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Question

# Chlorine has an average atomic mass of 35.45 amu. The two naturally occurring isotopes of chlorine are chlorine-35 and chlorine-37. Why does this indicate that most chlorine atoms contain 18 neutrons?

Solution

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Since the atomic number Z of Chlorine is 17 and the mass number A is 35.45, we can calculate the number of neutrons N, as the difference between these 2:

\begin{aligned} \ N &= A - Z \\ \ &= 35.15 - 17 \\ \ &= 18.15 \\ \ &\approx 18 \\ \end{aligned}

Therefore the number of neutrons is:

$\boxed{\therefore N = 18}$

Since we used the average atomic mass to calculate the number of neutrons, this is why we can conclude that most Chlorine atoms contain 18 neutrons.

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