Earth Science Lecture: Atoms and Chemical Bonds

Introduction

• Topic: Atoms and chemical bonds related to minerals and rocks.
• Importance: This lecture supplements textbook material.

Early Atomic Theory

• Democritus (462-362 BC)
  • Universe is composed of indivisible atoms.
  • Atoms are indestructible, always in motion, exist in empty space, and differ in shape and size.
• Resistance: Plato and Aristotle did not accept atomic theory.
• Reintroduction: John Dalton (early 1800s) revived atomic theory.

Dalton's Atomic Theory

1. Matter is made of indivisible atoms.
2. Atoms of an element are identical (now known isotopes exist).
3. Atoms of different elements differ in mass.
4. Atoms combine in whole-number ratios to form compounds.
5. Atoms are neither created nor destroyed in chemical reactions.

Discovery of the Electron

• Late 19th Century: Study of electric currents with cathode ray tubes.
• JJ Thompson (1897): Discovered electrons (negatively charged particles).
• Deflection Experiments: Showed cathode rays deflected by electric and magnetic fields, indicating negative charge.
• Consequence: Disproved indivisibility of atoms (Dalton's theory).

Rutherford and the Atomic Nucleus

• 1907: Ernest Rutherford's Gold Foil Experiment
  • Discovered dense, positively charged nucleus.
  • Proposed atom mostly empty space with dense nucleus.
  • Identified protons and speculated on neutrons (discovered by James Chadwick in 1932).

Atomic Structure

• Protons: Positively charged.
• Electrons: Negatively charged, move in orbitals (Heisenberg Uncertainty Principle).
• Neutrons: No charge, similar mass to protons.
• Atomic Number: Defines the element (number of protons).
• Isotopes: Atoms of the same element with different numbers of neutrons.
• Periodic Table: Organizes elements by atomic number and properties.

Periodic Table Organization

• Rows (Periods): Increase in atomic number from left to right.
• Columns (Groups): Elements with similar chemical properties.
• Valence Electrons: Electrons in the outermost shell determining chemical behavior.
• Octet Rule: Atoms seek a stable configuration of 8 valence electrons.

Types of Chemical Bonds

• Ionic Bonds: Transfer of electrons, forming positive and negative ions (e.g., Sodium Chloride - NaCl).
• Covalent Bonds: Sharing of electrons between atoms.
• Metallic Bonds: Electrons move freely among atoms (responsible for electrical conductivity and malleability of metals).

Ionic Bonds

• Example: Sodium and Chlorine
  • Sodium loses an electron, becoming a positive ion.
  • Chlorine gains an electron, becoming a negative ion.
  • Positive and negative ions attract, forming a crystalline solid.

Covalent Bonds

• Example: Two Hydrogen Atoms
  • Achieve stability by sharing electrons.
  • Form molecules (e.g., H₂).

Metallic Bonds

• Characteristics:
  • Valence electrons move freely among metal atoms.
  • Good electrical conductivity.
  • Malleability and ductility of metals.

Summary and Importance

• Chemistry in Earth Science: Understanding atomic structure and bonding is crucial for studying minerals and rocks.

Questions

1. What determines the physical and chemical properties of an atom?
   • Answer: Number of protons.
2. Electrons have a __________ charge and a mass that is __________ than that of protons.
   • Answer: Negative, smaller.
3. Atomic bonding which involves two atoms sharing one or more electrons is known as __________ bonding.
   • Answer: Covalent.
4. When an atom loses an electron, it becomes a __________ charged ion.
   • Answer: Positively.

Conclusion

• Relevance: Basic chemistry principles are essential for understanding earth science topics such as minerals and rocks.
• Encouragement: Engage with these foundational concepts to enhance overall scientific literacy.