The Atom - Intro
Unit 2: Atoms
Infinitely Small
The discovery of atoms, among the smallest units of matter, posed challenges.
Historically, such breakthroughs often involved collaboration among numerous scientists.
Electrons – J.J. Thomson
Thomson's investigations centered on demonstrating the existence of electrons.
He utilized electric currents flowing from negative to positive, creating rays.
By manipulating these rays with external electric fields, he confirmed they contained negatively charged particles, named electrons.
Electrons – J.J. Thomson (Continued)
Confirmed that electrons are negatively charged.
Given that atoms are neutral, he theorized they consist of negative electrons distributed within a positively charged cloud, which maintains overall neutrality.
Electrons – Robert Millikan
While Thomson established the existence of negatively charged particles, Millikan sought to quantify them.
His famous oil drop experiment involved balancing the gravitational and electric forces on charged oil droplets to measure the electron's mass accurately.
Nucleus - Ernest Rutherford
Rutherford aimed to validate Thomson's atomic model through experimentation.
He directed alpha particles at a thin gold foil; predicted outcomes were based on Thomson's model—particles should pass through with minimal deflection.
Nucleus - Ernest Rutherford (Continued)
Contrary to expectations, many alpha particles were reflected, indicating a dense, positively charged nucleus rather than an entirely empty positive cloud.
The deflections demonstrated the presence of a central nucleus around which electrons orbit.
Nucleus - Ernest Rutherford (Continued)
To balance the positive charge of the nucleus, the existence of uncharged particles, called neutrons, was proposed by Rutherford and James Chadwick, leading to the classical atomic model.
Problem with Rutherford’s Model
The notion of electrons orbiting the nucleus like planets raised stability concerns.
Electrons moving in circular paths would lose energy due to acceleration and spiral inward ultimately.
Henri Becquerel - Radiation
Becquerel discovered that uranium salts emit radiation capable of exposing photographic plates without light.
This marked the discovery of particles capable of emitting radiation through various media.
Quantum Theory
Radiation discovery prompted explorations into matter and energy behaviors.
Visible light is but a tiny segment of the broader spectrum of electromagnetic radiation.
A significant debate persists regarding whether light behaves as waves or particles, categorized under the electromagnetic spectrum.
Electromagnetic Spectrum – James Maxwell
Maxwell suggested light comprises electromagnetic waves that form a continuous spectrum, though only a small part is visible to human observation.
Photoelectric Effect - Heinrich Hertz
Investigated whether light behaves as energy or matter.
Hertz exposed metal surfaces to different light frequencies, discovering that higher frequencies emit electrons, while lower frequencies do not.
Photoelectric Effect - Heinrich Hertz (Continued)
Findings indicate electron release depends on the light frequency rather than intensity; electrons respond only when frequency exceeds a certain threshold.
The relationship between frequency and wavelength noted.
Photoelectric Effect - Heinrich Hertz (Continued)
Different frequencies: Radio waves (low frequency) and gamma rays (high frequency).
Quantum Hypothesis – Max Planck
Planck postulated that matter exchanges energy in discrete amounts (quanta), not continuously.
Developed the equation ∆E = hf for calculating energy changes related to frequency.
Observations of radiation revealed that emitted energy occurs in bursts, termed "quanta."
Photons – Albert Einstein
Einstein proposed electromagnetic radiation consists of particles called photons, encapsulating units of light energy.
Photons interact with electrons—the energy transfer potentially releases an electron if sufficient.
Photons – Albert Einstein (Continued)
Electrons, held by electrostatic forces, are freed only if incoming photon energy exceeds a certain threshold.
This concept led to defining Threshold Frequency, indicating that mere exposure to multiple lower-energy photons won’t cause electron release.
Atomic Spectra – Fingerprinting the atom
Spectroscopy involves analyzing light patterns emitted by substances.
Hydrogen was among the first elements studied, showing that when energized, it releases a unique atomic spectrum, serving as its fingerprint.
Absorption and Emission
White light yields a continuous spectrum, while elements typically exist in ground states.
Energy absorption excites electrons; they revert to the ground state, emitting unique line spectra particular to each element.
Visualizing Emission of a Photon
Emission and absorption spectra can be observed via spectrophotometry, visualizing wavelengths of light that are lost or emitted by excited atoms.
Model of the Atom - Niels Bohr
Utilizing emission spectra, Bohr formulated the Quantum Model of the atom.
Suggested that electrons reside in fixed energy levels, jumping to higher states upon gaining energy before returning and emitting light.
Model of the Atom - Niels Bohr (Continued)
Electrons transition between levels release energy, observable as light; Bohr’s model primarily applies to hydrogen.
Further research was required to explain exceptions, particularly regarding transition metals.
Electron Waves - Louis de Broglie
De Broglie hypothesized that electrons exhibit wave properties, oscillating in standing waves instead of circular orbits.
Standing waves restrict energy levels to defined values, negating destructive interference.
Electron Waves - Louis de Broglie (Continued)
Conceptualized visuals of standing waves akin to guitar strings, where only fixed multiples of wavelengths exist.
Location of Electron - Erwin Schrodinger
Schrodinger devised the wave equation to ascertain electron energy levels, defining regions with high probabilities of electron presence, termed orbitals.
Orbitals exhibit varied shapes correlating with different electron energy levels.
Location of Electron - Werner Heisenberg
Developed the Uncertainty Principle: precise knowledge of an electron's position compromises the ability to judge its momentum, and vice versa.
Saying Goodbye to Bohr
Contrast between Bohr's orbits and contemporary understandings of electrons is made; orbitals represent probability regions rather than fixed paths.
Quantum Mechanics
Quantum mechanics quantifies the likelihood of electron presence (in orbitals) but does not describe motion.
Electron Probability Density Graphs depict 3D regions surrounding a nucleus, indicating probable locations, originating with the hydrogen atom.
Homework
Complete a chart summarizing scientists and their contributions.
Refer to pages 142 #5 and 152 #1,3 for additional assignments.