Atomic Structure
Section: Nuclear Physics | Syllabus: Cambridge AS Level Physics 9702
The Nuclear Model of the Atom Understanding the structure of the atom is fundamental to nuclear physics. Structure of the Atom An atom consists of: Nucleus: A very small, dense, positively charged centre Electrons: Negatively charged particles in orbit around the nucleus Exam Tip Remember the key phrase: "positively charged nucleus with negatively charged electrons in orbit around it" - this is exactly how the syllabus describes atomic structure.
Key Fact If an atom were the size of a football stadium, the nucleus would be the size of a marble at the center! Evidence for the Nuclear Model: Alpha Particle Scattering The scattering of alpha (α) particles by a thin sheet of metal provides evidence for the nuclear model of the atom.
The Experiment A beam of alpha (α) particles was fired at a thin sheet of gold foil A detector screen surrounding the foil recorded where the alpha particles went Alpha particles are positively charged and relatively heavy FIG 5.1: Alpha particle scattering experiment setup Radioactive source emitting alpha particles, Narrow beam directed at thin gold foil, Circular fluorescent detector screen, Paths of scattered particles at various angles.
Evidence from Alpha Scattering Observation Conclusion Evidence For Most alpha particles passed straight through undeflected The atom is mostly empty space (a) A very small nucleus surrounded by mostly empty space Very few alpha particles (about 1 in 8000) bounced back or were deflected through large angles They hit something very small, dense and massive (b) A nucleus containing most of the mass of the atom Some alpha particles were deflected through small angles They passed close to something positive and were repelled (like charges repel) (c) A nucleus that is positively charged Exam Tip Learn the three key pieces of evidence: (a) small nucleus + mostly empty space, (b) nucleus contains most of the mass, (c) nucleus is positively charged.
Questions often ask you to link observations to conclusions. Composition of the Nucleus The nucleus is composed of protons and neutrons . These particles are collectively called nucleons . Relative Charges and Masses Particle Symbol Location Relative Charge Relative Mass Proton p Nucleus +1 1 Neutron n Nucleus 0 1 Electron e⁻ Orbiting nucleus −1 Negligible Exam Tip You must know the relative charges: proton = +1 , neutron = 0 , electron = −1 .
These exact values are required by the syllabus. FIG 5.2: Atomic structure diagram Detailed nucleus showing protons (red, marked +) and neutrons (grey, marked n) clustered together, with electron shells surrounding it.
Proton Number and Nucleon Number Every element is uniquely defined by the number of protons in its nucleus. Proton Number (Atomic Number), Z The number of protons in the nucleus of an atom. Nucleon Number (Mass Number), A The total number of protons and neutrons in the nucleus.
Calculating the Number of Neutrons The number of neutrons in a nucleus can be calculated using: Number of neutrons = A − Z Key Relationships Relative charge of nucleus = +Z (equal to the proton number, since each proton has charge +1 and neutrons have charge 0) Relative mass of nucleus ≈ A (equal to the nucleon number, since protons and neutrons each have relative mass 1) Exam Tip The proton number determines the relative charge on the nucleus.
The nucleon number determines the relative mass of the nucleus. Nuclide Notation We represent atoms using standard nuclide notation: Example: Nuclide Notation A Z X Where: X = chemical symbol of element A = nucleon number (mass number) - top number Z = proton number (atomic number) - bottom number Example: Carbon Atom Notation Example 1: Carbon atom 12 6 C Proton number (Z) = 6 Nucleon number (A) = 12 Number of protons = 6 Number of neutrons = 12 − 6 = 6 Number of electrons (in neutral atom) = 6 Example: Uranium Atom Notation Example 2: Uranium atom 238 92 U Proton number (Z) = 92 Nucleon number (A) = 238 Number of protons = 92 Number of neutrons = 238 − 92 = 146 Number of electrons (in neutral atom) = 92 Isotopes An element may have more than one isotope.
Isotopes Atoms of the same element with the same number of protons but different numbers of neutrons (same proton number Z, different nucleon number A). Key Properties of Isotopes Same number of protons (same Z) - so they are the same element Different number of neutrons (different A) Same chemical properties (same electron configuration) Different physical properties (different masses, some may be radioactive) Example: Isotopes of Carbon 12 6 C: 6 protons, 6 neutrons (stable, most common) 13 6 C: 6 protons, 7 neutrons (stable, rare) 14 6 C: 6 protons, 8 neutrons (radioactive, used in carbon dating) All three are carbon because they all have 6 protons, but they have different nucleon numbers.
FIG 5.3: Isotopes of carbon Three carbon isotope nuclei side by side: C-12 with 6 protons (red) + 6 neutrons (grey), C-13 with 6 protons (red) + 7 neutrons (grey), C-14 with 6 protons (red) + 8 neu…
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