HW From textbook “CHEMISTRY a Contemporary Approach”

• Read “Atomic Mass and Isotopes” p63

• Do exercise 2.2 p64

• Make a four questions quiz (with answers) based on class notes

Atoms of an element that contain the same number of protons but a different number of neutrons are called isotopes of that element.

Example

The isotopes of neon are Ne-20, Ne-21, and Ne-22.

 

                          

Problems

Draw the isotopes of the following elements as in the example above.

The isotopes of hydrogen are H-1, H-2 (deuterium), and H-3 (tritium)

The isotopes of carbon are C-12 and C-14

The isotopes of boron are B-10 and B-11

The average atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.

Example

The isotopes of chlorine are Cl-35 and Cl-37

In nature 75 % of all Cl atoms have a mass of 35 amu (Cl-35)

In nature 25 % of all Cl atoms have a mass of 37 amu (Cl-37)

Therefore, the average atomic mass of Cl is (75%) (35 amu) + (25%) (37 amu) = 35.5 amu

 

The average atomic mass of an element is close to the mass of the most abundant isotope.

Example

The average atomic mass of H is 1.00794 amu and it is calculated from H-1, H-2, and H-3. Therefore, H-1 is the most abundant isotope for hydrogen since 1 amu is the closest value to 1.00794 amu.

 

Problems

1. Element X has two natural isotopes. The isotope with mass 10.012 amu has a relative abundance of 19.91%. The isotope with mass 11.009 amu has a relative abundance of 80.09%.

What is this element? Calculate the average atomic mass to verify your answer.

2. Carbon has two natural isotopes. The isotope with mass X amu has a relative abundance of 98.89 %. The isotope with mass 13 amu has a relative abundance of 1.108 %.

What is mass (X) of the first isotope?

TEST YOUR UNDERSTANDING

1/07

8/07

1/06

6/06

1/05

Base your answers to questions 66 and 67 on the information below.

Naturally occurring elemental carbon is a mixture of isotopes. The percent composition of the two most abundant isotopes is listed below.

• 98.93% of the carbon atoms have a mass of 12.00 atomic mass units.

• 1.07% of the carbon atoms have a mass of 13.00 atomic mass units.

66 In the space provided in your answer booklet, show a correct numerical setup for calculating the average atomic mass of carbon.

67 Describe, in terms of subatomic particles found in the nucleus, one difference between the nuclei of carbon-12 atoms and the nuclei of carbon-13 atoms. The response must include both isotopes.

6/05

3 Which two notations represent atoms that are isotopes of the same element?   

1/04

2 Isotopes of an element must have different (1) atomic numbers (2) mass numbers (3) numbers of protons (4) numbers of electrons

6/04

Base your answers to questions 59 through 61 on the data table below, which shows three isotopes of neon.

59 In terms of atomic particles, state one difference between these three isotopes of neon.
60 Based on the atomic masses and the natural abundances shown in the data table show a correct numerical setup for calculating the average atomic mass of neon.

61 Based on natural abundances, the average atomic mass of neon is closest to which whole number?

8/04

3 The atomic mass of an element is the weighted average of the masses of

                (1) its two most abundant isotopes  (2) its two least abundant isotopes

                (3) all of its naturally occurring isotopes (4) all of its radioactive isotopes

12 Which statement concerning elements is true?

                (1) Different elements must have different numbers of isotopes.

                (2) Different elements must have different numbers of neutrons.

                (3) All atoms of a given element must have the same mass number.

                (4) All atoms of a given element must have the same atomic number.

1/03

1 An atom of carbon-12 and an atom of carbon-14 differ in  (1) atomic number   (2) mass number (3) nuclear charge (4) number of electrons

2  Hydrogen has three isotopes with mass numbers of 1, 2, and 3 and has an average atomic mass of 1.00794 amu. This information indicates that

(1) equal numbers of each isotope are present (3) more isotopes have an atomic mass of 1 than of 2 or 3

(2) more isotopes have an atomic mass of 2 or 3 than of 1 (4) isotopes have only an atomic mass of 1

6/03

The atomic mass of an element is calculated using the

                (1) atomic number and the ratios of its naturally occurring isotopes (3) masses and the ratios of its naturally occurring isotopes

                (2) atomic number and the half-lives of each of its isotopes (4) masses and the half-lives of each of its isotopes

8/03

65 The table below gives information about two isotopes of element X.

Calculate the average atomic mass of element X.
° Show a correct numerical setup.

° Record your answer.

° Express your answer to the correct number of significant figures.

6/02

1 Atoms of the same element that have different numbers of neutrons are classified as

         (1) charged atoms  (2) charged nuclei  (3) isomers (4) isotopes

2 All the isotopes of a given atom have

(1) the same mass number and the same atomic number (3) different mass numbers but the same atomic number

(2) the same mass number but different atomic numbers (4) different mass numbers and different atomic numbers

8/02

What is the total number of protons in the nucleus of an atom of potassium-42?  (1) 15      (2) 19       (3) 39       (4) 42

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