Chapter 5 Electrons in Atoms

Section 5.2 Quantum Theory and the Atom

Objectives•1. Compare the Bohr and quantum mechanical

models of the atom.

•2. Explain the impact of de Broglie's wave particle duality and the Heisenberg uncertainty principle on the current view of electrons in atoms.

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

Atomic emission spectrum

As you recall from the section you’ve just read

-Hydrogen emission spectra is discontinuous ( made up of select frequencies of light)

But why are elements atomic emission spectra discontinuous rather than continuous ?

A young physicist named Nicholas Bohr proposed a quantum model for hydrogen that seemed to answer this question

I. Bohr's Model of the Atom

•Bohr correctly predicted the frequency lines in hydrogen’s atomic emission spectrum.

•Building off Planck’s and Einstein's concepts of quantized energy( only certain values are allowed)

•He proposed that the hydrogen atom only has certain allowable energy states

•The lowest allowable energy state of an atom is called its ground state.

•When an atom gains energy, it is referred to as being in an an excited state.

I. Bohr's Model of the Atom (continued)

•Bohr suggested that an electron moves around the nucleus only in certain allowed circular orbits.

•The smaller the orbit , the lower the atom’s energy level and vice versa

I. Bohr's Model of the Atom (continued)

•In his model:

•Each orbit was given a number, called the quantum number( n).

•For the first orbit n=1 and the second n =2 and so on …

I. Bohr's Model of the Atom (continued)

•Hydrogen’s single electron is in the n = 1 orbit in the ground state (lowest allowable energy)

• no energy is radiated at ground state.

•When energy is added ( from an outside source) , the electron moves to the n = 2 orbit (the first excited state).

•As “n” increases: energy levels become more spacious

• When an atom is in an excite state it can drop from a higher energy orbit to a lower one – thus resulting in the release of a photon

I. Bohr's Model of the Atom (continued)

I. Bohr's Model of the Atom (continued)

The four electron transitions account for visible line in hydrogen's atomic emission spectra: for example an electron dropping from orbit number 3 to orbit number 2 produces the red line

These transitions account for all of hydrogen's visible lines. Or electron transition are not visible but fall in UV or infrared range

Problem with Bohr's Model of the Atom

•Bohr’s model explained the hydrogen’s spectral lines, but failed to explain any other element’s lines.

•The behavior of electrons is still not fully understood, but it is known they do not move around the nucleus in circular orbits.

The Quantum Mechanical Model of the Atom

•Louis de Broglie (1892–1987) hypothesized that particles, including electrons, could also have wavelike behaviors.

•He questioned since wave can have particle like behavior can it be thought of n the reverse?

II. The Quantum Mechanical Model of the Atom (cont.)

•The de Broglie equation predicts that all moving particles have wave characteristics.

λ represents wavelengthsh is Planck's constant.m represents mass of the particle.ν represents frequency.

II. The Quantum Mechanical Model of the Atom (cont.)

•Heisenberg (a German physicist) showed it is impossible to take any measurement of an object without disturbing it. (hence like trying to find a balloon in a dark room ex pg. 131)

•The Heisenberg uncertainty principle states that it is fundamentally impossible to know precisely both the velocity and position of a particle at the same time.

•The only quantity that can be known is the probability for an electron to occupy a certain region around the nucleus.

II. The Quantum Mechanical Model of the Atom (cont.)

•Erwin Schrödinger an Austrian physicist furthered de Broglie’s theory

• In his theory he treated electrons as waves in a model called the quantum mechanical model of the atom.

•Schrödinger’s equation was the first which applied equally well to elements other than hydrogen.

• Unlike Bohr’s model , this one even makes an attempt to describe

• and electron’s path around the nucleus

• A three dimensional region around the nucleus call an atomic orbital describes the elctron’s probable location

• Describe loosely before with electron cloud model in which the electron will spend about 90% of its time near the positively charged nucleus

II. The Quantum Mechanical Model of the Atom (cont.)

•The wave function predicts a three-dimensional region around the nucleus called the atomic orbital.

III. Hydrogen Atomic Orbitals

•Principal quantum number (n) indicates the relative size and energy of atomic orbitals.

•n specifies the atom’s major energy levels, called the principal energy levels.

• Similar to Bohr’s equation

III. Hydrogen Atomic Orbitals

• “ n” indicates relative size and energies of atomic orbitals

• As n increase the orbitals become larger , electrons spend more tome from the nucleus and energy increases.

• Read paragraph on principal energy levels and subshells on page 133

• and look at table on following page

III. Hydrogen Atomic Orbitals (continued)

•Each energy sublevel relates to orbitals of different shape.

Models displaying the location of electrons in oxygen (continued)