Atomic and Nuclear Physics is an exciting branch of physics that helps us understand the fundamental structure of matter and the immense energy contained within atomic nuclei. In this unit, students will study key topics including:
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Atom and Atomic Nucleus – The basic structure of atoms and their central nuclei.
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Natural Radioactivity & Background Radiations – Understanding radioactive decay and the radiation present in our environment.
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Nuclear Transmutations – How elements can change into other elements through nuclear reactions.
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Half-Life and Its Measurement – Techniques to calculate the decay of radioactive substances.
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Radioisotopes and Their Uses – Practical applications in medicine, industry, and research.
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Fission and Fusion Reactions – Nuclear reactions that release enormous energy.
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Hazards of Radiations and Safety Measures – Precautions to safely handle radioactive materials.
These notes provide a complete guide with MCQs, short questions, and long questions, making it easier for students to prepare for exams effectively.
Important MCQs – Unit 18 Atomic and Nuclear Physics
1. Which among the following radiations has more penetrating power?
(a) Alpha particle
(b) Gamma ray ✅
(c) Beta particle
(d) All have the same ability
2. The central part of an atom is called:
(a) Proton
(b) Nucleus ✅
(c) Electron
(d) Neutron
3. Atoms of the same element having same protons but different neutrons are called:
(a) Isobars
(b) Isotopes ✅
(c) Nuclides
(d) Radioisotopes
4. In 1896, the scientist who discovered natural radioactivity was:
(a) Rutherford
(b) Marie Curie
(c) Becquerel ✅
(d) Roentgen
5. The half-life of carbon-14 is:
(a) 573 years
(b) 5730 years ✅
(c) 57,300 years
(d) 573,000 years
Short Questions – Unit 18 Atomic and Nuclear Physics
1. Differentiate between Atomic Number (Z) and Atomic Mass Number (A).
Answer:
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Atomic Number (Z): The atomic number is the number of protons in the nucleus of an atom. It determines the identity of the element. For example, carbon has Z = 6, meaning it has 6 protons in its nucleus.
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Atomic Mass Number (A): The atomic mass number is the total number of protons and neutrons in the nucleus of an atom. It determines the mass of the atom. For example, carbon-12 has A = 12, meaning it has 6 protons and 6 neutrons.
Key Point:
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Z = defines the element
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A = defines the mass of the atom
2. What are isotopes? Give one example.
Answer:
Atoms of the same element that have the same atomic number (same number of protons) but different numbers of neutrons are called isotopes.
Example: Hydrogen has three isotopes:
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Protium (¹H) – 1 proton, 0 neutrons
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Deuterium (²H) – 1 proton, 1 neutron
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Tritium (³H) – 1 proton, 2 neutrons
Note: Isotopes have identical chemical properties but different atomic masses.
3. What is natural radioactivity?
Answer:
The spontaneous emission of radiations from unstable atomic nuclei is called natural radioactivity.
Explanation:
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Certain elements like uranium, thorium, and radium have nuclei that are unstable.
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These nuclei emit radiation naturally without any external cause.
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The emitted radiation can be alpha (α), beta (β), or gamma (γ) rays.
4. Define ionization.
Answer:
Ionization is the process by which radiations split matter into positive and negative ions.
Explanation:
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When radiation such as alpha, beta, or gamma rays passes through matter, it can knock out electrons from atoms.
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This creates positively charged ions (atom losing an electron) and negatively charged electrons, making the matter ionized.
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Ionization is an important property of radioactive radiation and is used in radiation detectors.
5. What are radioisotopes?
Answer:
Radioisotopes are artificially produced radioactive elements obtained by bombarding stable nuclei with protons, neutrons, or alpha particles.
Explanation:
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Radioisotopes do not occur naturally; they are created in laboratories or nuclear reactors.
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They emit radiation like natural radioactive elements.
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Uses: Medical diagnosis and treatment, sterilization of equipment, tracing elements in research, and industrial applications.
Long Questions – Unit 18 Atomic and Nuclear Physics
Q1: Explain Rutherford’s discovery of the nucleus. Describe the structure of an atom with its main parts.
Answer:
Rutherford’s Discovery of the Nucleus
In 1911, Ernest Rutherford, a New Zealand-born physicist, performed the gold foil experiment to investigate the structure of the atom:
1. Experiment Setup:
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Rutherford directed a beam of alpha particles (positively charged, fast-moving particles) at a very thin sheet of gold foil.
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A fluorescent screen was placed around the foil to detect the deflection of alpha particles.
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2. Observations:
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Most alpha particles passed straight through the gold foil.
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A small number were deflected at large angles.
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Very few bounced back toward the source.
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3. Conclusions:
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Most of the atom is empty space, allowing most alpha particles to pass through.
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A tiny, dense, positively charged center exists in the atom, called the nucleus, containing most of the atom’s mass.
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Electrons revolve around the nucleus in orbits, keeping the atom electrically neutral overall.
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Significance:
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Rutherford’s experiment disproved the “plum pudding model” of J.J. Thomson, which assumed that positive charge was spread uniformly.
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It led to the nuclear model of the atom, which forms the basis of modern atomic theory.
Structure of an Atom
1. Nucleus:
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The central part of the atom, very small and dense.
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Contains protons (positively charged) and neutrons (neutral).
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Together, protons and neutrons are called nucleons.
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Responsible for most of the atom’s mass.
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2. Electrons:
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Negatively charged particles that revolve around the nucleus in nearly circular orbits.
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Electrons are very light compared to protons and neutrons.
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Their movement creates electrical balance in the atom.
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3. Overall Atom:
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The atom is electrically neutral because the number of electrons equals the number of protons.
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It consists of a small, dense nucleus at the center and electrons moving around it, with mostly empty space between the nucleus and electrons.
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Summary: Rutherford’s discovery showed that atoms have a dense positively charged nucleus surrounded by orbiting electrons, and most of the atom’s volume is empty space. This is the foundation of the modern nuclear model of the atom.
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