grade 9-chemistry_ethiofetenacom_7278

Structure of the Atom

Anode Rays

  • Definition:

    • Anode rays, also known as canal rays, are streams of positively charged particles (ions) that travel from the anode towards the cathode in a vacuum tube.

  • Other Name:

    • Anode rays are also referred to as canal rays.

  • Properties of Anode Rays:

    • Composition: Comprise positively charged ions produced by the ionization of gas in the discharge tube.

    • Direction: Move from the anode to the cathode.

    • Charge: Carry a net positive charge.

    • Behavior in Electric Fields: They are deflected in electric and magnetic fields towards the negative terminal due to their positive charge.

Discovery of the Electron

  • Context:Students should discuss the following points regarding electric bulbs and their behavior:

    • What happens when the switch is turned on?

    • What occurs inside the bulb?

    • Reasons for color differences (yellowish vs. white lights).

Crooke’s Discharge Tube

  • Invention: Developed by Heinrich Geissler in 1855, leading to the creation of vacuum tubes. Sir William Crookes modified these tubes to produce cathode rays, crucial in the discovery of electrons.

    • Design: A glass tube evacuated of air with two metal electrodes at either end connected to a battery.

    • Electrodes:

      • Anode: Connected to the positive terminal.

      • Cathode: Connected to the negative terminal.

    • Current Flow: Requires high voltage (10,000 - 20,000 volts) and low pressure for electricity to flow, generating cathode rays and producing visible light (e.g., greenish glow at anode).

Further Experiments by J.J. Thomson

  • Experiments Conducted:

    • Path of Cathode Rays:

      • Cathode rays travel in straight lines; observed by placing an object between the cathode and anode, creating a shadow on the opposite side.

    • Paddle Wheel Experiment:

      • A paddle wheel placed in the path of cathode rays rotates, demonstrating that these rays have particle mass and kinetic energy.

    • Charge Determination:

      • Cathode rays deflected towards the positive plate when passing through electric fields, indicating they are negatively charged.

    • Observations of Magnetic Fields:

      • Cathode rays deflected by magnetic fields confirm their negative charge.

    • Properties of Cathode Rays:

      • Travel straight, cause mechanical motion, their properties are independent of gas type, and they can ionize gases.

Thomson’s Atomic Model

  • Plum Pudding Model (1904):

    • Electrons embedded in a positively charged sphere, analogous to blueberries in a muffin.

    • This model explained electrical neutrality but lacked support for the shielding of protons by electrons.

Millikan’s Oil Drop Experiment (1909)

  • Purpose: Determined the charge of an electron.

    • Method: Using atomizer to produce charged oil droplets in an ionized chamber exposed to an electric field.

    • Observations:

      • Charge on oil drops is integral multiples of approximately 1.59 x 10^-19 coulombs; hence considered the charge of an electron.

    • Mass Calculation: Derived from the charge-to-mass ratio found by Thomson.

Discovery of the Nucleus

  • Rutherford’s Experiment:

    • Conducted with α-particles directed at a thin gold foil.

    • Observed that some particles deflected at large angles, indicating concentrated mass and charge, leading to the theory of the atomic nucleus.

    • Implication: Most mass and positive charge reside in a small central nucleus with electrons in surrounding space.

    • Stability of Nucleus:

      • Predicted the presence of neutrons to neutralize proton repulsion, enhancing nuclear stability.

Rutherford’s Atomic Model

  • Description:

    • Atoms consist of a tiny, dense nucleus surrounded by electrons in vast space; primarily empty. Nucleus has mass and positive charge.

    • Significant steps toward atomic understanding although incomplete with respect to electron behavior.

Discovery of the Neutron

  • Chadwick’s Experiment (1932):

    • Bombarded beryllium with α-particles revealing loosely-bound neutral particles (neutrons) that were not deflected in electric fields.

    • Conclusion: Demonstrated the existence of neutrons, thus refining the atomic model to account for both protons and neutrons in the nucleus.

Composition of Atoms and Isotopes

  • Exploration of Isotopes:

    • Atoms of the same element with different neutron counts result in variations in atomic mass and properties. Understanding the arrangement of sub-atomic particles is vital to categorizing isotopes.