Electromagnetic Radiation Ms. Marshall Chemistry WW-P

Properties of Radiation How does electromagnetic energy present itself? What parameters can be measured? How was quantum theory developed?

Einstein’s Photoelectric Effect Quantum Theory Einstein’s Photoelectric Effect What is the photoelectric effect? The frequency of radiation is what matters – not the intensity Nobel prize in 1919 Experimental support Planck’s quantum theory. Quantum is a “packet” or “bundle” of energy Wave-particle duality – radiation has properties of both particles and waves

Radiation Properties Radiation is classified as electromagnetic energy Electromagnetic radiation is energy traveling through space that has wave-like properties. Parameters: frequency (ν), wavelength (λ), amplitude, energy (E) What is the relationship between λ and ν? How are both variables related to E? All ER waves travel at the speed of light!! 2.998 x 108 m/s.

Why does this matter? Atomic structure was identified by looking at spectra because they are unique we can identify elements based on their spectra We use what we have learned about the atoms using their spectra to classify them in the Periodic Table

Electromagnetic Spectrum Source: Wilbraham, A.C., et. Al. Chemistry, 2005

Essential Question Why don’t atoms give off a continuous spectra?

Bohr’s Model of the Atom 1913 – Rutherford’s Nuclear Model of the atom was changed. New discoveries about how the energy of an atom changes when it absorbs or emits light were included Bohr’s model is based on Hydrogen atoms only Proposed that electrons travel only in specific circular paths, or orbits, around the nucleus. Worked well for Hydrogen, not for other atoms.

Bohr’s Model of the Atom Each possible e- orbit has a fixed energy called energy levels

Check for Understanding If an electron absorbs energy to change energy levels, how much energy will it absorb. Less than the amount of energy of the “new” energy level More energy than the amount of energy required for the “new” energy level Just enough energy to have the same energy as the “new” energy level An amount of energy equal to the energy of the new energy level

Bohr’s Model of the Atom The amount of energy an e- gains or loses will not always be the same. Energy levels are not equally spaced This model failed to explain the energies absorbed and emitted for atoms other than hydrogen.

The Quantum Mechanical Model Erwin Schrodinger Based on new theoretical calculations and experimental results Created and solved a mathematical equation describing the behavior of the e- in an H atom Quantum mechanical model The modern description, primarily mathematical, of the behavior of electrons in atoms comes from solutions to Schrodinger’s equation

Quantum Mechanical Model Differences from Bohr’s Model Not an exact path for the e- to travel around the nucleus Determines the allowed energies an e- can have and the probability of finding the e- in a location Described by probability (4 marble examples)

How do these pictures relate to the probability of an e- being in certain place? Think-Pair-Share (1.5 minutes)

What is Quantum Mechanics 1905 – Albert Einstein Explained experimental data that proposed that light could be described as quanta of energy. Photons – a quantum of light; 1924 – Louis de Broglie Light behaves as waves and particles, so can particles of matter behave as waves? 1927 – Clinton Davisson & Lester Germer @ Bell Labs in NJ (confirmed de Broglie’s hypothesis) Noticed that e- reflected from metal surfaces when bombarded with beams of e-. Odd patterns – like x-rays reflecting from metal surfaces.

What is quantum mechanics? Reflected as if they were waves. This property of particles is used to create clear, enlarged images of very small objects. Size of object needs to be very small to observe the wavelength.

What is quantum mechanics? Classical mechanics Describes the motions of objects much larger than atoms adequately Quantum mechanics Describes the motion of subatomic particles and atoms as waves

Heisenberg Uncertainty Principle Werner Heisenberg (German physicist) It is impossible to know exactly both the velocity and position of a particle at the same time. Does not apply to large objects Cars, airplanes

Atomic Orbitals Solving Schrodinger’s equation tells you the energies an e- can have. Also leads to a mathematical expression atomic orbital A region of space in which there is a high probability of finding an electron Determined by the solutions to Schrodinger’s equation.

Where are the electrons ? Ground state – most stable region closest to nucleus Excited state – when energy is applied, electron absorbs energy and jumps to a region farther away from the nucleus; the electron immediately returns to stable ground state and emits energy (in the form of light)

Atomic Orbitals Principal quantum numbers (n) Labels for energy levels n = 1, 2, 3, 4, … Each principal energy level can have a number of orbitals with different shapes at different energy levels. (sublevels) Key Point Each energy sublevel corresponds to an orbital of a different shape, which describes where the electron will most likely be found.

Sublevels Due to the spherical shape of the s orbital, the probability of finding an e- at a given distance does not depend on direction.

Sublevels

Sublevels

Electrons!!! If all atoms are composed of the same fundamental building blocks, how is it that different atoms have vastly different chemically behaviors?