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The proton radius puzzle is an unanswered problem in physics relating to the size of the proton. [1] Historically the proton charge radius was measured by two independent methods, which converged to a value of about 0.877 femtometres (1 fm = 10 −15 m).
The size, or root mean squared (RMS) charge radius, of the proton (the smallest nuclide) has a 2018 CODATA recommended value of 0.8414 (19) fm (10 −15 m), although values may vary by a few percent according to the experimental method employed (see proton radius puzzle). Nuclide size ranges up to ≈ 6 fm.
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The proton is a "bare charge" with only about 1/64,000 of the radius of a hydrogen atom, and so is extremely reactive chemically. The free proton, thus, has an extremely short lifetime in chemical systems such as liquids and it reacts immediately with the electron cloud of any available molecule.
An inconsistency between proton charge radius measurements made using different techniques [12] was known as the proton radius puzzle, but more recent measurements show consistent results. [13] The CODATA recommended values are: proton: 8.4075(64) × 10 −16 m [14] deuteron: 2.127 78 (27) × 10 −15 m [15]
The results of these measurements diverged from the then accepted value giving rise to the so called proton radius puzzle. Later this puzzle found its resolution when new improved measurements of the proton radius in the electronic hydrogen became available. [20] Muonic helium is created by substituting a muon for one of the electrons in helium ...
The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm. [15] The shape of the atomic nucleus can be spherical, rugby ball-shaped (prolate deformation), discus-shaped (oblate deformation), triaxial (a combination of oblate and prolate deformation) or pear-shaped.
Charge quantization is the principle that the charge of any object is an integer multiple of the elementary charge. Thus, an object's charge can be exactly 0 e, or exactly 1 e, −1 e, 2 e, etc., but not 1 / 2 e, or −3.8 e, etc. (There may be exceptions to this statement, depending on how "object" is defined; see below.)