Electromagnetic Radiation

1. Chapter 9: Electrons in atoms and the periodic table Electromagnetic Radiation • light is one of the forms of energy • technically, light is one type of a more general form of energy called electromagnetic radiation • electromagnetic radiation travels in waves • every wave has four characteristics that determine its properties – wave speed, height (amplitude), length, and the number of wave peaks that pass in a given time

2. velocity = c = speed of light • its constant! = 2.997925 x 108 m/s (m•sec-1) in vacuum • all types of light energy travel at the same speed • amplitude = A = measure of the intensity of the wave, “brightness” • height of the wave • wavelength = λ = distance between crests • generally measured in nanometers (1 nm = 10-9 m) • same distance for troughs or nodes • frequency = ν = how many peaks pass a point in a second • generally measured in Hertz (Hz), • 1 Hz = 1 wave/sec = 1 sec-1

3. scientists in the early 20th century showed that electromagnetic radiation was composed of particles we call photons • Max Planck and Albert Einstein • photons are particles of light energy • each wavelength of light has photons that have a different amount of energy and different colors • the longer the wavelength, the lower the energy of the photons

4. Electromagnetic Spectrum

5. Light’s Relationship to Matter • Atoms can acquire extra energy, but they must eventually release it • When atoms emit energy, it always is released in the form of light • However, atoms don’t emit all colors, only very specific wavelengths • in fact, the spectrum of wavelengths can be used to identify the element

7. 1922, Bohr: The orbits for the hydrogen atom are quantized n = principal quantum number Excited State ( n>1 ) Ground State ( n=1 )

8. Ground and Excited States • in the Bohr Model of hydrogen, the lowest amount of energy hydrogen’s one electron can have corresponds to being in the n = 1 orbit – we call this its ground state • when the atom gains energy, the electron leaps to a higher energy orbit – we call this an excited state • the atom is less stable in an excited state, and so it will release the extra energy to return to the ground state- either all at once or in several steps

9. The Quantum-Mechanical Model of the Atom Erwin Schrödinger applied the mathematics of probability and the ideas of quantitization to the physics equations that describe waves – resulting in an equation that predicts the probability of finding an electron with a particular amount of energy at a particular location in the atom. These regions of probability are called orbitals instead of orbits.

11. s orbital shape p orbital shape < d orbital shape

12. Electron Configuration • At ground state, the electrons fill the lowest-energy orbitals available • No orbital can have more than 2 electrons 1s 1 2 2p 2p 2p 2s 3 3d 3p 3d 3d 3d 3d 3p 3p 3s 4d 4p 4d 4d 4d 4d 4p 4p 4 4s 4f x 7

13. The electron configuration is a listing of the subshells in order of filling with the number of electrons in that subshell written as a superscriptKr = 36 electrons = 1s22s22p63s23p64s23d104p6 • A shorthand way of writing an electron configuration is to use the symbol of the previous noble gas in [] to represent all the inner electrons, then just write the last setRb = 37 electrons = 1s22s22p63s23p64s23d104p65s1= [Kr]5s1

14. Valence Electrons • the electrons in all the subshells with the highest principal energy shell are called the valence electrons • electrons in lower energy shells are called core electrons • chemists have observed that one of the most important factors in the way an atom behaves, both chemically and physically, is the number of valence electrons

15. Valence Electrons - Example Rb = 37 electrons= 1s22s22p63s23p64s23d104p65s1 • the highest principal energy shell of Rb that contains electrons is the 5th, therefore Rb has 1 valence electron and 36 core electrons Kr = 36 electrons= 1s22s22p63s23p64s23d104p6 • the highest principal energy shell of Kr that contains electrons is the 4th, therefore Kr has 8 valence electrons and 28 core electrons

16. Electrons Configurations andthe Periodic Table

18. The Noble Gas Electron Configuration • the noble gases have 8 valence electrons • except for He, which has only 2 electrons • we know the noble gases are especially nonreactive • He and Ne are practically inert • the reason the noble gases are so nonreactive is that the electron configuration of the noble gases is especially stable

19. Everyone Wants to Be Like a Noble Gas! The Alkali Metals • the alkali metals have one more electron than the previous noble gas • in their reactions, the alkali metals tend to lose their extra electron, resulting in the same electron configuration as a noble gas • forming a cation with a 1+ charge

20. Everyone Wants to Be Like a Noble Gas!The Halogens • the electron configurations of the halogens all have one fewer electron than the next noble gas • in their reactions with metals, the halogens tend to gain an electron and attain the electron configuration of the next noble gas • forming an anion with charge 1- • in their reactions with nonmetals they tend to share electrons with the other nonmetal so that each attains the electron configuration of a noble gas

22. Trends in Ionization Energy