Isotopes

Isotopes represent atoms of an element that contain the same atomic number (same number of protons), but different mass numbers (different number of neutrons). 

Atomic Mass

The atomic mass of an element that you find on the periodic table represents the average masses of its isotopes. 

Recall that the mass number or atomic mass of an element represents the total number of its protons and neutrons, while the atomic number represents the total number of its protons. 

Concept: The atomic masses listed for the elements on the periodic table are the average masses of their isotopes. 

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Video Transcript

Whether you call it atomic mass or mass number or atomic weight, we're going to say that all it tells us is the combined mass of the protons and the neutrons in our element. Remember that the protons and the neutrons can be found within our nucleus. We’re going to say atomic mass listed for the elements on the periodic table are just the average masses of their isotopes. We're going to say that isotopes are elements with the same number of protons but different number of neutrons.
A good example is carbon-12 and carbon-13. Both of them are carbon so they both have the same atomic number. They both have six protons. Now if we subtract the atomic mass by the atomic number, this tells us the number neutrons. They're both carbon because they both have six protons, they have the same atomic number but when you subtract their atomic masses by their atomic number, we get different number of neutrons. This is what isotopes are. If you look at your periodic table, you’ll see that the atomic masses, almost all of them are not whole numbers. They all have decimals after them that's because they're the average mass when you take into account all the isotopes for a given element.

Isotopes are elements with the same number of protons, but different number of neutrons.

Concept: Calculating the average mass of an element

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Video Transcript

Here we can find out the atomic mass of an element based on the masses of its isotopes. We're going to say that the atomic mass or average mass that we see on the periodic table is equal to the mass of isotope 1 times the fractional abundance of isotope 1. Fractional abundance, it's super simple. Let's say you have 25% of isotope 1. Its fractional abundance would just be that percentage divided by 100. It's percent abundance is 25% but its fractional abundance is 25% divided by 100 or 0.25.
Then we’re going to say plus the mass of isotope 2 times its fractional abundance. The mass here that we usually have are either in grams over moles or amu, atomic mass units. That's what we mean by mass here. This is the formula we're going to have to use anytime we’re dealing with isotopes.

Example: Antimony has two common isotopes. If one of the isotopes 121Sb has an isotopic mass of 120.9038 amu and a natural abundance of 57.25%, what is the isotopic mass (to 4 significant figures) of the other isotope? The atomic mass of antimony is 121.8 g/mol. 

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Example: The atomic mass of an imaginary element A is 251.7 amu. If element A consists of two isotopes that have atomic masses of 250 and 253 respectively, what is the natural abundance of each isotope?

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A lesser known term referred to as isotones involves atoms that have the same number of neutrons, but different number of protons

Isotopes Additional Practice Problems

An unknown element, X, has three naturally occurring isotopes. The masses of the threee isotropes are 22.356, 23.492, and 26.106. The first two have a percent abundance of 55.230% and 28.015%, respectively. What is the average mass of the element?

A) 23.303

B) 26.242

C) 23.986

D) 18.929

E) 24.934

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Bromine has two naturally occurring isotopes. One of them, bromine-79, has a mass of 78.9183 amu and a natural isotopic abundance of 50.69%. Calculate the m/m % of the other isotope, bromine-81.

Note: m/m % stands for the mass of Br-81 in grams contained in 100 grams of bromine element. 

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Assume that a new element has only two isotopes E-182 and E-186 and its atomic mass is 184.3 amu. Which isotope is more abundant?

a. E-182

b. E-186

c. I need more information to decide. 

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Suppose the element Z contains only two isotopes, 29Z (atomic mass 28.933 amu, abundance 43.03%) and 31Z (atomic mass 30.916 amu). What would be the atomic mass of Z that would appear in the periodic table?

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Which of the following statements about isotopes is TRUE?

A) Isotopes of the same element don't usually have the same properties.

B) An isotope of an atom with a larger number of neutrons is larger than an isotope of the same atom that contains fewer neutrons.

C) Isotopes of the same element differ only in the number of electrons they contain.

