1 Objectives Waves and Electromagnetic Spectrum Emission Spectrum Bohr’s Model and Electron Cloud Model Valence Electrons and Lewis Dot Diagrams Section 2.2 (p. 69) Electrons in Atoms

2 H Modern atomic models grew out of the study of light. H Light consists of electromagnetic waves, (with electric and magnetic properties). Helectromagnetic waves travel through empty space. (see picture) H Waves transfer energy Electrons are most important for reactivity of atoms because… When two atoms come near each other, their outermost electrons will interact.

3 simulation 13 HWavelength ( [lambda] ) is the distance between the crests. HFrequency ( [nu] ) is the number of wave cycles per unit of time (usually per second, s -1 = Hertz (Hz)) HFind crest, amplitude, origin, etc. in the diagram

4 Electromagnetic Spectrum Electromagnetic radiation includes gamma rays, x-rays, ultraviolet light (UV), visible light, infrared (IR), microwaves, and radio waves. They all travel at the speed of light of 3.0 x 10 8 m/s. We only see visible light. High energy High Frequency Short wavelength Low energy Low Frequency Long wavelength

5 Emission spectrum animation 16 An emission spectrum shows only the frequencies of light that are emitted (in those colors), the rest is black.

6 The emission spectrum is unique for each element. We know the atmosphere of the sun from measuring the emission spectrum Noble gases when energized give of colored light: helium pink, argon lavender, krypton whitish, xenon blue and neon orange

7 He explained simple emission spectra: After first absorbing energy, the electrons “jump” to a higher energy level (orbit further outside). The electron jumps back soon after, releasing the same amount of energy in form of light. Neils Bohr (1885 – 1962) suggested that electrons orbit around the nucleus at specific distances, which relates to the electron’s energy (energy level). The farther out, the higher the energy. Without this motion and energy, electrons would be drawn into the positive nucleus

8 compare to climbing up and down a ladder: you must step on a rung. You can’t step between the rungs.

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10 a) Thomson b) Rutherford c) Bohr d) Schrödinger Cookie dough nucleus electrons in Electron clouds model orbits Scattered scattered with distinct learn more electrons electrons energies later no nucleus Summary of the Historical Development of the Modern Atom Theory

11 Bohr’s Model is limited Bohr could not explain 1.more complicated emission spectra 2.how atoms form bonds Electron clouds Continuous research later revealed that energy levels are not exact planet-like orbits (Bohr), but spherical regions around the nucleus, = Electron clouds Schrodinger and others

12 Probability (Electron cloud)  Electron clouds are 3-dimensional areas of probability to find the fast moving electron. (Think of moving blades of a fan)

13 The ways in which electrons are arranged around the nuclei of atoms are called electron configurations. The maximum number of electrons that can occupy one energy level is given by the formula 2n 2 (n = energy level). Electron clouds higher energy level (n) = higher energy = farther from the nucleus = larger energy level (n) max. # of e - n=12 n=28 n=318 n=432 2n 2

14 Valence Electrons  are the electrons in the outermost occupied energy level.  The number of valence electrons for all A-groups in the periodic table is equal the group number (1A = 1, 2A = 2, etc.)  Valence electrons are the ones interacting with other atoms, thus being responsible for many properties and the chemical reactivity of the atoms.  The maximum number of valence electrons is 8.

15 show valence electrons as dots around the four sides of the element’s symbol. Lewis Electron Dot Structures Your turn: Cl H Al Ca Xe P.

16 Question 1 How does the atomic number of an element differ from its mass number? Answer The atomic number of an element is the number of protons in the nucleus. The mass number is the sum of the number of protons and neutrons.

17 A. Chlorine Write a Lewis dot diagram for each of the following. Question 2 C. Potassium B. Calcium

18 A. Chlorine Answer C. Potassium B. Calcium

19 A. Ultraviolet light Give an example for each type of electromagnetic energy listed below. Question 3 C. Visible light B. Infrared light

20 Sample answers: A. ultraviolet light: Answer the spectrum of light we see as color C. visible light: B. infrared light: part of sunlight radiant heat