The Atom | Physics for GCSE/IGCSE - Year 11 PDF Download

Atomic Structure

• Atoms constitute the fundamental units of all substances.
• They possess minute dimensions, with a radius of merely 1 × 10^-10 m.
  • Approximately one hundred million atoms could align along your thumbnail's width.
• Atoms feature a minuscule yet dense nucleus positioned at their core, orbited by electrons.
• The nucleus, which is over 10,000 times smaller than the atom itself, contains nearly all of the atom's mass.
• Atoms consist of compact, positively charged nuclei, encompassed by negatively charged electrons.

Rutherford's Experiment

Overview of the Experiment 

• In 1909, a team of scientists led by physicist Ernest Rutherford conducted an experiment to investigate the Plum Pudding model of the atom. Rutherford directed two of his students, Hans Geiger and Ernest Marsden, to carry out the experiment.
• The experiment involved the scattering of alpha (α) particles by a thin metal sheet, supporting the nuclear model of the atom.
• A beam of alpha particles (He 2 ions) was aimed at a thin gold foil, with the expectation that the alpha particles would pass through the foil, possibly changing direction slightly.
• Surprisingly, the results revealed that:
  • Most of the alpha particles passed straight through the foil.
  • Some alpha particles changed direction but still passed through the foil.
  • A few alpha particles rebounded off the gold foil.
• The unexpected behavior of the alpha particles, especially the rebounding, contradicted the Plum Pudding model, necessitating the development of a new model. This marked a significant milestone in understanding the structure of the atom.

• When α-particles are directed at thin gold foil:
  • Most α-particles pass through without deviation (A).
    This occurrence is attributed to the predominant empty space within the atom.
  • Some α-particles undergo deflection at slight angles (B).
    This phenomenon arises due to the repulsion between the positively charged α-particles and the positively charged nucleus, which houses the majority of the atom's mass.
  • A minute fraction of α-particles recoil directly backward (C).
    This rare event occurs owing to the nucleus's exceedingly minute size.

Atoms & Ions

• Atoms and ions are tiny particles that are the building blocks of matter.
• Ions are atoms or groups of atoms that have an electric charge due to the loss or gain of electrons.
• When atoms lose electrons, they become positively charged ions, as there are more protons than electrons.
• Conversely, when atoms gain electrons, they become negatively charged ions, with more electrons than protons.
• Atoms undergo these changes to achieve stability.
• A stable atom has an equal number of protons (positive charge) and electrons (negative charge), making it electrically neutral.
• For example, sodium (Na) can lose an electron to become a positively charged sodium ion (Na⁺).
• Similarly, chlorine (Cl) can gain an electron to become a negatively charged chloride ion (Cl^-).