Models of the Atom Notes

Models of the Atom

• Development of the atomic model shows modern chemistry's impact.

• Atomic models help describe key features of atoms, though direct observation is not possible.

Historical Perspectives

Ancient Greece - Democritus (430 BC)

• Proposed that matter is composed of particles called "atomos" (atoms).

• Identified four elemental substances: Air, Fire, Earth, Water.

John Dalton’s Theory (1808)

• Dalton argued that atoms are the basic unit of matter.

• Key concepts:

  • All elements consist of indivisible atoms.

  • Atoms of each element are identical.

  • Atoms of different elements differ.

  • Compounds arise from combinations of different elements' atoms.

Advancements in Atomic Theory

Problematic Concepts

• Experimental studies revealed that atoms are divisible with smaller components.

Thomson’s Model (1897)

• J.J. Thomson discovered electrons: low mass, negatively charged particles.

• Concluded atoms must contain positive charge to neutralize electron charge.

• Proposed the Plum Pudding Model: uniform positive charge with electrons embedded within.

Rutherford’s Model (1911)

• Conducted experiments with gold foil.

• Observations:

  • Most alpha particles passed through.

  • Some deflected slightly; few greatly or bounced back.

• Conclusions:

  • Atoms are mostly empty space.

  • Nucleus is densely packed, with positive charge and most atom mass.

Bohr Model (1913)

• Niels Bohr focused on electron arrangement within atoms.

• Suggested electrons exist in fixed orbitals around a dense nucleus.

• Energy levels of electrons:

  • 1st level: 2 electrons.

  • 2nd level: 8 electrons.

  • 3rd level: up to 18, often limited to 8.

Chadwick’s Model (1932)

• Discovered neutrons: neutral particles in the nucleus alongside protons.

• Confirmed electrons orbit the nucleus in defined energy levels.

Modern Atomic Models

Electron Cloud Model

• Electrons do not orbit in fixed paths but exist in a fuzzy, three-dimensional space surrounding the nucleus.

Wave Mechanical Model

• Describes electrons' probable location as a cloud rather than fixed orbits.

Structure of the Atom

• Atoms consist of a dense, positively charged nucleus externally surrounded by electrons.

Nucleus Components

• Includes protons (positive charge) and neutrons (neutral).

Subatomic Particles

• Protons: mass of 1.67x10^-24 g (1 amu).

• Neutrons: approximately the same mass as protons.

• Each atom's atomic number = number of protons in its nucleus.

• Mass number = protons + neutrons.

• Electrons: lower mass than protons/neutrons; negatively charged, surrounding nucleus.

Isotopes

• Isotopes are variations of an element differing in neutron count.

  • Example: Hydrogen isotopes have varying numbers of neutrons (H-1, H-2, H-3).

• Isotope symbols denote element and mass number (e.g., C-14).

Atomic Mass

• Atomic masses on periodic tables reflect average weights of all naturally occurring isotopes.

Energy Levels and Electron Behavior

• Electrons move between energy levels by absorbing or emitting photons.

• Lower orbit = ground state; higher = excited state.

Summary of the Bohr Model

• Energy is quantized: electrons can only exist in fixed orbits and move between them by absorbing/emitting defined energy quantities.