CIE IGCSE Physics (0625) The Nucleus Study Notes - New Syllabus
CIE IGCSE Physics (0625) The Nucleus Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of The Nucleus
Key Concepts:
- Composition of the Nucleus and Relative Charges
- Atomic Structure
- Isotopes and its uses
- Nuclear Fission and Fusion
Composition of the Nucleus and Relative Charges
Composition of the Nucleus and Relative Charges
Composition of the Nucleus:
The nucleus is the tiny, dense central part of an atom.
- It contains:
- Protons: Positively charged particles.
- Neutrons: Neutral particles (no charge).
- Almost all of the atom’s mass is concentrated in the nucleus.
- The nucleus has an overall positive charge due to the protons.
Relative Charges of Subatomic Particles:
- Proton: Charge = +1
- Neutron: Charge = 0 (neutral)
- Electron: Charge = −1
Example :
An atom contains 11 protons, 12 neutrons, and 10 electrons.
(a) What is the name of this element?
(b) What is the overall charge of this atom?
▶️ Answer/Explanation
(a) Identify the Element:
The number of protons (atomic number) is 11, which corresponds to sodium (Na) on the periodic table.
(b) Calculate the Overall Charge:
Protons = +11
Electrons = −10
Net charge = \( +11 + (-10) = +1 \)
So, this is a sodium ion (Na⁺) with a +1 charge.
Atomic Structure
Atomic Structure
Proton Number (Atomic Number) – Z:
- The number of protons in the nucleus of an atom.
- It determines the identity of the element.
- Also equals the number of electrons in a neutral atom.
Nucleon Number (Mass Number) – A:
- The total number of protons and neutrons in the nucleus.
- It gives the relative mass of the atom.
Number of Neutrons:
Number of neutrons = \( A – Z \)
Charge of the Nucleus:
- Only protons carry charge; neutrons are neutral.
- So, the total positive charge of the nucleus is \( +Z \).
Mass of the Nucleus:
- Protons and neutrons both contribute to the mass.
- So, the total mass of the nucleus is approximately \( A \) atomic mass units (u).
Nuclide Notation:
Written as: \( \large{^A_Z\text{X}} \)
- \( \text{X} \) = Chemical symbol
- \( Z \) = Proton number (bottom)
- \( A \) = Nucleon number (top)
Example:
The nuclide notation for an oxygen atom is written as \( ^{17}_8\text{O} \).
(a) How many protons, neutrons, and electrons are present in this atom?
(b) What is the charge on its nucleus?
(c) What is the approximate relative mass of the nucleus?
▶️ Answer/Explanation
(a) Subatomic Particles:
- Protons = 8 (from the proton number \( Z \))
- Neutrons = \( A – Z = 17 – 8 = 9 \)
- Electrons = 8 (same as protons in a neutral atom)
(b) Nuclear Charge:
Only protons carry charge, so total nuclear charge = \( +8 \)
(c) Relative Mass of the Nucleus:
Mass ≈ number of protons + neutrons = \( 8 + 9 = \boxed{17 \, \text{u}} \)
Isotopes and its uses
Isotopes
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
- They have the same proton number (Z) but different nucleon numbers (A).
- All isotopes of an element have the same chemical properties because they have the same number of electrons.
- They may have different physical properties, such as mass or radioactivity.
- An element can have more than one naturally occurring isotope.
| Isotope | Protons | Neutrons | Nucleon Number (A) |
|---|---|---|---|
| \( ^{12}_6\text{C} \) | 6 | 6 | 12 |
| \( ^{13}_6\text{C} \) | 6 | 7 | 13 |
Both are carbon isotopes. They have the same number of protons (6), but different numbers of neutrons (6 and 8). Hence, their mass numbers differ.
Uses of Isotopes
1. Medical Uses:
- Radiotherapy: Isotopes like Cobalt-60 emit gamma rays used to destroy cancer cells.
- Medical tracers: Radioactive isotopes like Technetium-99m are used to trace the movement of substances in the body (e.g., blood flow, organ function).
