Ch. 4-1 Models of the Atom

POINT > Describe some basic properties of light POINT > Describe the emission-line spectrum of hydrogen POINT > Identify how Bohr explained electron orbitals POINT > Describe how Bohr’s atomic model explained the emission-line spectrum of hydrogen

Dalton

Thomson

Rutherford Problem Could not explain chemical properties of elements or locate electrons

WB CHECK: Who’s experiment demonstrated the existence of the atomic nucleus? Who’s experiment discovered the electron? Who’s experiment gave us the “plum pudding” model of the atom?

Visible light is a small part of the energy range of the electromagnetic spectrum (a continuous spectrum) Light has properties of both waves and particles

Waves have a frequency, v (cycles per second, or Hertz, Hz) Waves have a wavelength, λ

c = λ v c is a constant (the speed of light), so frequency and wavelength are inversely related Higher frequencies = higher energy

WB CHECK: As the wavelength of electromagnetic radiation decreases, what happens to the frequency? As the wavelength of electromagnetic radiation decreases, what happens to the energy level?

Which has more energy, red light or blue light?

Light Bulb Hydrogen Lamp Not continuous Continuous Spectrum

< Emission-line spectrum of H

Student of Rutherford Proposed electrons can only be found in specific paths (orbits) around the nucleus POINT > Identify how Bohr explained electron orbitals

Niels Bohr used the emission spectrum of hydrogen to develop a quantum model for H A quantum is a discrete, minimum amount of energy that an electron can absorb or emit Central idea: Electrons can circle the nucleus only in certain, defined energy levels (orbitals)

Each electron has a fixed energy called an energy level (like a rung on a ladder) To move between levels, a quantum of energy must be absorbed or emitted The further from the nucleus, the higher the energy level (usually)

POINT > Describe how Bohr’s atomic model explained the emission-line spectrum of hydrogen

WB CHECK: The emission-line spectrum of hydrogen is discontinuous because a)Electrons and protons have opposite charges b)Electrons can absorb or emit a continuous range of energies c)Electrons are fixed in orbitals that cannot change d)Electrons can absorb or emit only discrete amounts of energy

WB CHECK: As electrons move farther from the nucleus, do they have more or less energy? What must happen for an electron to move from one orbital to another?

Bohr’s model (1913) Electrons orbit the nucleus in circular paths of fixed energy (energy levels)

Again, a quantum is the amount of energy required tomove an electron from one energy level to another Electrons absorb energy to go to an excited state Electrons emit energy to return to the ground state

Bohr Problem: This explained hydrogen, but was inaccurate with more than one electron

Homework: Read 4.1 pages F.A. page 97 #1-6