Below is a detailed description of the educational document or worksheet provided in the image, focusing on the topic “4.4 Radiation from the nucleus and nuclear energy.” The document targets key concepts in nuclear physics, including isotopes, nuclear forces, radioactive decay, and half-life. It is likely intended for high school or introductory college students studying physics or chemistry, given its structured format and question types. The document is divided into two main sections: a list of questions (numbered 1–11) and a section titled “4.2 Exam questions” with multiple-choice and short-answer questions (numbered 1–5). Here’s a comprehensive breakdown of its layout and content.
Visual and Layout Description
The document features a clean, professional design typical of educational materials:
- Background and Colors: The background is a light beige or off-white, providing a neutral base for readability. A gray sidebar runs vertically along the left edge, containing the topic title “TOPIC 4 Radiation from the nucleus and nuclear energy” and the page number “13” in white text.
- Font and Text: The text is presented in a sans-serif font, likely Arial or a similar typeface, with a size of approximately 11–12 points. All text is black and left-aligned, ensuring clarity and consistency.
- Structure: The content is split into two distinct sections. The top section lists questions 1–11 in a numbered sequence, while the bottom section, headed “4.2 Exam questions,” contains questions 1–5 with circular bullet points (e.g., ⮕) and specified mark values (1–3 marks).
- Visual Elements: There are no images, graphs, or diagrams—just text-based questions and options, making it a straightforward worksheet focused on conceptual understanding and calculations.
- Additional Notes: At the bottom of the “4.2 Exam questions” section, a note reads, “More exam questions are available in your learnON title,” indicating this document is part of a broader digital or textbook learning platform.
The layout is well-spaced and organized, enhancing readability and usability for students working through the material.
Detailed Content Description
Top Section: Questions 1–11
This section consists of 11 questions designed to reinforce understanding of nuclear physics concepts. Below is a detailed description of each question, including explanations and answers where applicable:
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Name the isotope that has an atomic number of 11 and contains 12 neutrons.
- The atomic number (Z) is the number of protons, so Z = 11 means 11 protons. The number of neutrons is given as 12. The mass number (A) is the sum of protons and neutrons: A = Z + neutrons = 11 + 12 = 23. This isotope is Sodium-23, written as ( \ce{^{23}_{11}Na} ).
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How many protons and neutrons are there in one atom of each of the following isotopes?
- a. Hydrogen-2 (deuterium): Hydrogen has Z = 1 (1 proton). Mass number A = 2, so neutrons = A - Z = 2 - 1 = 1. Answer: 1 proton, 1 neutron.
- b. Americium-241: Americium has Z = 95 (95 protons). Mass number A = 241, so neutrons = 241 - 95 = 146. Answer: 95 protons, 146 neutrons.
- c. Europium-164: Europium has Z = 63 (63 protons). Mass number A = 164, so neutrons = 164 - 63 = 101. Answer: 63 protons, 101 neutrons.
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What force acts between a proton and a neutron and under what conditions does it act?
- The strong nuclear force acts between nucleons (protons and neutrons). It operates only at very short distances, typically within the nucleus (about 10⁻¹⁵ meters), and is much stronger than the electromagnetic force at these ranges, binding the nucleus together.
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The half-life of caesium-134 is 2.06 years. What fraction of caesium is left after 10.3 years?
- Calculate the number of half-lives: ( n = \frac{10.3}{2.06} = 5 ). After 5 half-lives, the fraction remaining is ( \left(\frac{1}{2}\right)^5 = \frac{1}{32} ). Answer: 1/32 of the original amount remains.
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What is meant when an isotope is described to be stable?
- A stable isotope does not undergo radioactive decay. Its nucleus has a balanced configuration of protons and neutrons, where the strong nuclear force effectively counteracts the electromagnetic repulsion between protons, preventing emission of radiation.
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If the strong nuclear force acts between all nucleons and is much stronger than the electromagnetic force at the distance of two neighboring nucleons in a nucleus, explain how a nucleus might be unstable.
- Despite the strong nuclear force’s strength, a nucleus can become unstable if there are too many protons, increasing electromagnetic repulsion, or an imbalanced neutron-to-proton ratio. These conditions can overpower the strong force, leading to radioactive decay.
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100 grams of a radioactive isotope is delivered to a hospital. Three hours later there is only 6.25 grams left. What is the half-life of the isotope?
