A New Atomic Model Section 4.1

Problems with Rutherford’s Model Did not explain where the electrons were arranged Did not explain why the negatively charged electrons did not get drawn into the positively charged nucleus

The Wave Description of Light Electromagnetic radiation: a form of energy that exhibits wavelike behavior as it travels through space Visible light is a form of electromagnetic radiation Electromagnetic spectrum: all the forms of electromagnetic radiation

Electromagnetic Spectrum

More Facts about Light Light travels at 3.00 x 108 m/s through a vacuum Wavelength: (λ) is the distance between corresponding points on adjacent waves Is a distance measurement (nm or m) Frequency: (ν) is the number of waves that pass a given point in a specific time Measured in waves/s which is a Hertz (Hz)

Wave (a) has a longer wavelength and a lower frequency than wave (b)

The Photoelectric Effect Photoelectric effect: the emission of electrons from a metal when light shines on the metal The frequency of the light striking the metal must be on a certain level or no electrons were emitted, even if the light was very bright

More about Light The wave theory of light predicted that light of any frequency would supply enough energy to eject an electron Max Planck, a German physicist, proposed that light must travel in small bundles of energy Albert Einstein expanded on this theory by introducing the dual wave-particle theory

The Wave- Particle Description of Light Quantum: (of energy) is the minimum quantity of energy that can be lost or gained by an atom Photon: a particle of electromagnetic radiation having zero mass and carrying a quantum of energy The energy of a particular photon depends on the frequency of the radiation

Einstein’s Explanation of the Photoelectric Effect Electromagnetic radiation is absorbed by matter only in whole numbers of photons There is a minimum amount of energy required to knock an electron loose If the photon’s frequency is below the min, then the electrons stay put

More Electrons in different metals require different minimum frequencies to exhibit the photoelectric effect

Ground State vs. Excited State Ground state: the lowest energy state of an atom Excited state: a state in which the atom has a higher potential energy than it has in the ground state Excited atoms will return to the ground state Only one ground state for an atom, many excited states

Emission-Line Spectrum When an atom returns to its ground state, it emits light Emission-line spectrum: a diagram or graph that indicates the degree to which a substance emits radiant energy with respect to wavelength

Hydrogen Atom Emission-Line Spectrum Investigators passed an electric current through H2 at low pressure There was a pinkish light emitted When the light was passed through a prism, it was separated into 4 specific colors of visible light This was the emission-line spectrum

Hydrogen Atom Emission-Line Spectrum

More about Hydrogen There were other series of lines in the uV and IR regions of hydrogen’s emission-line spectrum The series are called Lyman (UV), Balmer(visible), and Paschen (IR) after their discoverers

Hydrogen Atom Emission-Line Spectrum

Why did H2 only give off specific frequencies of light? Continuous spectrum: the emission of a continuous range of frequencies, like a rainbow Scientists had expected H2 to emit a continuous spectrum The energy differences between the atoms’ energy states must be fixed

The Bohr Model Niels Bohr was a Danish physicist He linked the atom’s electron to photon emission According to his model, the electrons can circle the nucleus only in allowed paths, or orbits The electron is in its lowest energy state when it is closest to the nucleus Hydrogen only has one electron

Bohr’s Model of the Atom

More The orbit is separated from the nucleus by an empty space where the electron cannot exist When the atom is in the excited state, the electron is in a higher orbit farther from the nucleus There are only certain orbits available, like the rungs of a ladder

When a hydrogen atom is in an excited state, the electron is in a higher orbit When the atom returns to the ground state, it emits energy in the form of a photon This is called emission Energy must be added to the atom to move an electron to a higher level This is called absorption

Visual of Absorption and Emission

More about Bohr’s Model Bohr related the possible energy-level changes to the lines in the hydrogen emission-line spectrum He calculated mathematically values for the lines Unfortunately, Bohr’s model did not explain the emission-line spectrum of atoms with more than one electron

Emission-Line Spectra The emission spectrum of each element is unique Emission spectra are used to identify unknown samples Also used to determine the relative composition of stars

Some Emission-Line Spectra