CHAPTER 4

Electrons in Atoms

To understand electrons, we must understand waves!• Electromagnetic radiation - Energy that travels through

space in waves.• Electromagnetic spectrum - All forms of electromagnetic

radiation.

• Wavelength - Distance between similar points on adjacent waves

• Frequency - # of waves that pass a fixed point per second

Frequency and Wavelength are inversely proportional to each other. Their product is the speed of light!

C = ln“C” = speed of light; 3.00 x 108 m/s“l” = wavelength; m“n” = frequency; Hz

Examples: What is the frequency of a wave whose wavelength is

4.55 x 10-12 m?

What is the wavelength of a wave whose frequency is 6.52 x 1017 Hz?

“Wave – Particle Duality”

Electrons are both a particle and a wave.

The Double-Slit Experiment

Observed:As electrons pass through 2 thin openings, they hit the screen not in 2 locations (as expected), but in 5 locations!

Conclusion: Electrons must travel in wave motion. As waves cross, an interference pattern is created which causes the 5 locations.

The Photoelectric Effect Observed:

When light is shined at a piece of metal, electrons are emitted from the surface of the metal – but only if the light has enough energy.

Conclusion:The light seen coming from the electrons is composed of photons (packets of energy) that each contain a quantum of wave energy (specific amount of wave energy).

The Energy of a photon is directly proportional to the frequency of the wave.

E = hn

“E” = energy; J“h” = Planck’s constant ; 6.63 x 10-34 Js“n” = frequency; Hz

Examples: What is the energy of a wave whose frequency is 9.25 x 1015 Hz?

What is the frequency of a wave whose energy is 8.34 x 10-14 J?

What is the energy of a wave whose wavelength is 3.21 x 10-19 m?

Energy and ElectronsElectrons exist in energy levels that surround the nucleus.

“Ground State ” = lowest energy level (E1)

“Excited State ” = higher energy levels (E2E4)

Electrons are restricted to only certain energies.Electrons can never be located between energy levels.

Electrons become excited by absorbing energy. (A & B)

Electrons return to a lower energy state by emitting energy in the form of electromagnetic radiation. (C, D, &E)

The Hydrogen AtomElectron transitions are quantized. They involve specific amounts of energy.

The energy absorbed or emitted by an electron is equal to the difference between energy levels.

The Balmer Series represents the transitions that are visible to the human eye.

When the light coming from an atom is split by a prism, a Bright-line spectrum (emission spectrum) is created.

Bright-line Spectrum – series of specific frequencies of visible light

Each line of color represents a photon of light emitted by the electrons.The black space between each line represents the absence of light.

In comparison, when sunlight is split by a prism a Continuous Spectrum is formed.

Continuous spectrum - a complete range of frequencies of visible light

Each color blends into the next. All frequencies of light are present. (None absent)

The Quantum Model of the Atom• Electrons are both particles and waves.• Electrons do not travel around the nucleus in defined orbits.• Heisenberg Uncertainty Principle : It is impossible to know

the exact location of an electron. analogy

• An electron’s “orbital” is its most probable location.• Shrodinger’s equation: • The most accurate description of an electron’s position is

provided by a set a quantum numbers.

Quantum Numbers• There are 4 quantum numbers : n, l, m and s• used to specify the properties of orbitals and the

electrons that occupy them.

n = Principal Quantum Number - Indicates the energy level occupied

by an electron

- Can range in value from 1-7.

- “n” increases as you move away from the nucleus.

l = Angular Momentum Quantum Number -also called “sublevel”-indicates the shape of the orbital-there are 4 shapes : s, p, d, and f

m = Magnetic Quantum Number - Indicates the 3-D orientation of an orbital around

the nucleus. (x, y, or z plane)- Each orbital shape has a specific number of

possible orientations.

s = 1 orientation p = 3 orientations

d = 5 orientations f = 7

orientations

s = Spin Quantum Number – Indicates the direction of spin for a single electron– 2 directions possible:• Clockwise : + ½ or ↑• Counterclockwise : - ½ or ↓

Electron Configurations Organization within the Electron Cloud

• 7 s ppp ddddd fffffff• 6 s ppp ddddd fffffff• 5 s ppp ddddd fffffff• 4 s ppp ddddd fffffff• 3 s ppp ddddd • 2 s ppp• 1 s

Each orbital can hold 2 electrons!!

Number = energy level (“n”)Letter = sublevel (“l”)Square = orbital (“m”)Arrow = electron (“s”)

An orbital diagram indicates where all of the electrons in an atom can be found.2s

6s

5s

4s

3s

7s

2p

6p

5p

4p

3p

7p

5d

4d

3d

6d

4f

5f

7d 7f

6f

1s

3 rules govern how electrons are organized:

1. Aufbau Principle: electrons occupy lower energy orbitals before occupying higher energy orbitals.

The “diagonal rule” to filling orbitals-

Start at 1s.Follow the arrows.When you get to the top

of an arrow, move to the bottom of the next .

2. Hund’s Rule – Within a sublevel, each orbital must contain one electron before pairing up.

3. Pauli Exclusion Principle – electrons in the same orbital must spin in opposite directions.

p sublevel

The orbital diagram for Lithium.

The electron configuration would be written:

The noble-gas notation would be written:

2s

6s

5s

4s

3s

7s

2p

6p

5p

4p

3p

7p

5d

4d

3d

6d

4f

5f

7d 7f

6f

1s

The orbital diagram for Sulfur.

The electron configuration would be written:

The noble-gas notation would be written:

2s

6s

5s

4s

3s

7s

2p

6p

5p

4p

3p

7p

5d

4d

3d

6d

4f

5f

7d 7f

6f

1s

The orbital diagram for Silver.

The electron configuration would be written:

The noble-gas notation would be written:

2s

6s

5s

4s

3s

7s

2p

6p

5p

4p

3p

7p

5d

4d

3d

6d

4f

5f

7d 7f

6f

1s

The orbital diagram for Tungsten.

The electron configuration would be written:

The noble-gas notation would be written:

2s

6s

5s

4s

3s

7s

2p

6p

5p

4p

3p

7p

5d

4d

3d

6d

4f

5f

7d 7f

6f

1s

The Periodic Table is an orbital diagram!