D) Isotopes of the same element have the same mass.

E) Some elements have 3 or more naturally occurring isotopes.

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Fill in the requested information.

a) How many protons, electrons and neutrons does tin(IV)-120 have?

 


b) Write the atomic symbol for the tin isotope in a).

 


c) What is the mass number of the isotope of rubidium that has 49 neutrons?

 


d) Which neutral element could have an isotope with 38 neutrons for a mass number of 72?

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Gallium consists of two naturally-occurring isotopes, 69Ga and 71Ga, with masses of 68.926 and 70.926 amu, respectively. Given that 69Ga is 60.3% abundant, what is the average molar mass of Ga?

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The element antimony has an atomic weight of 121.757 amu and only two naturally occurring isotopes. One isotope has an abundance of 57.3% and an isotopic mass of 120.904 amu. What is the mass of the other isotope?

A) 52.479 amu

B) 121.757 amu

C) 122.393 amu

D) 122.610 amu

E) 122.902 amu

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Iron has four stable isotopes; their masses and natural abundances are listed in the table.  Calculate the average atomic mass of iron, in amu. 

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Assume that potassium (atomic mass = 39.10) has only two isotopes, one of which is potassium-39 (isotopic mass = 38.9637 amu, abundance = 93.258%). What is the mass of the second isotope?

A. 43.06 amu

B. 40.95 amu

C. 42.05 amu

D. 39.97 amu

E. 41.88 amu

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The fictional element kryptonite (lets give it the symbol Kt) associated with Superman's home planet of Krypton could be said to consist of two isotopes with the following abundances. Estimate the approximate atomic mass of kryptonite.

Kt  285; 70.0%

Kt  289; 30.0%

a. 288.2 amu

b. 286.2 amu

c. 284,6 amu

d. 282.6 amu

e. 280.3 amu

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Atoms of the same element with different mass numbers are called

a. allotropes

b. chemical families

c. nucleons

d. neutrons

e. isotopes

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There are three isotopes of hydrogen, differing with respect to

a. atomic mass.

b. atomic number.

c. nuclear charge.

d. electron configuration.

e. number of protons.

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I think I discovered element 120, which would be the last element in the 2A column. It consists of two isotopes. The abundance of the first isotope is 68.00% and its atomic mass is 294.6 u. The abundance of the second isotope is _________ (you can figure this out since there are only two isotopes) and its atomic mass is 297.8 u. What is the average atomic mass of the element?

a. 296.2 u

b. 295.0 u

c. 296.8 u

d. 295.6 u

e. 297.4 u

 

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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.

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An element has three isotopes. Given the abundances and relative masses, calculate the average atomic mass and determine (from the periodic table) which element it is.

 

 

 

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The element antimony has an atomic weight of 121.757 amu and only two naturally occurring isotopes. One isotope has an abundance of 57.30% and an isotopic mass of 120.904. Based on these data, what is the mass of the second isotope?

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Consider the following three pairs of species:

I. 55Fe3+ and 56Fe2+

II. Na+ and Ne

III. 13C and 14C

Which of the following statements is  NOT true?

 

A) I and III both contain pairs of isotopes.

B) II shows two species with different number of electrons.

C) III shows two species with different mass numbers, but the same atomic numbers.

D) I shows two species with the same number of protons.

Watch Solution

Calculate the atomic mass of element "X", if it has 2 naturally occurring isotopes with the following masses and natural abundances:

X-45          44.8776 amu         32.88%

X-47          46.9443 amu         67.12%

 

A) 46.26 amu

B) 45.91 amu

C) 46.34 amu

D) 46.84 amu

E) 44.99 amu

Watch Solution

Calculate the atomic mass of silver if silver has 2 naturally occurring isotopes with the following masses and natural abundances:

Ag-107                           106.90509                      amu 51.84%

Ag-109                           108.90476                      amu 48.46%

A) 108.19 amu

B) 107.90 amu

C) 108.32 amu

D) 108.00 amu

E) 107.79 amu

Watch Solution

Determine the number of protons, neutrons and electrons in the following:

