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 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 number and kinds of atomic orbitals depend on the energy sub level. The maximum number of electrons that can occupy a principle energy level is 2n 2. (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) = 3x10 8 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? 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 = h ν Atomic spectra are like fingerprints: no two are alike!