Chemistry Lesson 2: Atoms and Molecules

John Dalton’s Atomic Model

• Introduced in 1803 as a theory of chemical combination.
• Postulates:
  • Elements are made of indivisible particles called atoms.
  • Atoms of the same element are identical; different elements have different atoms.
  • Atoms cannot be created or destroyed.

Scientific Laws

• Law of Conservation of Mass: Matter cannot be created or destroyed.
• Law of Definite Proportions: A given chemical compound contains the same elements in the same mass proportions.
• Law of Multiple Proportions: If two elements form multiple compounds, the mass ratios of one element that combine with a fixed mass of the second element are rational numbers.

Subatomic Particles

• Atoms are the smallest unit of an element retaining its properties.
• Atoms consist of subatomic particles:
  • Protons (positively charged)
  • Electrons (negatively charged)
  • Neutrons (neutral)

History of Atomic Structure

• Thomson’s Plum Pudding Model: Described atoms as a positively charged "soup" with negatively charged electrons embedded.
• Rutherford’s Gold Foil Experiment: Demonstrated that atoms are mostly empty space with a dense, positively charged nucleus.

Discovery of Protons

• Eugen Goldstein (1886) observed bright rays from the anode of a cathode tube, identifying them as streams of positively charged particles known as protons.

Discovery of Neutrons

• James Chadwick discovered neutrons in 1932 by firing alpha radiation at beryllium, producing uncharged radiation detected in paraffin wax.

Atoms, Molecules, and Ions

• Atoms: Single neutral particles.
• Molecules: Neutral particles composed of two or more bonded atoms.
• Ions: Charged particles (either positively or negatively).

Atomic Number and Mass Number

• Atomic Number: Represents the number of protons in an atom's nucleus; also equal to the number of electrons.
• Mass Number: Total of protons and neutrons in the nucleus.
  • Calculated as: Mass Number = Number of Protons + Number of Neutrons.
  • Number of Neutrons = Mass Number - Number of Protons.

Example Elements Summary

• Oxygen (O): Mass number 16, atomic number 8, 8 protons, 8 electrons, 8 neutrons.
• Silicon (Si): Mass number 28, atomic number 14, 14 protons, 14 electrons, 14 neutrons.
• Nitrogen (N): Mass number 14, atomic number 7, 7 protons, 7 electrons, 7 neutrons.

This quiz covers Lesson 2 on Atoms, Molecules, and Ions, focusing on John Dalton's Atomic Model and its key postulates. Explore the foundational concepts of chemical combination and the essential laws of conservation of mass and definite proportions. Test your understanding of these important principles in chemistry.