A) p+= 18 n° = 22 e- = 40

B) p+= 40 n° = 22 e- = 18

C) p+= 18 n° = 22 e- = 18

D) p+= 18 n° = 18 e- = 2

E) p+= 22 n° = 18 e- = 18

Watch Solution

Vanadium has two naturally occurring isotopes, 50V with an atomic mass of 49.9472 amu and 51V with an atomic mass of 50.9440. The atomic weight of vanadium is 50.9315. The percent abundances of the vanadium isotopes are _______ % 50V and __________% 51V

a. 0.25, 99.75

b. 99.75, 0.25

c. 49, 51

d. 1.25, 98.75

e. 99, 1.0

Watch Solution

Fill in the blank spaces and write out all the symbols in the left hand column in full, in the form AZX  (i.e., include the appropriate values of Z and A as well as the correct symbol X).

 

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A naturally occurring sample of an element contains only two isotopes. The first isotope has a mass of 68.9255 amu and a natural abundance of 60.11%. The second isotope has a mass of 70.9247 amu. Find the atomic mass of the element.

a) 70.13 amu

b) 69.72 amu

c) 84.06 amu

d) 69.93 amu

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The number of ____defines the element; and isotopes differ in the number of ______.

A. electrons; neutrons

B. protons; neutrons

C. protons; electrons

D. neutrons; protons

E. electrons; protons

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Silver has an atomic mass of 107.868 amu and only two naturally-occurring isotopes. The Ag-109 isotope (108.905 amu) has a natural abundance of 48.16%. What is the mass (in amu) of the other isotope? Express your final answer using 6 significant figures.

A. 106.911 amu

B. 106.908 amu

C. 106.903 amu

D. 106.905 amu

E. 106.899 amu

Watch Solution

Which of the following pairs of atoms represent isotopes?     

A) H-1, O-18     

B) N-14, C-14     

C) Br-79, Cl-35     

D) C-12, C-14     

E) Se-77, F-19

Watch Solution

Which of the following represent isotopes?

A) A and D     

B) A and B     

C) C and D     

D) A and C

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Chlorine has 2 naturally occurring isotopes, Cl-35 and Cl-37.  Which is generally true? 

a)  Cl-35 has the greater abundance.

b)  The relative abundances are the same.

c)  The relative abundances are never known.

d)  Cl-37 has the greater abundance.

e)  There has to be a third naturally occurring isotope. 

Watch Solution

Which of the following  contains a pair of isotopes?

Watch Solution

Which of the following represent isotopes?

I: 3215X     II: 3216X     III: 3115X     IV: 3417X

a. I and II

b. I and IV

c. I and III

d. III and IV

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There are two isotopes of the element gallium that appear in nature, 69Ga (M = 68.926 amu) and 71Ga (M = 70.924 amu). Given that the aveage atomic mass for gallium is M = 69.72 amu, what is the percent by number of the 69Ga isotope in naturally occuring gallium?

A. 39.7%
B. 97.0%
C. 50.0%
D. 60.3%
E. 98.9%

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An unknown element, X, has three naturally occurring isotopes. The masses of the three isotopes are 54.335, 59.345 and 62.349 The two have a percent abundance of 35.528% and 38.136% respectively. What is the average mass of the element?

a. 58.676
b. 65.713
c. 58.481
d. 58.356
e. 58.873

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Which of the following symbols represent isotopes of nitrogen?

a. i only

b. ii only

c. iii only

d. i and ii

e. i and iii

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An unknown element E is found to have two isotopes. The atomic mass of E is 57.15 amu. The mass of the first isotope E - 56 is 56.482 amu, and its natural abundance is 69.19%. Calculate the mass of the second isotope of E?

a. 68.38 amu

b. 54.98 amu

c. 58.65 amu

d. 57.82 amu

e. 56.82 amu

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The element X has three naturally occuring isotopes;  aX, bX, and cX, where a, b, and c represent the mass number. The masses (amu),except aX, and the percent abundances of the isotopes are given in the table below. The average atomic mass of X is 17.20 amu. What is the mass of aX in amu?

a. 16.60

b. 16.67

c. 19.01

d. 18.11

e. 9.99

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Which box below represent isotopes of each other (each box represents a single atom)?

