session 2 stud

Water Molecule

• Representation of water molecule as H₂O with oxygen (O) atom and two hydrogen (H) atoms.

Historical Context of Chemistry

Mendeleev's Contributions

• Year: 1870

• Place: Saint Petersburg, Russia

• Instructor: Dimitri Mendeleev

• Focus: Introductory Chemistry, foundation of periodic table.

Mendeleev's Discoveries

Element Compositions

• Used combinations of sulfur and iron to discover different compounds:

  • 10 g of iron + 11.5 g of sulfur = Pyrite

  • 10 g of iron + 7.7 g of sulfur = Greigite

  • 10 g of iron + 5.7 g of sulfur = Troilite

• Noted compositions:

  • FeS2 (Pyrite)

  • Fe3S4 (Greigite)

  • FeS (Troilite)

Mendeleev’s Periodic Table (1871)

• Aim: Systematic teaching of chemistry utilizing all 56 known elements.

• Ordered elements by increasing atomic mass (protons not yet discovered). ↑

• Arranged elements in columns based on similar chemical and physical properties (periodic properties).

• Included empty spaces for elements not yet discovered.

Limitations of Mendeleev's Understanding

• Lack of knowledge regarding atoms and their structure:

  • Uncertainty about atoms' existence, understanding mass equivalents but not the nature of a mole.

  • Ignored the atomic nucleus and electron role, lack of typical oxidation states.

Features of Mendeleev’s Table

• Elements listed in order of mass and properties.

• Did not list elements alphabetically or solely by atomic number.

J.J. Thomson's Discoveries

Cathode Ray Experiment (1897)

• Used a cathode ray tube to identify negatively charged particles (electrons).

• Found that the particles were deflected towards positively charged plates, indicating their charge.

• Determined a constant mass-to-charge ratio without pinpointing exact values.

Plum-Pudding Model (1890s)

• Conceptual model proposing that electrons are scattered within a positively charged 'pudding'.

Robert Millikan's Experiment

Millikan Oil Drop Experiment (1909)

• Experiment to measure the charge and mass of an electron:

  • Charge of electron: e− = −1.602 × 10−19 C

  • Mass of electron: me = 9.109 × 10−28 g

Rationale Behind Millikan’s Experiment

• Necessary to address inconsistencies in Thomson’s findings (effects of charge and mass on deflection).

Types of Radiation in Early Chemistry

Types of Radiations Identified

• Beta particles: high-energy electrons or positrons.

• Alpha particles: +2 charge, equivalent to helium nucleus.

• Gamma radiation: high-energy photons beyond x-rays.

Rutherford’s Nuclear Model

Gold Foil Experiment

• Designed to test Thomson's model by bombarding gold foil with alpha particles.

• Observed particle behavior:

  • Some particles deflected indicating a nucleus, while others passed through unhindered.

The Atomic Structure

• Components of an atom include:

  • Nucleus: small, positively charged center containing most mass of the atom.

  • Electrons: negatively charged particles surrounding the nucleus.

• Nucleus is approximately 1/10,000 the size of the entire atom.

Conclusion

• Significant progression in understanding atomic structure from Mendeleev to Rutherford, with the introduction of the electron and nucleus conceptions.