2. Industrial Uses:
- Leak detection: Radioactive isotopes are added to pipelines. If there’s a leak, the radiation can be detected from outside the pipe.
- Thickness control: Beta emitters like Strontium-90 are used in paper or metal production to ensure the correct thickness.
3. Archaeological Uses:
- Carbon dating: \( ^{14}_6\text{C} \) is used to estimate the age of ancient biological materials like wood, bones, or fossils.
Example:
Two atoms of chlorine are represented as \( ^{35}_{17}\text{Cl} \) and \( ^{37}_{17}\text{Cl} \).
(a) Explain why they are isotopes.
(b) Compare their number of protons, electrons, and neutrons.
(c) Would they behave differently in a chemical reaction? Explain.
▶️ Answer/Explanation
(a)
Both atoms have the same proton number (17), so they are the same element (chlorine). However, they have different nucleon numbers (35 and 37), meaning different numbers of neutrons. Therefore, they are isotopes.
(b)
| Isotope | Protons | Electrons | Neutrons |
|---|---|---|---|
| \( ^{35}_{17}\text{Cl} \) | 17 | 17 | 18 |
| \( ^{37}_{17}\text{Cl} \) | 17 | 17 | 20 |
(c)
No, both isotopes will behave the same in chemical reactions because chemical behavior is determined by the number of electrons, and both have 17 electrons.
Nuclear Fission and Fusion
Nuclear Fission and Fusion
Nuclear Fission:
Fission is the process where a large nucleus splits into two smaller nuclei, usually after absorbing a neutron.This process releases energy due to the loss in total mass (mass is converted to energy).
- Free neutrons are also released, which can trigger a chain reaction.
Example Nuclide Equation for Fission:
\( ^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3\, ^{1}_{0}\text{n} \)
- Uranium-235 absorbs a neutron and splits into barium-141 and krypton-92, releasing 3 neutrons and energy.
Qualitative Mass & Energy Change:
- The total mass of the products is slightly less than the mass of the reactants.
- This “missing” mass is converted into a large amount of energy (mass–energy equivalence).
Nuclear Fusion:
Fusion is the process where two small nuclei combine to form a larger nucleus.This occurs under very high temperature and pressure (like in stars).
- It releases even more energy than fission per reaction, also due to a small mass loss.
Example Nuclide Equation for Fusion:
\( ^{2}_{1}\text{H} + ^{3}_{1}\text{H} \rightarrow ^{4}_{2}\text{He} + ^{1}_{0}\text{n} \)
- Deuterium and tritium (two isotopes of hydrogen) fuse to form helium-4 and a neutron, releasing energy.
Qualitative Mass & Energy Change:
- The combined mass of the helium and neutron is less than the original hydrogen nuclei.
- This difference in mass is again released as energy.
Example:
Two nuclear reactions are given below:
Reaction A: \( ^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3\, ^{1}_{0}\text{n} \)
Reaction B: \( ^{2}_{1}\text{H} + ^{3}_{1}\text{H} \rightarrow ^{4}_{2}\text{He} + ^{1}_{0}\text{n} \)
(a) Identify which is fission and which is fusion.
(b) In both reactions, the total mass of the products is slightly less than the mass of the reactants. What happens to this “missing” mass?
(c) Why does fusion require high temperature and pressure?
▶️ Answer/Explanation
(a)
Reaction A is fission: A large nucleus (uranium-235) splits into two smaller nuclei.
Reaction B is fusion: Two small hydrogen nuclei (deuterium and tritium) combine to form a helium nucleus.
(b)
The “missing” mass is converted into energy according to Einstein’s equation \( E = mc^2 \).
This is why both processes release large amounts of energy.
(c)
Fusion requires high temperature and pressure to give the positively charged nuclei enough kinetic energy to overcome electrostatic repulsion (since like charges repel).
Only at very high conditions (like inside stars) can nuclei get close enough to fuse.