Chapter 4 Electron Configurations

Early thoughts Much understanding of electron behavior comes from studies of how light interacts with matter. Early belief was that light traveled as a wave, but some experiments showed behavior to be as a stream of tiny fast moving particles

Waves Today scientists recognize light has properties of waves and particles Waves: light is electromagnetic radiation and travels in electromagnetic waves.

4 Characteristics of a wave: 1) amplitude - height of the wave. For light it is the brightness 2) Wavelength ( )– distance from crest to crest. For light – defines the type of light Visible light range – nm

Properties continued 3) Frequency ( )– measures how fast the wave oscillates up and down. It is measured in number per second. Hertz = 1 cycle per second FM radio = 93.1 MHz or 93.1 x 10 6 cycles per second Visible light = 4 x cycles per second to 7 x cycles per second 4) speed – 3.00 x 10 8 m/s

Short wavelength, high frequency Long wavelength, low frequency Visible Spectrum ROY G BIV Longer wavelength shorter wavelength

Electromagnetic spectrum (meters) gamma x-rays UV visible light infrared microwave 1 TV

Wavelength and frequency Wavelength and frequency are inversely related!! = c/ Where is the wavelength, c is the speed of light and is the frequency Speed of light = Constant = 3.00 x 10 8 m/sec

Example Example: An infrared light has a wavelength of 2.42 x m. Calculate the frequency of this light. = c/ = 3.0 x 10 8 m/sec = 2.42 x m = 1.2 x waves/sec

Wavelength and frequency ****Remember and are inverse. Therefore short wavelength = high frequency!!

Quantum Theory Needed to explain why certain elements when heated give off a characteristic light (certain color) Max Planck – idea of quantum

Quantum Theory - The amount of energy (electromagnetic radiation) an object absorbs/emits occurs only in fixed amounts called quanta (quantum) - Quanta – finite amount of energy that can be gained or lost by an atom.

1905 Einstein’s theory Einstein explained photoelectric effect by proposing that light consists of quanta of energy called PHOTONS Photon = discrete bit of energy Consider light traveling as photons

Energy equation Amount of energy of a photon described as E = h Where E = energy = frequency h = Planck’s constant = x J s Joule = SI unit for energy

Photoelectric effect – electrons are ejected from the surface of a metal when light shines on the metal. The frequency determines the amount of energy. The higher the frequency, the more energy per photon.

Photoelectric effect Ex. X-rays have a high frequency; therefore can damage organisms while radio waves have a low frequency.

Dual nature of radiant energy Photons act both like particles and waves. Line Spectra: A spectrum that contains only certain colors, or wavelengths

Question: How are electrons arranged in atoms? Note: All elements emit light when they are vaporized in an intense flame or when electricity is passed through their gaseous state.

How are electrons arranged in atoms? Explanation: Bohr atom: 1911 postulated that atoms have energy levels in which the electrons orbit energy levels and/or orbits are labeled by a quantum number, n. lowest energy level = ground state n=1

How are electrons arranged in atoms? when an electron absorbs energy, it jumps to a higher level (known as the excited state) n = 2,3 or 4 Bohr model of an atom Hydrogen Red- e falls from 3 to 2 Blue e falls from 4 to 2 Violet e falls from 6 to 2

1924 – Louis DeBroglie – If waves of light can act as a particle, then particles of matter should act like a wave. Found to be true.

DeBroglie Matter waves = wavelike behavior of particles. From equation – wave nature is inversely related to mass therefore we don’t notice wave nature of large objects. However, electrons have a small mass so they have a larger wave characteristic

Schroedinger’s wave equation predicted probability of finding an electron in the electron cloud around nucleus. Gave us four quantum numbers to describe the position.

Heisenberg’s Uncertainty Principle The position and momentum of a moving object cannot simultaneously be measured and known exactly. Cannot know where it is and where its going at the same time.

Quantum mechanical model of an atom – Treats the electrons as a wave that has quantized its energy Describes the probability that electrons will be found in certain locations around the nucleus.

Orbitals Orbitals – atomic orbital is a region around the nucleus of an atom where an electron with a given energy is likely to be found (high probability) Have characteristic shapes, sizes and energies. Four different kinds of orbitals s, p, d, f.

S and p orbitals

Orbitals and energy Principal Energy Level = quantum number n. Principal Quantum number Value of n = 1,2,3,4,5,6,7 Tells you the distance from the nucleus

sublevels Each level is divided into sublevels # of sublevels = value of n N=1 1 sublevel n=2 2 sublevels s,p,d or f shows the shape

Orbital shape Each sublevel has a certain number of orbitals which are directed three dimensionally S = 1 sphere P = 3 figure 8 (along the x,y or z axis) D = 5 figure 4-27 p. 145 F = 7

Each electron in an orbital will have a spin – 2 options clockwise, vs. counter clockwise. Electron configuration Pauli Exclusion Principle – each orbital in an atom can hold at most 2 electrons and their electrons must have opposite spin.

Aufbau Principle- electrons are added one at a time to the lowest energy orbitals available Hund’s Rule – electrons occupy equal energy orbitals so that the maximum number or unpaired electrons result. Occupy singly before pairing

Diagonal Rule: for order of sublevels: must remember 1s, 2s, 2p, 3s, 3p, 4s Exceptions to Aufbau Principle Cr Z=24 Cu Z = 29

Energy levels Number of electrons per energy level 1st = 2 2nd = 8 3rd = 18 4th = 32

Number of electrons per orbital 2 Difference between paired and unpaired electrons Paired = 2 electrons in the same orbital Unpaired = 1 electron in the orbital