1-THE-STRUCTURE-OF-THE-ATOM

The History of the Atomic Theory

ATOMS

  • All matter is made up of atoms.

  • Atomic Parts include:

    • Protons: positive (+) charge

    • Neutrons: no charge (0)

    • Electrons: negative (-) charge

HOW DOES THE ATOM LOOK LIKE?

  • Structure of an atom varies by atomic theory.

History of the Atom Timeline

  • Timeline of significant milestones in atomic theory:

    • Democritus: 460 BC

    • Dalton: 1603 AD

    • Thomson: 1897

    • Rutherford: 1912

    • Bohr: 1913

    • Quantum Cloud Model: post 1930

ATOMIC MODELS

  • The atomic model evolved significantly over centuries, beginning in 400 BC:

    • Initially conceptualized similarly to a billiard ball.

Key Figures in Atomic Theory

  • Introduction to important scientists who shaped atomic understanding:

    • Their pursuit of knowledge greatly advanced the view of the universe.

Democritus

  • Greek philosopher who initiated the quest for matter's nature over 2400 years ago.

  • Proposed a fundamental question:

    • Could matter be indefinitely divided or was there a limit?

Democritus’ Theory

  • Matter cannot be endlessly divided; a smallest piece (indivisible) exists.

  • Named this smallest piece "atomos," meaning "not to be cut."

  • Proposed that atoms are small, hard particles made of the same material but of various shapes and sizes.

  • Stated that atoms are infinite, always in motion, and capable of bonding.

Influence of Philosophers

  • Democritus’ ideas were largely ignored for over 2000 years due to the dominance of Aristotle and Plato.

  • They favored a four-element theory: earth, fire, air, and water, leading to the neglect of the atomic theory.

Dalton’s Model

  • John Dalton’s early 1800s experiments paved the way for atomic acceptance.

Dalton’s Theory

  • All elements consist of atoms.

  • Atoms are indivisible and indestructible.

  • Atoms of the same element are identical, while different elements vary.

  • Compounds arise from the combination of atoms from two or more elements.

Thomson’s Plum Pudding Model

  • Joseph John Thomson's 1897 discovery revealed atoms consist of smaller particles.

Thomson Model Description

  • Proposed the "Plum Pudding" model:

    • Atoms contain a positively charged medium with scattered negatively charged electrons (raisins).

  • Introduced the concept of "corpuscles" (now known as electrons).

Rutherford’s Nuclear Model

  • Ernest Rutherford developed the nuclear model of the atom, identifying electron orbits around a nucleus.

Rutherford's Gold Foil Experiment

  • Conducted an experiment firing a stream of charged particles through thin gold foil:

    • Most passed through unimpeded, while some were deflected.

  • Concluded that atoms consist mostly of open space with a dense, positively charged nucleus that repels other positive charges.

  • Renamed the dense center to "nucleus."

Rutherford’s Findings

  • Positively charged particles (protons) reside in the nucleus.

  • Negatively charged particles exist outside the nucleus.

Bohr Model

  • Niels Bohr proposed an enhanced model in 1913.

Bohr Model Features

  • Electrons exist in specific energy levels or orbits around the nucleus, similar to planets around the sun.

  • Each orbit corresponds to certain distances from the nucleus.

Quantum Model

  • Proposed by Werner Heisenberg and Erwin Schrödinger.

Quantum Theory Overview

  • Electrons do not travel in fixed paths but occupy probabilistic locations based on energy levels.

  • The nucleus remains small and positively charged, amidst a larger electron cloud.

Key Characteristics of Quantum Model

  • Describes electron positions as shells or energy levels labeled s, p, d, and f.

  • Electrons whirl around the nucleus and their locations are energy-dependent.

Electron Cloud

  • A region where electrons are likely found.

  • Electrons are not randomly moving but confined energetically.

  • Electrons with lower energy occupy levels closer to the nucleus, while those with higher energy are found in outer levels.

Key Definitions

  • Indivisible: Refers to the concept introduced by Democritus.

  • Electron: Introduced by Thomson, now known as the negatively charged constituent of the atom.

  • Nucleus: Center of the atom, housing protons and neutrons.

  • Proton: Positively charged particle identified by Rutherford.

  • Neutron: Discovered by James Chadwick in 1932.