Section 5-2 Quantum Theory and the Atom. Objectives Compare the Bohr and quantum mechanical models...

Section 5-2

Quantum Theory and the Atom

Objectives• Compare the Bohr and quantum

mechanical models of the atom• Explain the impact of de Broglie’s

wave-particle duality and the Heisenberg uncertainty principle on the modern view of electrons in atoms

• Identify the relationships among a hydrogen atom’s energy levels, sublevels, and atomic orbitals

Bohr Model of the Atom

• In 1913 Niels Bohr proposed quantum model for the H atom

•Based on Planck and Einstein’s ideas of quantitized energy

• Bohr proposed H atom has only certain allowable energy states

•Lowest state= ground state•Gaining energy = excited state

Bohr Model of the Atom

• Electrons move in certain, specific, circular orbitals

• Smaller the orbit = lower the energy level

• Assigned the allowable electron orbitals the principle quantum number, n.

•1st orbit= lowest energy: n=1 •2nd orbit= 2nd lowest energy: n=2

Bohr Model of the Atom

• Energy is added to an atomelectron moves to higher energy level in an “excited state”

• Electron in “excited state” drops to a lower energy orbit emits a photon

E = E higher-energy orbit – E lower-energy orbit= E photon= h

Bohr Model of the Atom

• Problems with Bohr’s model– Only explained the atomic emission

spectra of H– Did not explain why electrons

should only be allowed certain, specific energy levels

Quantum Mechanical Model of the Atom

• In 1924 Louis de Broglie proposed that electrons, like light also had a particle-wave dual nature.

• de Broglie noticed only multiples of half wavelengths are allowed in circular orbits

1 half-wavelength

2 half-wavelengths

3 half-wavelengths

Quantum Mechanical Model of the Atom

• de Broglie formulated an equation for the wavelength, mass, and velocity of a particle

• Moving particles’ wave characteristics decrease as mass increases.

mv

h

Quantum Mechanical Model of the Atom

• Broglie’s revelation still could not accurately model electron behavior of elements more complex than Hydrogen.

Heisenberg Principle Uncertainty

– Fundamentally impossible to know precisely both the velocity AND position of a particle at the same time.

–Cannot measure an object without disturbing it

Schrödinger Wave Equation

• In 1926 Erwin Schrödinger created the quantum mechanical model.– Limited electrons to only certain energy

levels – Atomic orbital: 3 dimensional area around

the nucleus that predicts the 90 % PROBABLE location of an electron

Electron Density Diagram

Quantum Mechanical Model

• Assigns principal quantum numbers (n) relative to sizes and energies of orbitals

• (n) specifies atom’s major energy levels= principle energy levels

• Lowest level= ground state= n= 1•H has 7 energy levels, n= 1 to 7

Quantum Mechanical Model

• Principal energy levels contain energy sublevels

•Principal energy level 1 has 1 sublevel

•Principal energy level 2 has 2 sublevels

•Principal energy level 3 has 3 sublevels

Look at Figure 5-14 (p. 132)

Energy Sublevels

• s, p, d, and f• Labeled according to shapes of

orbitals•s = spherical•p = dumbbell•d and f = not all have same shape

s and p orbitals

Three p orbitals

d orbitals

Energy Sublevels• Each orbital contains 2 electron at most

• Principal energy level 1 has 1 sublevel: 1s orbital• Principal energy level 2 has 2 sublevels: 2s and 2p

• 2p sublevel has 3 dumbbell-shaped p orbitals (2px, 2py, and 2pz)

• Principal energy level 3 has 3 sublevels: 3s, 3p, and 3d• d sublevels have 5 orbitals

• Principal energy level 4 has 4 sublevels: 4s, 4p, 4d, and 4f

• f sublevels have 7 orbitals

Look at Table 5-2 (p. 134)

Assignment• P.134 #13-17

– Make sure to answer ALL parts of the question and use complete sentences!!