Atomic Emission Spectrum & Bohr Model

Development of Atomic Models empty space Indivisible Identical React in fixed ratios + stuff – electrons +

Development of Atomic Models Rutherford’s Model: couldn’t explain some properties like light colors. Rutherford’s model fails to explain why objects change color when heated. As the temperature of this horseshoe is increased, it first appears black, then red, then yellow, and then white. The observed behavior could be explained only if the atoms in the iron gave off light in specific amounts of energy. A better atomic model was needed to explain this observation.

A hot, opaque object, like the filament in an incandescent light bulb, emits a continuous spectrum, having light of all wavelengths. emission spectrum has emission lines at specific wavelengths. These discrete lines are due to the quantum nature of the atom. Florescent lights, neon signs, and many streetlights are examples of emission spectrum sources.

Atomic Emission Spectra white light  continuous spectrum prism Demo Atomic Emission Spectra elements  discrete lines of E & f helium (He) lamp Demo – 4 lamps (show He, Ne, Ar, then H) prism (only specific colors of energy & frequency)

One element at a time, let’s energize it and view the AES (atomic emission spectra) emitted. Ar How many e-’s in He? (2) How many lines? (10) How? Why more lines in Ne and Ar than He? (more e-’s) H

Hydrogen Emission Spectrum A mystery for Niels Bohr.

Bohr’s Shell Model (1913–Niels Bohr) electrons occupy only specific levels (or shells) of “quantized” energy (& wavelength & frequency) Electrons as Waves quantized into specific multiples of wavelengths, but none in between.

e–’s absorb (+) energy, move to outer levels Bohr’s Shell Model e–’s emit (–) energy, move back to inner levels (n=5 to n=2) EXCITED state e–’s absorb (+) energy, move to outer levels (n=2 to n=5) GROUND state has least energy 5 4 3 2 ∆E 2 2 Which transition shows a light wave of the greatest energy? n=5 to n=2

Development of Atomic Models These illustrations show how the atomic model has changed as scientists learned more about the atom’s structure.

Quick Quiz! 1. Rutherford's model of the atom could NOT explain… any properties of elements. some properties like light color. the distribution of mass in an atom. the distribution of positive and negative charges in an atom.

Quick Quiz. 2. Bohr's model of the atom proposed that electrons are found… embedded in a sphere of positive charge. in fixed positions surrounding the nucleus. in fixed orbits of specific energy. orbiting the nucleus in a single fixed circular path.

Quick Quiz. 3. The lines in the emission spectrum for an element are caused by… the movement of electrons from lower up to higher energy levels. the movement of electrons from higher down to lower energy levels. the electrons in energy levels in the ground state. the electron locations of an atom.

n is energy level Quick Quiz. 4. Which transition in an excited hydrogen atom will emit the lowest energy of light? A. n=2 to n=4 B. n=4 to n=1 C. n=3 to n=2 D. n=3 to n=4 n is energy level longest wavelength