Question

Chlorine has two natural isotopes, chlorine-35 and chlorine-37.

Describe the effect these isotopes have on the relative atomic mass of chlorine

Answer 1

The relative atomic mass of chlorine depends on the ratio between the abundance of these two naturally-occurring isotopes.

Step-by-step explanation:

The relative atomic mass of an element is a weighted average of the atomic mass of its naturally-occurring isotopes. The relative abundance of each isotope gives its weight in this weighted average.

For these two naturally-occurring isotopes of chlorine:

• The relative atomic mass of
  `^(35){\rm Cl}` is approximately
  `34.969` daltons. The relative abundance of this isotope in nature is approximately
  `0.758`.
• The relative atomic mass
  `^(37){\rm Cl}` is approximately
  `36.966` daltons. The relative abundance of this isotope in nature is approximately
  `0.242`.

`\begin{array}\cline{1-3} \text{Isotope} & \text{Atomic Mass} & \text{Relative Abundance}\\ \cline{1-3} ^(35){\rm Cl} & \approx 34.968\; \rm Da} & \approx 0.758 \\ \cline{1-3} ^(37){\rm Cl} & \approx 36.966\; \rm Da & \approx 0.242 \\ \cline{1-3}\end{array}`.

`\begin{aligned}&\text{relative atomic mass of Cl} \\ &= \text{atomic mass of $^(35){\rm Cl}$} * \text{relative abundance of $^(35){\rm Cl}$} \\&\quad + \text{atomic mass of $^(37){\rm Cl}$} * \text{relative abundance of $^(37){\rm Cl}$} \\ &\approx 34.968\; \rm Da * 0.758 + 36.966\; \rm Da * 0.242 \\ &\approx 35.45\; \rm Da \end{aligned}`.

The relative abundance of
`^(35){\rm Cl}` is much higher than that of
`^(37){\rm Cl}`. Consequently, the relative atomic mass of the element
`\rm Cl` is closer to the atomic mass of
`^(35){\rm Cl}\!` than that of
`^(37){\rm Cl}\!`.