- The decay sequence is: 100 g → 50 g → 25 g → 12.5 g → 6.25 g, which is 4 half-lives in 3 hours. Thus, each half-life is ( \frac{3}{4} = 0.75 ) hours. Answer: 0.75 hours.
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Sketch a graph of the decay of the isotope in question 7.
- The graph would be an exponential decay curve:
- At t = 0 hours: 100 g
- At t = 0.75 hours: 50 g
- At t = 1.5 hours: 25 g
- At t = 2.25 hours: 12.5 g
- At t = 3 hours: 6.25 g
- The y-axis represents mass (g), and the x-axis represents time (hours), showing a steep, downward curve leveling off as time increases.
- The graph would be an exponential decay curve:
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Why are half-lives used for radioactive isotopes rather than just stating how long they take to decay?
- Radioactive decay is a random, exponential process where the amount of isotope never fully reaches zero. Half-life provides a consistent measure of decay rate—the time for half the atoms to decay—making it more practical than an indefinite total decay time.
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The activity (decays per second) of a radioactive isotope halves with each half-life. Explain why that would be the case.
- Activity is the rate of decay, proportional to the number of radioactive atoms. As the number of atoms halves with each half-life, the activity also halves, reflecting the reduced number of decaying nuclei. -
You need 20 grams of a radioactive isotope with a half-life of 3 hours. How much would you need to buy if it takes 24 hours to deliver it?
- Number of half-lives in 24 hours: ( n = \frac{24}{3} = 8 ). After 8 half-lives, you need 20 g, so the initial amount is ( 20 \times 2^8 = 20 \times 256 = 5120 ) grams. Answer: 5120 grams.
Bottom Section: 4.2 Exam Questions
This section contains five exam-style questions, including multiple-choice and short-answer formats, each with assigned marks:
Question 1 (1 mark)
- MC: What is the atomic number (Z) of an element?
- A. The number of neutrons in the nucleus
- B. The number of protons in the nucleus
- C. The number of nucleons in the nucleus
- D. The total number of nucleons and electrons in the atom
- Answer: B. The number of protons in the nucleus (Z defines the element).
Question 2 (1 mark)
- MC: Which of the following best describes all the isotopes of an element?
- A. Same number of protons; different number of neutrons
- B. Same number of neutrons; different number of protons
- C. Same number of nucleons; different number of protons
- D. Same number of nucleons; different number of neutrons
- Answer: A. Same number of protons; different number of neutrons (isotopes vary in neutron count).
Question 3 (1 mark)
- MC: A radioactive element has a half-life of 6.0 days. A sample of this material initially contains 12,400 nuclei. How many radioactive nuclei are remaining after 12 days?
- A. 6200
- B. 3100
- C. 1550
- D. 775
- Calculation: 12 days = 2 half-lives (12 / 6 = 2). After 2 half-lives: ( 12,400 / 4 = 3100 ). Answer: B. 3100.
Question 4 (3 marks)
- A radioactive source has a half-life of 50 years. It currently contains 10,000 of the unstable nuclei. How many unstable nuclei were there in the source 200 years ago? Show your reasoning.
- Reasoning: 200 years ago = 4 half-lives (200 / 50 = 4). Going backward, multiply by 2 per half-life: ( 10,000 \times 2^4 = 10,000 \times 16 = 160,000 ).
- Answer: 160,000 unstable nuclei.
Question 5 (3 marks)
- The initial activity of a radioactive source is 8000 decays per second. After 1 day it has decreased to 1000 decays per second. What is the half-life of this source (in hours)? Show your reasoning.
- Reasoning: Decay from 8000 to 1000 is 3 half-lives (8000 → 4000 → 2000 → 1000). 1 day = 24 hours, so each half-life = ( 24 / 3 = 8 ) hours.
- Answer: 8 hours.
Summary
This educational document provides a detailed exploration of nuclear physics concepts through a mix of conceptual questions and quantitative problems. The top section (Questions 1–11) builds foundational knowledge and skills in identifying isotopes, understanding nuclear forces, and performing half-life calculations. The bottom section (“4.2 Exam questions”) tests this knowledge with exam-style questions, blending multiple-choice and short-answer formats to assess both recall and reasoning. The consistent formatting, clear text, and absence of visuals emphasize a focus on textual comprehension and calculation, making it an effective study tool for students learning about radiation and nuclear energy.