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MASSES OF LIGHT ATOMS
Recently Aston[^1] has again determined the masses of light atoms with the aid of a new mass spectrograph, attaining in his measurements (by the doublet method) appreciably greater accuracy than had previously been possible. Similar measurements were carried out for \(^1\mathrm{H}\), \(^2\mathrm{D}\), \(^4\mathrm{He}\), \(^ {12}\mathrm{C}\), \(^ {14}\mathrm{N}\). On the basis of these data Oliphant[^2] calculated the masses of other light atoms, proceeding from known nuclear reactions whose energy balance is now known with great accuracy. The values obtained by Oliphant are given in the second column of the table. The latest elements are radioactive; their masses are known only approximately. Asterisks mark Aston’s data, on the basis of which Oliphant made his calculations. The last column of the table contains Aston’s latest data[^3], published after the appearance of Oliphant’s article. The values for \(^ {10}\mathrm{B}\) and \(^ {19}\mathrm{F}\) completely coincide with the data calculated by Oliphant. Oliphant’s error in determining atomic masses should apparently not exceed 0.0003 mass units. It should be noted, however, that in Aston’s data for elements lying beyond boron (with the exception of C), the measurement accuracy is appreciably less than this value.
Of particular interest is the fact that, when the deviations of the masses of the elements under consideration from integer values are plotted (the mass of \(^ {16}\mathrm{O}\) being taken as equal to 16.0000) as a function of mass number, a smooth curve is obtained, oscillating with period 4 (Fig. 1). The curve shown indicates that, if one assumes the construction of stable nuclei by the successive addition of protons or neutrons, then the most—
more stable elements are \(^{4}\mathrm{He}\), \(^{8}\mathrm{Be}\), \(^{12}\mathrm{C}\), \(^{16}\mathrm{O}\), \(^{20}\mathrm{Ne}\), etc. This sequence suggests that in the nucleus there exist \(\alpha\)-particles or certain stable configurations arising after
| Element | Mass | Element | Mass | |
|---|---|---|---|---|
| n | 1.0091 | \(^{10}\mathrm{B}\) | 10.0161 | \(10.0161 \pm 0.0003\) |
| \(^{1}\mathrm{H}\) | 1.0081* | \(^{11}\mathrm{B}\) | 11.0128 | |
| \(^{2}\mathrm{D}\) | 2.0147* | \((^{12}\mathrm{B})\) | (12.0153) | |
| \(^{3}\mathrm{T}\) | 3.0171 | \((^{11}\mathrm{C})\) | (11.0143) | |
| \(^{3}\mathrm{He}\) | 3.0171 | \(^{12}\mathrm{C}\) | 12.0036* | |
| \(^{4}\mathrm{He}\) | 4.0039* | \(^{13}\mathrm{C}\) | 13.0073 | |
| \(^{6}\mathrm{Li}\) | 6.0167 | \((^{13}\mathrm{N})\) | (13.0096) | |
| \(^{7}\mathrm{Li}\) | 7.0180 | \(^{14}\mathrm{N}\) | 14.0073* | |
| \((^{8}\mathrm{Li})\) | (8.0190) | \(^{15}\mathrm{N}\) | 15.0048 | |
| \(^{8}\mathrm{Be}\) | 8.0078 | \(^{16}\mathrm{O}\) | 16.0000 | |
| \(^{9}\mathrm{Be}\) | 9.0149 | \(^{17}\mathrm{O}\) | 17.0046 | |
| \(^{10}\mathrm{Be}\) | 10.0164 | \((^{17}\mathrm{F})\) | (17.0073) | |
| \(^{19}\mathrm{F}\) | 19.0045 | \(19.0045 \pm 0.0006\) | ||
| \(^{20}\mathrm{Ne}\) | \(19.9986 \pm 0.0006\) | |||
| \(^{29}\mathrm{Si}\) | \(28.9864 \pm 0.0008\) | |||
| \(^{40}\mathrm{A}\) | \(39.9754 \pm 0.0014\) |
each addition of an \(\alpha\)-particle. However, the fact that \(^{8}\mathrm{Be}\) is a stable element and has a mass equal to two \(\alpha\)-particles does not speak in favor of the separate existence of \(\alpha\)-particles inside the nucleus.
Fig. 1.
Regarding the relation of radioactive elements (\(^{8}\mathrm{Li}\), \(^{11}\mathrm{C}\), \(^{12}\mathrm{B}\), \(^{13}\mathrm{N}\), \(^{17}\mathrm{F}\)) to the curve shown, nothing definite can be said in view of the inaccurate values of their masses.
It should also be noted that for the mass of the neutron, taken as the mean of three different transformations, one obtains a value noticeably larger than has usually been assumed.
L. Groshev, Moscow
References
- Aston, Nature, 137, 357, 1936.
- Oliphant, Nature, 137, 396, 1936.
- Aston, Nature, 137, 613, 1936.