⚛️ CIE IGCSE Physics – The Atom
Understanding the atom is fundamental to nuclear physics, radioactivity, electricity, and chemistry.
1️⃣ Structure of the Atom
🔑 Basic Structure
An atom consists of:
- A positively charged nucleus
- Negatively charged electrons orbiting the nucleus
🧱 Inside the Nucleus
The nucleus contains:
- Protons (positive charge)
- Neutrons (no charge)
⚛️ Outside the Nucleus
- Electrons move around the nucleus in energy levels (shells).
- They have negative charge.
- They have very small mass compared to protons and neutrons.
📊 Relative Charges and Masses
| Particle | Charge | Relative Mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | –1 | 1/1836 |
🔎 Key Points
- Most of the atom is empty space.
- The nucleus is very small compared to the atom.
- Nearly all the mass is in the nucleus.
2️⃣ Formation of Ions
Atoms are normally neutral:
- Number of protons = number of electrons
🔋 Positive Ions (Cations)
Formed when an atom loses electrons.
Example:
Sodium atom:
- 11 protons
- 11 electrons (neutral)
If it loses 1 electron:
- 11 protons
- 10 electrons
- Net charge = +1
Becomes:
🔋 Negative Ions (Anions)
Formed when an atom gains electrons.
Example:
Chlorine atom:
- 17 protons
- 17 electrons
If it gains 1 electron:
- 17 protons
- 18 electrons
- Net charge = –1
Becomes:
🎯 Important Rule
- Losing electrons → positive ion
- Gaining electrons → negative ion
Protons NEVER change in ordinary chemical reactions.
3️⃣ Alpha Particle Scattering Experiment
This was carried out by Ernest Rutherford and his team.
It is also known as the Gold Foil Experiment.
🧪 Experimental Setup
- Alpha particles fired at a very thin sheet of gold.
- Detector placed around foil.
- Most famous interpretation by Rutherford.
📊 Observations
- Most alpha particles passed straight through.
- Some were deflected slightly.
- Very few were deflected backwards.
🧠 Conclusions from the Experiment
(a) Atom is Mostly Empty Space
Since most alpha particles passed straight through:
- There is mostly empty space in atoms.
(b) Nucleus is Very Small and Contains Most of the Mass
Since a few particles were deflected at large angles:
- There must be a small, dense centre.
- This centre contains most of the mass.
Alpha particles only deflect strongly if they hit something very massive.
(c) Nucleus is Positively Charged
Alpha particles are positively charged.
They were repelled when close to the nucleus.
This proves:
- The nucleus is also positively charged.
Like charges repel.
🔥 Why Was This Important?
Before this experiment, the “plum pudding model” suggested:
- Positive charge spread throughout atom.
- Electrons embedded inside.
Rutherford’s experiment disproved this.
📐 Nuclear Model of the Atom
After the experiment:
- Tiny, dense, positive nucleus at centre.
- Electrons orbit around it.
- Most of atom is empty space.
This is called the nuclear model.
🧠 Important Terms
| Term | Meaning |
|---|---|
| Nucleus | Central positive region of atom |
| Proton | Positively charged particle |
| Neutron | Neutral particle |
| Electron | Negatively charged particle |
| Ion | Charged atom |
| Alpha particle | Helium nucleus (2 protons, 2 neutrons) |
| Scattering | Deflection of particles |
📝 Exam Tips
1️⃣ When describing atomic structure:
Always mention:
- Positively charged nucleus
- Negatively charged electrons
- Mostly empty space
2️⃣ For ion questions:
Remember:
- Electrons change.
- Protons stay the same.
3️⃣ For Rutherford experiment:
Structure your answer like this:
Observation → Conclusion
Example:
Most alpha particles passed through → Atom mostly empty space.
4️⃣ Common Mistakes
❌ Saying electrons are in the nucleus
✔ Electrons orbit nucleus
❌ Saying nucleus is large
✔ It is very small compared to atom
❌ Saying protons move to form ions
✔ Only electrons are gained or lost
📊 Typical 4–6 Mark Question
Often asks:
- Describe experiment
- State observations
- Explain how evidence supports nuclear model
Make sure to:
✔ Mention most passed through
✔ Mention small number deflected
✔ Mention positive nucleus
✔ Mention most mass concentrated in nucleus
⭐ Quick Summary
- Atom has tiny positive nucleus.
- Electrons orbit nucleus.
- Atom mostly empty space.
- Positive ions form by losing electrons.
- Negative ions form by gaining electrons.
- Alpha scattering proved nuclear model.
⚛️ CIE IGCSE Physics – The Nucleus
This topic focuses on the structure of the nucleus, isotopes, and nuclear reactions (fission and fusion).
