Simple Model Of The Atom
Key idea: Describe the nuclear atom model, define proton number Z and nucleon number A, and use nuclide notation for isotopes (A Level Physics).
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The core idea
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Learning objectives
- Interpret nuclear structure, isotopes and Rutherford scattering.
1. Definitions (Must Know)
A. Nuclear atom model
In the nuclear atom model:
- most of the atom’s mass is in a tiny, positively charged nucleus,
- the nucleus contains protons and neutrons (nucleons),
- electrons occupy the space around the nucleus,
- the atom is mostly empty space.
Typical size scales:
- nucleus radius ∼ 10⁻¹⁵ m
- atom radius ∼ 10⁻¹⁰ m
B. Proton number (atomic number), Z
The proton number, Z, is the number of protons in the nucleus.
C. Nucleon number (mass number), A
The nucleon number, A, is the total number of nucleons (protons + neutrons) in the nucleus.
A = Z + N where N is the number of neutrons.
D. Nuclide notation
A nuclide is written as: ^A_ZX where X is the element symbol.
E. Isotopes
Isotopes are atoms of the same element (same Z) with different neutron number N (so different A).
In nuclear equations, track A (nucleon number) and Z (charge/proton number) carefully. They are conserved in nuclear processes.
2. Key Ideas (What Earns Marks)
- The Rutherford experiment supports the nuclear atom: small nucleus, mostly empty space.
- Use ^A_ZX notation correctly:
- A counts nucleons (protons + neutrons)
- Z counts protons (and sets the element)
- Isotopes: same element ⇒ same Z; different A because N differs.
3. Detailed Explanations
A. Particle summary (useful constants)
| Particle | Relative charge | Mass (kg, approx.) |
|---|---|---|
| electron, e⁻ | -e | 9.11 × 10⁻³¹ |
| proton, p | + e | 1.67 × 10⁻²⁷ |
| neutron, n | 0 | 1.67 × 10⁻²⁷ |
B. Why “mostly empty space” is a big conclusion
If positive charge were spread throughout the atom (plum pudding), large deflections would be common. The observed rarity of large-angle scattering implies a tiny nucleus and a large empty region around it.
4. Common Mistakes
- Swapping A and Z in nuclide notation.
- Saying isotopes have different Z (they do not).
- Mixing “atomic number” (protons) with “mass number” (nucleons).
5. Exam Tips
- Always write nuclide notation with A on top and Z on bottom: ^A_ZX.
- Use the equation A = Z + N to find neutrons quickly.
- When balancing nuclear equations later: check both A and Z separately.
6. Worked Examples
Modelled example 1
Finding neutron number
Problem
Study the worked solution
Read the nuclide numbers
Method
A = 27 and Z = 13.Reason
The upper number counts all nucleons; the lower number counts protons.Working
^A_ZAl = ²⁷₁₃AlSubtract protons from nucleons
Method
The nucleus contains 14 neutrons.Reason
A = Z + N, so N = A-Z.Working
N = 27-13 = 14
Common misconception 2
Isotope identification
Learner claim
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View solution step by step
Apply the element condition
Method
Isotopes must have the same Z.Reason
Proton number defines the element.Working
same element ⇔ same ZReject the first pair
Method
Carbon-12 and nitrogen-14 are not isotopes.Reason
Their proton numbers are 6 and 7, so they are different elements.Working
Z_C = 6 ≠ Z_N = 7Identify the isotope pair
Method
Chlorine-35 and chlorine-37 are isotopes.Reason
Both have Z = 17 but different A, hence different neutron numbers.Working
N = 18 and N = 20
7. Mind Stretchers
Mind stretcher 1: Why does the element depend on Z rather than A?Extension
Show Answer
Z is the proton number, which sets the nuclear charge and determines the electron structure of a neutral atom. Changing A without changing Z only changes neutron number, giving an isotope of the same element.
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Course and syllabus information
- Course
- GCE A-Level H2 Physics
- Edition
- GCE A-Level H2 Physics 2027