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Your textbook says the atomic mass of chlorine is 35.5 and of neon about 20.2. If atoms are built from protons and neutrons, why are these not whole numbers? The answer came from Francis Aston, who built an instrument that weighed atoms one kind at a time. This post covers what he did, works through the numbers he made famous, and shows how the same ideas appear in your JEE and CBSE chemistry and physics.
Who was Francis Aston?
According to his Nobel Prize facts page, Francis William Aston was born on 1 September 1877 in Harborne, near Birmingham, and died on 20 November 1945 in Cambridge. His Nobel biography says he studied chemistry and physics at Mason College, Birmingham, and worked for three years as a brewery chemist before turning to physics. At the end of 1909 he joined J. J. Thomson at the Cavendish Laboratory in Cambridge to work on positive rays, the streams of charged atoms produced in a gas discharge tube.
The 1922 Nobel Prize in Chemistry went to him alone, "for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule."
What puzzle was he trying to solve?
In his Nobel lecture of 12 December 1922, Aston explains the starting point. In 1912, Thomson's apparatus showed that neon gave not one trace on the photographic plate but two: a bright one near mass 20 and a fainter one near 22. Neon's accepted atomic weight was 20.20. Was the faint trace a new element, or were there two kinds of neon atom?
Aston first tried to separate the two by chemistry and physics. Thousands of distillations over cold charcoal gave no change in density. Months of diffusion through pipeclay gave a small but real difference of about 0.7 per cent between the heaviest and lightest fractions. Then the First World War stopped the work.
How did the mass spectrograph work?
Back in Cambridge in 1919, Aston designed a new instrument. In the lecture he describes it: a thin ribbon of positive rays passes between charged plates, which spread it out by an electric field, then between magnet poles that bend it the other way. He chose the geometry so that ions with the same mass-to-charge ratio, but different speeds, all arrive at the same point on a photographic plate. Each kind of atom draws its own sharp line, as light of one colour does in an optical spectrograph. He called the result a mass spectrum.
The force doing the sorting is the one in your Class 12 book: a charged particle in electric and magnetic fields feels F = q(E + v × B). Our post on Hendrik Lorentz and the electron theory covers where that equation came from.
With neon in the instrument, Aston found lines at 20 and 22 to within about one part in a thousand. Chlorine gave lines at 35 and 37, and, as he put it, "There is no sign whatever of any line at 35.46." The fractional atomic weight was an average over isotopes. The Nobel biography says he went on to find 212 of the naturally occurring isotopes.
Worked example: why neon's atomic mass is 20.18
Modern values from the NIST isotopic composition table for neon:
Neon-20: mass 19.99244 u, abundance 90.48%
Neon-21: mass 20.99385 u, abundance 0.27%
Neon-22: mass 21.99139 u, abundance 9.25%
Multiply each mass by its fraction. 19.99244 x 0.9048 = 18.0892; 20.99385 x 0.0027 = 0.0567; 21.99139 x 0.0925 = 2.0342.
Add. 18.0892 + 0.0567 + 2.0342 = 20.1801 u.
Check against the table. NIST gives the standard atomic weight of neon as 20.1797. The small difference comes from rounding the abundances.
What is the whole number rule, and where does it break?
The lecture states the main result: apart from hydrogen, the weights of the atoms of all the elements he measured were whole numbers to about one part in a thousand. That was the clue that atoms are built from identical units.
Aston also saw why hydrogen is the exception. A helium nucleus weighs slightly less than four hydrogen nuclei, so, in his words, if hydrogen is turned into helium "a certain quantity of mass must be annihilated in the process." He linked this to Einstein's E = mc² and to the heat of the Sun.
You can check this with modern masses from NIST for hydrogen and helium:
Four hydrogen-1 atoms: 4 x 1.007825 = 4.031300 u.
One helium-4 atom: 4.002603 u.
Mass lost: 4.031300 - 4.002603 = 0.028697 u.
Energy: the NCERT Class 12 chapter Nuclei gives 1 u = 931.5 MeV/c², so 0.028697 x 931.5 = 26.7 MeV.
Using whole atoms is fine here because the electron masses on both sides cancel. Aston's 1922 estimate of the loss was 0.0077 per hydrogen atom; the modern figure is 0.028697 / 4 = 0.0072.
Which mistakes does this help you avoid?
Calling the mass number the atomic mass. Mass number (20, 22) counts nucleons. Atomic mass (19.99244 u) is measured and is never exactly whole, except carbon-12 by definition.
Forgetting that isotopes are chemically alike. Neon-20 and neon-22 behave the same in reactions, which is why Aston's chemical separation failed.
The JEE (Main) 2026 syllabus lists atomic masses in chemistry and mass defect and binding energy in physics, so these calculations can appear in either paper. Check the current year's syllabus before you plan.
What can a student take from Aston's way of working?
Build the tool the question needs. Older methods could not resolve masses finely enough, so he designed one that could.
Trust a negative result. "No line at 35.46" settled the chlorine question as firmly as a positive line would have.
Notice the small misfit. The tiny gap between helium and four hydrogens looked like a rounding error. It turned out to explain starlight.
For the step where a weighted average or a mass defect question goes wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer. If you want to see how isotopes fit into the nucleus first, read our post on Rutherford's gold foil experiment.
Try it now: look up the two isotopes of chlorine in the NIST table and calculate chlorine's atomic mass to two decimal places.
