# Chapter 7 - Electronic Structure - Questions and Exercises - Exercises - Page 285: 7.27

$1.4\times10^{7}s^{-1}$

#### Work Step by Step

λ = 21 m c = $2.997925\times10^{8} ms^{-1}$ ν = c/λ = $\frac{2.997925\times10^{8} ms^{-1}}{21 m}$ =$1.4\times10^{7} s^{-1}$

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