20.1.2 The Stability of Atomic Nuclei

Question 1

The Stability of Atomic Nuclei

Answer 1

• Stable nuclei form a band of stability on a plot of number of protons versus number of neutrons.
• Nuclei above the band of stability tend to undergo beta decay, while nuclei below the band of stability tend to undergo positron emission or electron capture.
• Heavy nuclei can undergo alpha decay.

Question 2

note

Answer 2

• Stable nuclei form a band of stability (black dots) on a plot of number of protons versus number of neutrons. Except for 1 H and 3 He, all stable nuclei have one or more neutrons per proton. All nuclei with 84 or more protons are radioactive.
• Nuclei with 2, 8, 20, 28, 50 or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons are generally more stable. These numbers are referred to as magic numbers.
• As a general rule, nuclei with even numbers of both protons and neutrons tend to be more stable than nuclei with odd numbers.
• Nuclei above the band of stability (too many neutrons) tend to undergo beta decay. For example, carbon-14 (with 6 protons and 8 neutrons) undergoes beta decay, converting a neutron to a proton, and emitting a beta particle. This results in nitrogen-14 (with 7 protons and 7 neutrons).
• Nuclei below the band of stability (more protons than
  neutrons) tend to undergo positron emission or electron capture. For example, carbon-11 (with 6 protons and 5 neutrons) undergoes positron emission, converting a proton to a neutron. This results in boron-11 (with 5 protons and 6 neutrons).
• Heavy nuclei (>84 protons) can undergo alpha decay. This decreases both the number of protons and the number of neutrons by two.
• In general, a heavy nucleus cannot undergo a single nuclear decay process to become a stable nucleus. It must instead undergo a chain of decays. For example, uranium-238 undergoes a series of alpha decays and beta decays before finally forming lead-206.

Question 3

Which statement about isotopes is not correct?

Answer 3

Isotopes have the same atomic number and mass number.

Question 4

Which statement about the mass number is not correct?

Answer 4

There are no stable isotopes that have both an odd number of protons and an odd number of neutrons.

Question 5

Which statement about “magic numbers” for nuclei is not correct?

Answer 5

Nuclei with 2, 8, 20, 28, 50, 82, and 126 protons are generally stable.

Question 6

Look at the chart that shows the stability of an isotope as a function of the number of neutrons and protons.

What would be the correct predicted behavior for a hypothetical isotope of 12 5 B?

Answer 6

It will decay to 12 6 C by beta- emission

Question 7

Which of the following statements about isotopes is not true?

Answer 7

Stable nuclei can be radioactive.

Question 8

Which statement about isotopes is not correct?

Answer 8

The “magic numbers” for the total number of protons and neutrons are 2, 8, 20, 28, 50, 82, and 126.

Question 9

Look at the chart that shows the stability of an isotope as a function of the number of neutrons and protons (protons are along the x-axis).

Which of the following isotopes is unstable?

Answer 9

15 8 O

Question 10

Which of the following best defines isotopes?

Answer 10

Isotopes are atoms of the same element that have different atomic masses (due to different amounts of neutrons) but the same atomic number.

Question 11

Which statement about the decay of 14 C 6 is not correct?

Answer 11

Inside the nucleus of 14 6 C, a proton is changed into a neutron

Question 12

Which statement about isotopes and / or their nuclei is correct?

Answer 12

All nuclei with an atomic number of 84 or more are unstable.