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### Chapter 5

Electrons in Atoms

The Bohr Model

- An electron is found only in specific circular paths, or orbits, around the nucleus.
- Each orbit has a fixed energy. The orbits are called ‘energy levels.’

Energy Levels

- Energy levels are like
- the rungs of a ladder:
- You can move up or down by going from rung to rung.
- You can’t stand in-between rungs.

- For an electron to change energy levels it must gain or lose exactly the right amount of energy.

A Quantum

- A quantum of energy is the amount needed to move an electron from one energy level to another.
- The energy of an electron is said to be “quantized.”
- Energy levels in an atom are not all equally spaced.

An Airplane Propeller

- The blurry picture of an airplane propeller represents the area where the actual propeller blade can be found.

- Similarly, the electron cloud of an atom represents the locations where an electron is likely to be found.

The Model Quantum Mechanical

- Comes from the mathematical solution to the Schrodinger equation.
- Determines allowed energies an electron can have & how likely it is to find the electron in various locations around the nucleus.
- Uses probability

Atomic Orbitals

- A region in space in which there is a high probability of finding an electron.
- Energy levels of electrons are labeled by principal quantum numbers (n)
n = 1, 2, 3, 4 …

s Orbitals

are spherical

p Orbitals

are dumbbell- shaped

d Orbitals

4 out of the 5 d orbitals have clover leaf shapes

f Orbitals

are more complicated

Atomic Orbitals

The number and kinds of atomic orbitals depend on the energy sub level.

- N=1 has 1 sublevel called 1s
- N=2 has 2 sublevels called 2s and 2p
- N=3 has 3 sublevels called 3s, 3p, and 3d
- N=4 has 4 sublevels 4s, 4p, 4d, and 4f

The maximum number of electrons that can occupy a principle energy level is 2n2.

(n=principle quantum #)

Electron Configurations

- Electrons in an atom try to make the most stable arrangement possible (lowest energy)
- The Aufbau Principle, the Pauli Exclusion Principle, and Hund’s Rule are guidelines that govern electron configurations in atoms

Aufbau Principle

- Electrons occupy the orbitals of lowest energy first

Pauli Exclusion Principle

- An orbital can hold at most 2 electrons
- Does it make sense that two negatively charged particles will ‘want’ to share the same space?
- This phenomenon is made possible because electrons possess a quantum mechanical property called spin

Electron Spin

- Spin may be thought of as clockwise or counter-clockwise
- An arrow indicates an electron and its direction of spin
- An orbital containing paired electrons is written

Hund’s Rule

- When filling orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with similar spin

Hund’s Rule

- How would you put 2 electrons into a p sublevel?
- How would you put 7 electrons into a d sublevel?

Light

- Now that we understand how electrons are arranged in atoms, we can begin to look at how the frequencies of emitted light are related to changes in electron energies

Light

- Light waves properties:
- Amplitude – the wave’s height from zero to crest
- Wavelength – the distance between crests
- Frequency – the number of wave cycles to pass a given point per unit of time (Usually Hz = 1/s)

Light

- Wavelength has the symbol (λ) lambda.
- Frequency has the symbol (ν) nu.
- The speed of light is a constant (c) = 3x108 m/s
- c = λν

Light

- How are wavelength and frequency related?
- They are inversely related. As one increases, the other decreases
- How long are the wavelengths that correspond to visible light?
- 700-380 nanometers

Electromagnetic Spectrum

- Visible light is only a tiny portion of the electromagnetic spectrum which also includes radio waves, microwaves, infrared, visible light, ultra violet, X-rays, and gamma rays.
- If the entire electromagnetic spectrum was a strip of professional 16 mm movie film stretching from Los Angeles to Seattle, the portion of visible light would be only ONE frame of film.

Atomic Spectra

- When atoms absorb energy, electrons move to higher energy levels
- Electrons then lose energy by emitting light as they return to lower energy levels
- Atoms emit only specific frequencies of light that correspond to the energy levels in the atom
- The frequencies of light emitted by an element separate into discrete lines to give the atomic emission spectrum of the element

Atomic Spectra

- An electron with its lowest possible energy is in its ground state
- The light emitted by an electron is directly proportional to the energy change of the electron.
- E = hν
- Atomic spectra are like fingerprints: no two are alike!

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