Development of a New Atomic Model Properties of Light

Before 1900, scientists thought light behaved solely as a wave. We know know that light has a dual nature It behaves as a wave AND as a particle

Wave Properties of Light Light is energy! A form of energy that exhibits wavelike behavior as it travels through space is known as electromagnetic radiation Forms of electromagnetic radiation: Gamma rays, X rays, ultraviolet, visible, Infrared, microwaves, and radio waves

Electromagnetic Spectum

Wave Properties of Light All forms of light move at a constant speed speed of light = c c = 3.00 x 10 8 m/s or 3.00 x cm/s

Wave Properties of light The significant feature of wave motion is its repetitive nature Measurable properties Wavelength – ( ) the distance between corresponding points on adjacent waves Units: meter or some fraction of a meter such as nanometer or centimeter,

Wave Properties of Light Frequency-  ) the number of wave cycles that pass a given point in a given amount of time, usually one second. Unit: waves/second One wave/second is called a Hertz (Hz) 1/sec

Wave Properties of Light Wavelength and Frequency are mathematically related to each other c = wavelength x frequency c =  c =   c /  c /  c 

Wave Properties of Light Long Low  Low Energy Short High High Energy

The Photoelectric Effect Two experiments involving the interaction between light and matter that could not be explained by the wave theory of light. Photoelectric effect refers to the emission of electrons from a metal when light shines on the metal. Only light of a specific frequency could knock loose electrons from a metal.

The Particle Description of Light Max Plank studied the emission of light by hot objects Proposed that hot objects do not emit electromagnetic radiation continuously as would be expected if the energy emitted were in waves

The Particle Description of Light Objects emit energy in small, discrete packets of energy called quanta A quantum of energy is the minimum quantity of energy that that can be lost or gained by an atom.

The Particle Description of Light Energy and frequency are mathematically related Energy = planck’s constant x frequency E = h E = h h = x J.s

The Particle Description of Light 1905 Albert Einstein used Planck’s work to describe how light has a dual-nature [particle and wave] Light can be thought of as a stream of particles that carry a quantum of energy Einstein called these particles photons

The Particle Description of Light A photon is a particle of electromagnetic radiation having zero mass and carrying a quantum of energy. E photon = h E photon = h

The Particle Description of Light Electromagnetic radiation is absorbed by matter only in whole numbers of photons. In order for an electron to be ejected from the metal surface, it must be struck by a single photon possessing the minimum amount of energy required to knock the electron loose. This minimum energy relates to the minimum frequency.

Photoelectric Effect Photoelectric Effect Explained Photoelectric Effect Explained

The Hydrogen-Atom Line- Emission Spectrum Classical theory predicted that the hydrogen atoms would be excited by any amount of energy Expected the emission of a continuous range of frequencies of electromagnetic radiation, a rainbow of colors. Observations led to a new atomic theory: Quantum theory Bohr's bright-line spectra Bohr's bright-line spectra

Bohr Model of the Atom An electron will absorb a specific amount of energy and move from the ground state to the excited state. When it falls back down to the ground state it releases a photon of light equal to the energy difference between the energy levels.

Bohr Model of the Atom The greater the energy difference the higher the frequency of light being emitted. Ex: n = 6 - n = 2 emits a violet light because of the large energy difference n = 3 - n = 2 has a small energy difference thus a red light is emitted.