Answer
5 significant figures
Work Step by Step
For a neutral hydrogen atom \,^{1}\text{H} , the mass is mainly determined by the proton and the electron. The mass of a proton is significantly larger than the mass of an electron, so the difference between the mass of \,^{1}\text{H} and \,^{1}\text{H}^+ (the hydrogen cation) is only the mass of the missing electron. Relevant Data: • Mass of \,^{1}\text{H} atom: 1.007825 amu (from Table 2.1 of most periodic tables). • Mass of an electron: 9.10938356 \times 10^{-31} kg or approximately 0.0005486 amu. Thus, the mass of \,^{1}\text{H}^+ is the mass of a proton, because the electron has been removed. Calculation of the mass difference: \text{Mass difference} = \text{Mass of } \,^{1}\text{H} - \text{Mass of } \,^{1}\text{H}^+ = 1.007825 \, \text{amu} - (1.007825 \, \text{amu} - 0.0005486 \, \text{amu}) = 0.0005486 \, \text{amu} Now, we determine how many significant figures are required for the mass of \,^{1}\text{H} before this difference (the mass of the electron) becomes significant. We need enough significant figures so that this difference (0.0005486 amu) is visible. Thus, the number of significant figures required in the mass of \,^{1}\text{H} is 5 because the electron mass becomes significant starting from the fifth decimal place.