A. I and II

B. II and III

C. II and IV

D. III and IV

E. I and III

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Boron has two naturally occurring isotopes, 10B has a mass of 10.013 amu and 11B has a mass of 11.009 amu. What is the natural or % abundance of the  10B isotope? 

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Boron and Fluorine. 

Give the number of protons and neutrons in the following isotopes:

1. boron-10         _____ protons      _____neutrons

2. boron-11         _____ protons      _____neutrons

3. fluorine-19      _____ protons      _____neutrons

 

Watch Solution

Calculate the atomic mass of element ʺXʺ, if it has 2 naturally occurring isotopes with the following masses

and natural abundances:

X-45      44.8776 amu      32.88%

X-47      46.9443 amu      67.12%

A) 46.27 amu

B) 45.91 amu

C) 46.34 amu

D) 46.84 amu

E) 44.99 amu

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Gallium is a metal with a wide variety of uses (e.g., light-emitting diodes, lasers, medical imaging). Fill in the missing isotopic data on gallium in the table.

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Naturally occurring bromine consists of two isotopes, 79Br and 81Br with isotopic masses of 78.9183 and 80.9163, respectively. Given that the observed atomic mass of bromine is 79.904, calculate the percent abundance of 79Br and 81Br in naturally occurring bromine. 

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Conventional breeder reactors (oxymoron?) make more nuclear fuel from the isotopes they consume. For example, plutonium-239 converts thorium-232 to plutonium-239, which can be used to power another breeder reactor. 

Give the number of protons and neutrons in the following isotopes: 

thorium-239 ____ protons     _____ neutrons 

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Conventional breeder reactors (oxymoron?) make more nuclear fuel from the isotopes they consume. For example, plutonium-239 converts thorium-232 to plutonium-239, which can be used to power another breeder reactor. 

Give the number of protons and neutrons in the following isotopes: 

thorium-232 ____ protons     _____ neutrons 

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The atomic masses in the periodic table are not integral numbers. For example, carbon is listed as 12.01115 instead of 12.00000. What is the reason for this?

A. Atoms gain and lose electrons easily, significantly changing their masses.

B. Measurements taken of atomic samples are contaminated with other elements.

C. Technology does not allow for exact measurements of such small quantities.

D. The listed values are weighted averages of naturally occurring isotopes.

E. There is a theoretical uncertainty in the masses of atoms.

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Sulfur has four isotopes: S–32 (31.97 amu), S–33 (32.97 amu), S–34 (33.97 amu), and S–36 (35.97 amu). Which of these isotopes is the most abundant?

      1. S–32              2. S–33              3. S–34              4. S–36

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A 14-g sample of which isotope will have the largest number of atoms?

a. 13C

b. 14C

c. 13N

d. 14N

e. They would all have the same number of atoms.

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Only three isotopes of magnesium exist on earth. 24Mg is the most common form at 78.70% natural abundance with a mass of 23.98504 amu, 25Mg has a 10.13% natural abundance, while 26Mg has a natural abundance of 11.17% and a mass of 25.98259 amu. What is the mass of the 25Mg isotope?

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In which pair are the two species  both isoelectronic and isotopic? 

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Rubidium (Rb) has only two naturally occurring isotopes. The first isotope has a mass of exactly 86.909 amu and the second isotope has a percent abundance of 72.17%. What is the mass of the second isotope in amu?

a. 83.99 g/mol

b. 84.91 g/mol

c. 85.49 g/mol

d. 85.49 g/mol

e. 84.91 g/mol

Watch Solution

For the next three questions consider a 10 L sample of gaseous chlorine atoms in their natural relative abundances (3:1 35Cl : 37Cl). The Cl atoms react to form Cl2 gas.

Which is the most likely mass spectrum of the products?

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