The nucleus is responsible for:
- Most of the mass of an atom
- Radioactivity
- Nuclear energy
1️⃣ Composition of the Nucleus
🔑 Basic Structure
The nucleus contains:
- Protons
- Neutrons
These particles are called nucleons.
📊 Relative Charges
| Particle | Relative Charge |
|---|---|
| Proton | +1 |
| Neutron | 0 |
| Electron | –1 |
Electrons are NOT in the nucleus.
🔎 Key Idea
- Protons give the nucleus its positive charge.
- Neutrons add mass but no charge.
- Almost all the mass of the atom is in the nucleus.
2️⃣ Proton Number (Atomic Number) Z
🔑 Definition
The proton number (Z) is:
The number of protons in the nucleus of an atom.
It determines:
- The element.
- The chemical properties.
Example:
- Hydrogen → Z = 1
- Carbon → Z = 6
- Oxygen → Z = 8
If proton number changes → element changes.
3️⃣ Nucleon Number (Mass Number) A
🔑 Definition
The nucleon number (A) is:
The total number of protons and neutrons in the nucleus.
🔎 Finding Number of Neutrons
📌 Example
For Carbon-12:
- Z = 6
- A = 12
Neutrons:
So Carbon-12 has:
- 6 protons
- 6 neutrons
4️⃣ Nuclide Notation
Written as:
Where:
- A = nucleon number
- Z = proton number
- X = chemical symbol
📌 Example 1
Carbon-14:
- 6 protons
- 8 neutrons
📌 Example 2
Uranium-235:
- 92 protons
- 143 neutrons
5️⃣ Isotopes
🔑 Definition
Isotopes are:
Atoms of the same element with the same proton number but different nucleon numbers.
🔎 What This Means
- Same number of protons
- Different number of neutrons
- Same chemical properties
- Different masses
📌 Example
Carbon has isotopes:
All have:
- 6 protons
But different neutrons.
6️⃣ Relationship Between Proton Number and Nuclear Charge
Since each proton has charge +1:
Example:
Oxygen (Z = 8):
- Nuclear charge = +8
7️⃣ Relationship Between Nucleon Number and Nuclear Mass
Since protons and neutrons each have relative mass 1:
Electrons have negligible mass.
8️⃣ Nuclear Fission
🔑 Definition
Nuclear fission is:
The splitting of a large nucleus into two smaller nuclei, releasing energy.
📌 Example: Uranium-235
This produces:
- Two smaller nuclei
- Extra neutrons
- Large amount of energy
🔥 Why Energy Is Released
- Total mass after reaction is slightly less.
- Missing mass converted into energy.
- (Mass → Energy)
⚡ Chain Reaction
Released neutrons:
- Can cause further fission.
- This is called a chain reaction.
Used in:
- Nuclear power stations
- Atomic bombs (uncontrolled reaction)
9️⃣ Nuclear Fusion
🔑 Definition
Nuclear fusion is:
The joining of two small nuclei to form a larger nucleus, releasing energy.
📌 Example (Hydrogen Fusion)
Hydrogen isotopes combine to form helium.
🔥 Where Fusion Occurs
- In the Sun
- In stars
- In hydrogen bombs
⚠️ Important Difference
| Fission | Fusion |
|---|---|
| Splitting large nucleus | Joining small nuclei |
| Used in power stations | Occurs in stars |
| Easier to control | Very difficult to control |
🔬 Mass and Energy Changes (Qualitative)
In both fission and fusion:
- Total mass after reaction is slightly smaller.
- “Missing mass” becomes energy.
- Large amounts of energy are released.
No calculations needed at IGCSE level.
🧠 Important Terms
| Term | Meaning |
|---|---|
| Nucleus | Central part of atom |
| Proton number (Z) | Number of protons |
| Nucleon number (A) | Protons + neutrons |
| Isotope | Same element, different neutrons |
| Fission | Splitting nucleus |
| Fusion | Joining nuclei |
| Chain reaction | Self-sustaining fission reaction |
📝 Exam Tips
1️⃣ Always use:
2️⃣ In nuclear equations:
✔ Total proton number must balance
✔ Total nucleon number must balance
3️⃣ Common Mistakes
❌ Confusing proton number and nucleon number
❌ Forgetting electrons are NOT in nucleus
❌ Writing unbalanced nuclear equations
4️⃣ If asked difference between fission and fusion:
Clearly state:
- Fission = splitting
- Fusion = joining
- Both release energy
⭐ Quick Summary
- Nucleus contains protons and neutrons.
- Proton number (Z) determines element.
- Nucleon number (A) = protons + neutrons.
- Isotopes have same Z but different A.
- Nuclear charge = +Z.
- Nuclear mass ≈ A.
- Fission splits large nuclei.
- Fusion joins small nuclei.
- Both release large amounts of energy.