I. MODELS OF THE ATOM
A. The Development of Atomic Models
1. At Rutherford's time electrons were thought to be particles and light was thought to be a wave - these theories explained many properties.
2. In the early 1900s it was discovered that electrons have certain wave properties and light has certain particle-like properties.
3. Electrons and light both have dual wave-particle natures.
II. THE BOHR MODEL
A. Bohr Model of the Atom
1. In 1913, Niels Bohr, explained the hydrogen spectrum. His reasoning was this:
# 1: Energy is being added to the hydrogen gas in the form of electricity. # 2: Energy is leaving the tube in the form of light. # 3: The energy of the light leaving the tube is quantized - only certain frequencies of light are observed (the Balmer series). # 4: The hydrogen atoms in the tube must be absorbing energy, then releasing it in the form of specific frequencies of light. # 5: Therefore, the hydrogen atoms themselves must be quantized - that is, they exist only in certain, definite energy states (we call them energy levels). # 6: The atoms absorb specific amounts of energy and then exist for a short time in higher energy levels - these atoms are said to be excited. # 7: Excited hydrogen atoms will emit energy as they return to lower energy levels.
2. Viewing Energy Levels as a Staircase a. Consider a house cat sitting on the second step of a staircase. The cat's potential energy is considered to be small because it is close to the ground. If the cat climbs to a higher step, its potential energy increases. The energy change can be measured in specific values that correspond to the heights of the steps. If the cat moves from one step to a lower step, its energy decreases by a fixed amount.
b. The cat must always change positions by an integral number of levels - 1, 2, 3, etc. steps, but cannot move up or down by a half or a third of a step. Also, the change between two specific steps - for example, the second and fourth - always involves the same change in energy.
c. A hydrogen atom is made of a single electron-proton system. The electron is attracted to the proton. This attraction gives the electron-proton system potential energy. Bohr concluded that the greater the electron-proton distance, the greater the potential energy in the system, and the higher the energy level. (He incorrectly concluded that the electron in a given energy state keeps the same distance from the proton while moving around it in a circular path - the planetary model). He surmised that since the excited hydrogen atoms emit light energy of only specific frequencies, hydrogen atoms have specific energy levels and no others.
d. Unfortunately, while successful for the hydrogen atom, Bohr's model did not work for any atom with more than one electron.
B. The Quantum Mechanical Model of the Atom
1. The Schrodinger Wave Equation
a. In 1926, Erwin Schrodinger, devised an equation that treated electrons as waves - the Schrodinger Wave Equation (the Quantum Wave Equation).
b. This laid the foundation for the quantum theory, which mathematically describes the wave properties of electrons and other very small particles.
c. This equation works for all atoms (unlike Bohr’s equation).
2. In Schrodinger's model the nucleus is surrounded by orbitals, which, unlike Bohr's 2-dimensional orbits, are 3-dimensional regions in which a particular electron might be located.
a. Orbitals are clouds showing regions of probable electron location - part of orbital-cloud model (if electrons are waves, they cannot be precisely located).
D. Atomic Orbitals (Table 5.1, p.131)
1. Atomic orbital – the region of space in which there is a high probability of