nucleus:

A) 75 protons, 75 neutrons

B) 75 protons, 130 neutrons

C) 130 protons, 75 neutrons

D) 75 protons, 110 neutrons

Answer: D

Diff: 1

Section: 2-3

19) The masses on the periodic table are expressed in what units?

A) Grams

B) Amu’s

C) Tons

D) Pounds

Answer: B

Diff: 1

Section: 2-3

6

20) Of the following, the radioisotope most useful in treating disorders of the thyroid gland is:

A) C-14

B) Tc-99m

C) U-238

D) I-131

Answer: D

Diff: 2

Section: 2-4

21) Made up of helium nuclei traveling at 5-7% speed of light:

A) alpha particles

B) gamma rays

C) beta particles

D) neutrons

Answer: A

Diff: 1

Section: 2-4

22) The form of radioactivity that penetrates matter most easily is:

A) alpha particles

B) gamma rays

C) beta particles

D) protons

Answer: B

Diff: 1

Section: 2-4

23) The form of radioactivity that penetrates matter the least is:

A) alpha particles

B) gamma rays

C) beta particles

D) protons

Answer: A

Diff: 1

Section: 2-4

24) Form of radiation that has no mass:

A) alpha particles

B) beta particles

C) gamma rays

D) neutrons

Answer: C

Diff: 1

Section: 2-4

7

25) Made up of electrons from the nucleus traveling at 90-95% speed of light:

A) alpha particles

B) gamma rays

C) beta particles

D) neutrons

Answer: C

Diff: 1

Section: 2-4

26) The radioactive particles, alpha, beta and gamma ray, are called ionizing radiation because, as they

pass through an object, they:

A) repel ions

B) knock electrons off atoms or molecules in their path

C) decay into ions

D) attract ions

Answer: B

Diff: 2

Section: 2-4

27) What protective shielding is sufficent for gamma rays?

A) paper

B) aluminium sheet

C) lead sheet

D) all of the above

Answer: C

Diff: 1

Section: 2-4

28) What radioactive particle is missing in the following nuclear reaction?

Mo + ________ â†’ Mo

A) n

B) e

C) He

D) p

Answer: A

Diff: 2

Section: 2-5

8

29) What radioactive particle is missing in the following nuclear reaction?

Mo â†’ Tc + ________

A) n

B) e

C) He

D) p

Answer: B

Diff: 2

Section: 2-5

30) When Phosphorous-30 loses a positron what is the product of this radioactive decay?

A) Si

B) S

C) S

D) P

Answer: A

Diff: 1

Section: 2-5

31) The amount of a radioisotope that remains after two half-lives have passed is:

A) 98%

B) 75%

C) 50%

D) 25%

Answer: D

Diff: 2

Section: 2-6

9

32) In order to have 1/16 of a radioactive sample left how many half lives must the sample go through to

reach this amount?

A) 3

B) 4

C) 5

D) 6

Answer: B

Diff: 2

Section: 2-6

33) The half life of a specific radionuclide is 8 days. How much of an 80 mg sample will be left after 24

days?

A) 40 mg

B) 20 mg

C) 10 mg

D) 2.7 mg

Answer: C

Diff: 2

Section: 2-6

34) The half life of a specific Tc-99m is 6 hours. How much of an 192 mg sample will have decayed

after 30 hours?

A) 186 mg

B) 160 mg

C) 32 mg

D) 6 mg

Answer: A

Diff: 3

Section: 2-6

35) A fossil found in a cave was found to have a Carbon-14 ratio to carbon of 1/ 32 of a live object. If

the half life of Carbon-14 is 5730 yrs, how old is the object?

A) 5730 yrs

B) 28,650 yrs

C) 34,480 yrs

D) 17,190 yrs

Answer: B

Diff: 2

Section: 2-6

36) Using Table 2.7 in the text, identify which of the following radioactive nuclides would disappear

first given the same initial quantities.

A) U-238

B) C-14

C) I-123

D) F-18

Answer: D

Diff: 1

10

Section: 2-6

2.2 True/False

1) Different elements can contain the same atoms.

Answer: FALSE

Diff: 1

Section: 2-1

2) Atoms are made up of smaller particles called molecules.

Answer: FALSE

Diff: 1

Section: 2-1

3) The neutral atom contains equal numbers of protons and electrons.

Answer: TRUE

Diff: 1

Section: 2-1

4) The mass of the atom is almost all in the nucleus.

Answer: TRUE

Diff: 1

Section: 2-1

5) A neutron has no charge and negligible mass.

Answer: FALSE

Diff: 1

Section: 2-1

6) Atoms are electrically neutral.

Answer: TRUE

Diff: 1

Section: 2-1

7) An atom of K-40 contains 40 neutrons.

Answer: FALSE

Diff: 1

Section: 2-2

8) An atom of Pb-208 contains a total of 126 protons and neutrons.

Answer: TRUE

Diff: 1

Section: 2-2

9) Isotopes always have the same atomic nunber.

Answer: TRUE

Diff: 1

Section: 2-3

11

12

10) Isotopes always have the same mass nunber.

Answer: FALSE

Diff: 1

Section: 2-3

11) All elements with atomic number 83 or greater are radioactive

Answer: TRUE

Diff: 1

Section: 2-4

12) All isotopes of elements with atomic 82 or lower are not radioactive.

Answer: FALSE

Diff: 1

Section: 2-4

13) A positron is a positively charged electron

Answer: TRUE

Diff: 1

Section: 2-5

14) In beta decay the atomic number and mass number increase by 1.

