Atomic Theory: MCQ 3

Atomic Theory

• All matter is composed of tiny particles called atoms.
• Atoms are indivisible, meaning they cannot be broken into simpler particles.

John Dalton's Contributions

• Proposed the atomic theory in 1808, including the Law of Conservation of Mass.
• Atoms cannot be created or destroyed in a chemical reaction.

Cathode Rays

• Scientists discovered that invisible particles traveled from the cathode (- terminal) to the anode (+ terminal) in a discharge tube.
• These particles were later found to be negatively charged.

William Crookes' Experiments

• Conducted experiments to demonstrate the existence of cathode rays.
• Performed the Maltese Cross experiment, showing that cathode rays travel in straight lines.

JJ Thomson's Contributions

• Conducted experiments into the charge and mass of cathode rays in 1897.
• Found that cathode rays were negatively charged and lighter than the lightest known atom.
• Proposed the Plum Pudding Model, where a positively charged pudding (proton) was balanced by negatively charged fruit (electrons).

Electron Discovery

• Johnstone Stoney suggested the name "electron" for these negatively charged particles.
• Robert Millikan measured the charge on an electron using the oil drop experiment in 1911.

Rutherford's Experiment

• Assumed that positive α-particles would be deflected as they passed through thin gold foil in 1911.
• Observed that:
  • A large number of α-particles were not deflected at all.
  • Many particles were deflected at small angles.
  • A small number of particles were deflected at angles greater than 90°.
  • A smaller number bounced back towards their source.
• Concluded that:
  • The positive charge was concentrated into a dense nucleus in the centre of the atom.
  • The rest of the atom was empty space, allowing α particles to travel through undisturbed.

Rutherford's Atomic Model

• Proposed a new model for the atom, where electrons are arranged in concentric circular orbits at increasing distances from the nucleus.
• These different orbits were called energy levels.

Protons and Neutrons

• Rutherford discovered that positively charged particles identical to a hydrogen atom were emitted when an atom was bombarded.
• Concluded that these positively charged particles were protons and made up the positive part of all atoms.
• James Chadwick produced evidence for the existence of neutral particles, called neutrons, in the nucleus.

Neil Bohr's Contributions

• Proposed an atomic model in 1913, building on Rutherford's work.