Junior High School Second Year Chapter 1: Particle Model

Atomic Theory

• Definition of Atom:

  • An atom is a small and indivisible particle of matter.
  • Substances directly composed of atoms include metals, diamonds, graphite, etc.
  • The early philosopher Democritus proposed that everything in the universe is composed of tiny, indivisible particles called atoms.

• Development History:

  • Dalton: Atoms are indivisible spheres, and the type and quantity of different atoms remain unchanged in chemical reactions.
  • Thomson: Discovered that atoms have negatively charged electrons, and proposed that atoms consist of a nucleus and electrons.
  • Rutherford: Discovered that the nucleus is the center of the atom, concentrating all positive charge and mass, similar to the solar system model.
  • Bohr: Refined Rutherford's model, suggesting electrons can only move in specific orbits.

Molecules

• Composition of Molecules:

  • Composed of atoms and is the smallest particle that retains the chemical properties of a substance.
  • H2O is composed of two hydrogen atoms and one oxygen atom.
  • Ethanol is composed of carbon, hydrogen, and oxygen.

• Molecular Changes and Properties:

  • Water evaporation is a physical change where the distance between water molecules increases, but its chemical properties remain unchanged.
  • Electrolysis of water is a chemical change, decomposing into hydrogen and oxygen, altering water's chemical properties.

Atomic Structure

• Basic Structure:

  • The nucleus is positively charged, composed of protons and neutrons, with negatively charged electrons outside the nucleus.
  • Atoms are electrically neutral, with the number of protons equal to the number of electrons.

• Number of Protons, Neutrons, Electrons:

  • Hydrogen atom: 1 proton, no neutrons, 1 electron.
  • Carbon atom: 6 protons, 6 neutrons, 6 electrons.
  • The number of protons for each element from 1 to 20 must be memorized, corresponding to the same number of electrons.

Ions

• Conductors and Ions:

  • Conductors can conduct electricity because they have freely moving charges (ions) inside.
  • Ions in salt solutions can move freely to conduct electricity.

• Positive and Negative Ions:

  • Sodium ion (Na⁺): becomes a positive ion by losing an electron.
  • Chloride ion (Cl⁻): becomes a negative ion by gaining an electron.
  • Opposite charges attract to form compounds, such as sodium chloride.