Atomic models, particles and isotopes
Atomic structure and the periodic table • Atomic models and isotopes
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Scattering experiment and change in atomic model
A beam of alpha particles directed at a thin metal foil produces three observable outcomes: most alpha particles pass straight through, some deflect by small angles, and a very small fraction scatter at large angles. Cause: most of the atom is empty space. Effect: most alpha particles travel through with no interaction. Cause: concentrated positive charge in the atom produces strong electric fields near it. Effect: a few alpha particles deflect strongly or rebound when they approach this concentrated charge. The combination of these observations invalidates any model that spreads positive charge uniformly through the atom and supports a model with a tiny, dense, positively charged nucleus.
Plum pudding model versus nuclear model
The plum pudding model describes the atom as a diffuse positive 'pudding' with electrons embedded like 'plums' throughout. Limiting factor: the model predicts only minor deflections for high-energy, charged particles because positive charge spreads across the whole atom. The nuclear model describes the atom with a very small, dense nucleus containing positive charge and nearly all mass, surrounded by electrons occupying the remaining space. Cause: a concentrated nucleus produces strong, localized electric fields. Effect: rare, large-angle scattering of high-energy particles, which the nuclear model explains but the plum pudding model cannot.
Description of atoms using the nuclear model
Atoms consist of a nucleus containing protons and neutrons, with electrons arranged around the nucleus. Protons carry a +1 elementary charge; neutrons carry no net charge but contribute to mass. Electrons carry a −1 elementary charge and have negligible mass compared with protons and neutrons for most calculations. The nucleus radius is on the order of 10^−15 metres; the overall atom radius is on the order of 10^−10 metres. Atoms are electrically neutral when proton and electron counts are equal; ions form when electron count differs from proton count.
Calculating protons, neutrons and electrons
Atomic number (Z) equals the number of protons in an atom. Mass number (A) equals the total number of protons plus neutrons. Number of neutrons = mass number − atomic number (A − Z). For neutral atoms, electrons = protons. For ions, electrons = protons − charge for positive ions, and electrons = protons + magnitude of charge for negative ions. Worked examples: Carbon-12 (Z = 6, A = 12) → protons 6, neutrons 6, electrons 6. Magnesium ion Mg2+ (Z = 12, A = 24) → protons 12, neutrons 12, electrons 10.
Size and scale of atoms
Typical atomic radii lie around 100 pm (1 × 10^−10 m). Nuclear radii lie around 1 fm (1 × 10^−15 m). Cause: most of an atom's mass concentrates in the tiny nucleus while electrons occupy a much larger cloud. Effect: an atom is roughly 10^5 times larger in radius than its nucleus. Comparative scales: a typical virus (~100 nm) is about 1,000 times larger than an atom; a human hair (~100 μm) is about 10^6 times wider than a virus.
Relative atomic mass from isotope abundances
Relative atomic mass (Ar) for an element equals the weighted average of the masses of its isotopes, using their fractional abundances. Formula: Ar = Σ (isotope mass × fractional abundance). Fractional abundance equals percentage abundance divided by 100. Worked example: chlorine with 75% 35Cl and 25% 37Cl gives Ar = (35 × 0.75) + (37 × 0.25) = 26.25 + 9.25 = 35.5. Limiting factor: percent abundances must sum to 100% and isotope masses must use the same mass scale (typically relative atomic mass units).
Key notes
Important points to keep in mind