Lesson 1 of 11 · 7 min

Atomic mass unit and isotopes

NCERT §13.1, §13.2

Meera's exhibition poster opens with a question from her little brother: if an atom were blown up to fill their classroom, how big would its nucleus be? The answer, a pinhead, holds almost all the atom's mass.

The story this chapter follows: Meera's exhibition poster on energy

Meera is making a poster for the school science exhibition that follows one question, where does the energy come from, from a pinhead-sized nucleus in a classroom-sized atom to a nuclear reactor and the sun. Each panel needs a number she can defend: 931.5 MeV for every atomic mass unit, 127.5 MeV to take oxygen-16 apart, about 200 MeV per fission and 26.7 MeV from the sun's proton–proton cycle.

The lesson in notes

In short

A nucleus is about 10⁴ times smaller in radius than its atom, so its volume is about 10⁻¹² of the atom's. Blow an atom up to the size of a classroom and the nucleus would be a pinhead, yet it holds more than 99.9% of the atom's mass.

A ¹²C atom has a mass of 1.992647 × 10⁻²⁶ kg, too tiny for the kilogram to be handy. Atomic masses are instead given in the atomic mass unit u, one-twelfth of the mass of a ¹²C atom: 1 u = 1.660539 × 10⁻²⁷ kg (Eq. 13.1).

Atomic masses in u come out close to whole-number multiples of the hydrogen atom's mass, but with notable exceptions: chlorine's atomic mass is 35.46 u.

A mass spectrometer measures atomic masses precisely. It revealed isotopes: atoms of one element with the same chemistry but different masses. The Greek word means 'same place', since isotopes sit in one place in the periodic table. Nearly every element is a mixture of isotopes.

Chlorine is a mix of two isotopes: 34.98 u, making up 75.4%, and 36.98 u, making up 24.6%. Weighting each mass by its share, 0.754 × 34.98 + 0.246 × 36.98 ≈ 35.47 u, which is chlorine's atomic mass.

Hydrogen has three isotopes. The lightest, of mass 1.0078 u, makes up 99.985% of hydrogen; the others weigh 2.0141 u and 3.0160 u. Removing one electron (0.00055 u) from a light hydrogen atom (1.00783 u) leaves its nucleus, the proton, of mass 1.00727 u, or 1.67262 × 10⁻²⁷ kg (Eq. 13.2).

The heavier two are deuterium and tritium. Tritium nuclei are unstable, so tritium is not found in nature and is made artificially.