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The Atom
The Nuclear Model of the Atom: Learner Success Notes
These comprehensive notes cover everything you need to know about the nuclear model of the atom, organized according to the Cambridge syllabus requirements. Use these to master the concepts and prepare for exams.
5.1.1 The Atom
Core Basic Atomic Structure
An atom consists of:
- A positively charged nucleus at the center
- Negatively charged electrons orbiting around the nucleus
Key Point: The atom is mostly empty space between the nucleus and the electrons.
Supplement Rutherford's Alpha Scattering Experiment
This experiment provided evidence for the nuclear model:
What was done? Alpha particles (α) were fired at a very thin sheet of gold foil.
Observations and Conclusions:
- Most alpha particles passed straight through → Evidence that the atom is mostly empty space
- A few alpha particles were deflected at large angles → Evidence for a tiny, massive, positively charged nucleus
- A very small number bounced straight back → Evidence that the nucleus contains most of the mass of the atom
Ions and Electron Transfer
An ion is a charged atom formed when an atom gains or loses electrons.
- Positive ions (cations) are formed when an atom loses one or more electrons
- Negative ions (anions) are formed when an atom gains one or more electrons
Example:
Sodium atom (Na) loses 1 electron → Sodium ion (Na⁺)
Chlorine atom (Cl) gains 1 electron → Chloride ion (Cl⁻)
5.1.2 The Nucleus
Core Nuclear Composition
The nucleus is composed of two types of particles:
| Particle | Location | Relative Charge |
|---|---|---|
| Proton | Nucleus | +1 |
| Neutron | Nucleus | 0 (Neutral) |
| Electron | Orbits nucleus | -1 |
Key Definitions
Proton Number (Atomic Number), Z: The number of protons in an atom's nucleus.
This defines the element. (e.g., any atom with 6 protons is Carbon).
Nucleon Number (Mass Number), A: The total number of protons and neutrons in the nucleus.
Number of Neutrons, N: Calculated as: N = A - Z
Nuclide Notation
This is a shorthand way of representing atomic composition:
AZX
- X = Chemical symbol (e.g., Na, O, He)
- A = Nucleon (Mass) Number (protons + neutrons)
- Z = Proton (Atomic) Number (protons)
Example: 2311Na
- Proton number, Z = 11 (11 protons)
- Nucleon number, A = 23
- Neutron number, N = 23 - 11 = 12 neutrons
Relationships from Notation
- The relative charge of the nucleus is +Z (because each proton has a +1 charge)
- The relative mass of the nucleus is approximately A (protons and neutrons each have a relative mass of 1)
Isotopes
Isotopes are atoms of the same element (same number of protons, same Z) with different numbers of neutrons (different N, therefore different A).
Key Point: Isotopes have the same chemical properties (same electron configuration) but different masses.
Example: Chlorine Isotopes
- Chlorine-35: 3517Cl (17 protons, 18 neutrons)
- Chlorine-37: 3717Cl (17 protons, 20 neutrons)
Both are chlorine, but one has more neutrons and is heavier.
Supplement Nuclear Fission & Fusion
Nuclear Fission:
- A large, heavy nucleus (e.g., Uranium-235) splits into two smaller, lighter nuclei
- Releases a very large amount of energy
- Used in nuclear power plants
Nuclear Fusion:
- Two small, light nuclei (e.g., Hydrogen isotopes) join (fuse) together to form a larger nucleus
- Releases a massive amount of energy
- This is the process that powers stars like the Sun
Mass & Energy: In both processes, the total mass of the products is slightly less than the total mass of the reactants. This "lost" mass is converted into a huge amount of energy (as described by Einstein's equation E=mc²).
Quick-Check Summary
| Concept | Key Points |
|---|---|
| Atom Structure | Positive nucleus, negative electrons, mostly empty space |
| Ions | Lose e⁻ → positive ion. Gain e⁻ → negative ion |
| Alpha Scattering | Proved the nucleus is small, dense, positive, and massive |
| Proton (p⁺) | Charge: +1, In nucleus, defines the element |
| Neutron (n⁰) | Charge: 0, In nucleus, adds mass |
| Electron (e⁻) | Charge: -1, Orbits nucleus |
| Z (Atomic No.) | Number of protons |
| A (Mass No.) | Number of protons + neutrons |
| Isotopes | Same Z, different A (same element, different neutrons) |
| Fission | Big nucleus splits. Releases energy |
| Fusion | Small nuclei join. Releases energy |