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The Atom chapter 2

The Atom

2.1 Introduction

  • Matter is made up of small particles; this idea is known as the particulate nature of matter.

  • Diffusion: the spreading of gases or liquids due to the movement of their particles.

2.2 History of the Atom

  • Greek Philosophers (400 BC):

    • Proposed that matter was composed of small, indivisible particles called 'atomos'.

  • John Dalton (1808):

    • Atomic Theory: All matter is made up of indivisible atoms.

2.3 Discovery of the Electron

  • William Crookes (1875):

    • Cathode rays are emitted from the cathode (negative electrode) in a vacuum tube.

    • Cathode rays travel in straight lines, cause glass to fluoresce, and possess energy.

  • J.J. Thomson (1897):

    • Cathode rays consist of negatively charged particles called electrons.

    • Measured the charge to mass ratio of the electron.

2.4 Measuring the Charge on the Electron

  • Robert Millikan (1909):

    • Oil drop experiment determined the size of the charge on the electron: 1.6 x 10^{-19} coulombs.

    • Calculated the mass of the electron: 9.1 x 10^{-31} kg.

2.5 Thomson's Plum Pudding Model of the Atom

  • Atom is a sphere of positive charge with electrons embedded randomly.

2.6 Discovery of the Nucleus

  • Ernest Rutherford (1909):

    • Gold foil experiment using alpha particles.

    • Most alpha particles passed through undeflected, some deflected at large angles, and some reflected back.

    • Concluded that the atom has a small, dense, positive core called the nucleus.

2.7 Discovery of the Proton

  • Rutherford:

    • Discovered protons (positive particles) within the nucleus by bombarding light atoms with alpha particles.

    • Proposed that electrons exist in an 'electron cloud' surrounding the nucleus.

2.8 Discovery of the Neutron

  • James Chadwick (1932):

    • Bombarded beryllium with alpha particles.

    • Discovered neutrons, neutral particles with a mass similar to protons, located in the nucleus.

    • Neutrons help stabilize the nucleus by counteracting the repulsive forces between protons.

2.9 Properties of the Sub-Atomic Particles

  • Three sub-atomic particles: protons, neutrons, and electrons.

  • Protons and neutrons are in the nucleus; electrons are outside the nucleus.

  • Charge and mass:

    • Proton: charge = +1.6 x 10^{-19} C, relative charge = +1, relative mass = 1.

    • Neutron: charge = 0, relative charge = 0, relative mass = 1.

    • Electron: charge = -1.6 x 10^{-19} C, relative charge = -1, relative mass = 1/1838.