Answer: FALSE

Diff: 1

Section: 2-5

15) After four half-lives the activity of a radioactive sample has decreased to one quarter of the intial

value.

Answer: FALSE

Diff: 2

Section: 2-6

16) A nucleotide with a long half-life will decay more rapidly than one with a short half-life.

Answer: FALSE

Diff: 1

Section: 2-6

13

2.3 Short Answer

1) Write the symbolic notation ( X) for the following information.

A) 20 protons, 20 electrons, 20 neutrons _______________

B) 16 protons, 16 electrons, 16 neutrons _______________

C) 30 protons, 30 electrons, 35 neutrons _______________

D) 92 protons, 92 electrons, 146 neutrons _______________

Answer: Ca; S; Br; U

Diff: 1

Section: 2-2

2) Complete the following table:

Symbol Atomic # Mass # #p #n #e

I

________ ________ ________ ________ 53

________ 43 99 43 ________ 43

Cl

________ ________ 17 20 ________

________ 26 56 ________ ________ 23

Answer: Line 1: 53, 131, 53, 78

Line 2: Tc, 56

Line 3: 17, 37, 17

Line 4: Fe, 26, 30

Diff: 2

Section: 2-2

3) Write the symbol for the isotope of bismuth with 125 neutrons.

Answer: Bi

Diff: 1

Section: 2-2

14

4) The element rubidium (Rb) comprises two naturally occurring isotopes with mass numbers 85

(atomic mass 84.91 amu) and 87 (atomic mass 86.91 amu). The atomic mass of the element is 85.47

amu. From the atomic mass deduce which isotope is more abundant. Calculate the isotope distribution of

Rb.

Answer: The isotope with mass number 85 is more abundant.

Diff: 2

Section: 2-3

5) The element magnesium (Mg) comprises three isotopes with mass numbers 24 (atomic mass 23.99

amu), 25 (atomic mass 24.99 amu) and 26 (atomic mass 25.98 amu), with the natural abundance of

78.99%, 10.00% and 11.01%, respectively. Calculate the atomic mass of Mg.

Answer: The average atomic mass is 24.31 amu.

Diff: 3

Section: 2-3

6) Complete the following table.

Emission Symbol Charge

He

beta -1

Î³

e

neutron 0

Answer:

line 1: alpha, +2

line 2: e

line 3: gamma, 0

line 4: positron, +1

line 5: n

Diff: 1

Section: 2-4

15

7) Complete the following equations with the symbol for the atom or particle represented by the blank

space. Show the mass numbers and atomic numbers of the isotopes formed or the symbols of the

subatomic particles:

A) Po â†’ ________ + Pb

B) Pa â†’ ________ + e

C) ________ â†’ U + He

D) H + H â†’ He + ________

E) U + n â†’ Sb + ________ + 2 n

F) I â†’ Xe + ________

G) S + n â†’ p + ________

Answer: A) He; B) U; C) Pu; D) n; E) Nb; F) e; G) P

Diff: 2

Section: 2-5

8) Write a nuclear equation for the following processes:

A) Bismuth-214 undergoes beta decay.

B) Thorium-230 decays to a radium isotope.

Answer: A) Bi â†’ e + Po; B) Th â†’ He + Ra

Diff: 2

Section: 2-5

9) You obtain a new sample of cobalt-60, half-life 5.25 years, with a mass of 400 mg. How much cobalt60 remains after 15.75 years?

Answer: 50 mg

Diff: 2

Section: 2-6

10) The units of activity are known as the curie (Ci) and the becquerel (Bq). Write down the relationship

between them.

Answer: 1 Ci = 3.7 x 1010 Bq

Diff: 1

Section: 2-6

16

11) If a radioactive sample has an activity of 75 mCi. Calculate the activity in Bq.The units of activity

are known as the curie (Ci) and the becquerel (Bq). Write down the relationship between them.

Answer: Activity = 2.8 x 109 Bq

Diff: 2

Section: 2-6

12) Why can’t we use Carbon-14 dating techniques to date the age of a dinosaur bone?

Answer: Because after 10 half lives there is not enough C-14 left to measure it is less than 0.1 % of the

original value and dinosaurs lived over one billion years ago.

Diff: 2

Section: 2-6

13) A 100-mg technetium-99m sample is used in a medical study. How much of the Technetium-99m

sample remains after 24 hours? The half-life of Tc-99m is 6 hours.

Answer: 24 hours/6 hours = 4 half lives; 100 mg â†’ 50 mg â†’ 25 mg â†’ 12.5 mg â†’ 6.25 mg

Diff: 2

Section: 2-6

14) Krypton-81m is used for lung ventilation studies. Its half-life is 13 seconds. How long does it take

the activity of this isotope to reach one-quarter of its original value?

Answer: 1 â†’ 1/2 â†’ 1/4, so that is two half lives; therefore, 26 secs

Diff: 2

Section: 2-6

15) In order for a radionuclide to be used for medical diagnosis it must have certain properties. Name

two and explain why.

Answer: They must have a short half life in order to disappear from the body as soon as possible and

they should either be a beta or gamma ray emitter.

Diff: 2

Section: 2-7

16) What is the product of positron emission from F-18?

Answer: O-18

Diff: 1

Section: 2-7

17) Why is a radioactive nuclide which is an alpha emitter a bad choice in medical diagnoistics or

imaging?

Answer: Because of the short absorption depth, alpha particles cause the largest damage to tissue.

Diff: 2

Section: 2-7