Question: What has the lowest mass of any charged leptop?
About the article "Electron", section "Classification".

Candidate passages:
In the Standard Model of particle physics, electrons belong to the group of subatomic particles called leptons, which are believed to be fundamental or elementary particles.
Electrons have the lowest mass of any charged lepton (or electrically charged particle of any type) and belong to the first-generation of fundamental particles.
The second and third generation contain charged leptons, the muon and the tau, which are identical to the electron in charge, spin and interactions, but are more massive.
Leptons differ from the other basic constituent of matter, the quarks, by their lack of strong interaction.
All members of the lepton group are fermions, because they all have half-odd integer spin; the electron has spin .
The large majority of an atom's mass comes from the protons and neutrons that make it up.
The total number of these particles (called "nucleons") in a given atom is called the mass number.
The mass number is a simple whole number, and has units of "nucleons."
An example of use of a mass number is "carbon-12," which has 12 nucleons (six protons and six neutrons).
The actual mass of an atom at rest is often expressed using the unified atomic mass unit (u), which is also called a dalton (Da).
This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately .
Hydrogen-1, the lightest isotope of hydrogen and the atom with the lowest mass, has an atomic weight of 1.007825 u.
The value of this number is called the atomic mass.
A given atom has an atomic mass approximately equal (within 1%) to its mass number times the mass of the atomic mass unit.
However, this number will not be an exact whole number except in the case of carbon-12 (see below).
The heaviest stable atom is lead-208, with a mass of .
As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles.
One mole of atoms of any element always has the same number of atoms (about Avogadro constant).
This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element has a mass close to one gram.
Because of the definition of the unified atomic mass unit, each carbon-12 atom has an atomic mass of exactly 12 u, and so a mole of carbon-12 atoms weighs exactly 0.012 kg. 
The Bohr model gives almost exact results only for a system where two charged points orbit each other at speeds much less than that of light.
This not only includes one-electron systems such as the hydrogen atom, singly ionized helium, doubly ionized lithium, but it includes positronium and Rydberg states of any atom where one electron is far away from everything else.
It can be used for K-line X-ray transition calculations if other assumptions are added (see Moseley's law below).
In high energy physics,it can be used to calculate the masses of heavy quark mesons.
To calculate the orbits requires two assumptions: Classical mechanics The electron is held in a circular orbit by electrostatic attraction.
The centripetal force is equal to the Coulomb force.
:formula_5 where "m"e is the electron's mass, "e" is the charge of the electron, "k"e is Coulomb's constant and "Z" is the atom's atomic number.
This equation determines the electron's speed at any radius: : formula_6  It also determines the electron's total energy at any radius: : formula_7 The total energy is negative and inversely proportional to "r".
This means that it takes energy to pull the orbiting electron away from the proton.
For infinite values of "r", the energy is zero, corresponding to a motionless electron infinitely far from the proton.
The total energy is half the potential energy, which is true for noncircular orbits too by the virial theorem.
For positronium, "m"e is replaced by its reduced mass ().
Quantum rule The angular momentum is an integer multiple of "ħ": :formula_8 Substituting the expression for the velocity gives an equation for "r" in terms of n: :formula_9 so that the allowed orbit radius at any n is: :formula_10 The smallest possible value of "r" in the hydrogen atom is called the Bohr radius and is equal to: : formula_11 The energy of the "n"-th level for any atom is determined by the radius and quantum number: :formula_12 An electron in the lowest energy level of hydrogen () therefore has about 13.6 eV less energy than a motionless electron infinitely far from the nucleus.
The next energy level () is −3.4 eV.
The third ("n" = 3) is −1.51 eV, and so on.
For larger values of "n", these are also the binding energies of a highly excited atom with one electron in a large circular orbit around the rest of the atom.
The combination of natural constants in the energy formula is called the Rydberg energy ("R"E): formula_13 This expression is clarified by interpreting it in combinations which form more natural units:  formula_14 is the rest mass energy of the electron (511 keV)  formula_15 is the fine structure constant  formula_16 Since this derivation is with the assumption that the nucleus is orbited by one electron, we can generalize this result by letting the nucleus have a charge "q" = "Z e" where "Z" is the atomic number.
This will now give us energy levels for hydrogenic atoms, which can serve as a rough order-of-magnitude approximation of the actual energy levels.
So, for nuclei with "Z" protons, the energy levels are (to a rough approximation): formula_17 The actual energy levels cannot be solved analytically for more than one electron (see "n"-body problem) because the electrons are not only affected by the nucleus but also interact with each other via the Coulomb Force.
When "Z" = 1/"α" (Z ≈ 137), the motion becomes highly relativistic, and "Z"2 cancels the "α"2 in "R"; the orbit energy begins to be comparable to rest energy.
Sufficiently large nuclei, if they were stable, would reduce their charge by creating a bound electron from the vacuum, ejecting the positron to infinity.
This is the theoretical phenomenon of electromagnetic charge screening which predicts a maximum nuclear charge.
Emission of such positrons has been observed in the collisions of heavy ions to create temporary super-heavy nuclei.
The Bohr formula properly uses the reduced mass of electron and proton in all situations, instead of the mass of the electron: formula_18.
However, these numbers are very nearly the same, due to the much larger mass of the proton, about 1836.1 times the mass of the electron, so that the reduced mass in the system is the mass of the electron multiplied by the constant 1836.1/(1+1836.1) = 0.99946.
This fact was historically important in convincing Rutherford of the importance of Bohr's model, for it explained the fact that the frequencies of lines in the spectra for singly ionized helium do not differ from those of hydrogen by a factor of exactly 4, but rather by 4 times the ratio of the reduced mass for the hydrogen vs. the helium systems, which was much closer to the experimental ratio than exactly 4.0.
For positronium, the formula uses the reduced mass also, but in this case, it is exactly the electron mass divided by 2.
For any value of the radius, the electron and the positron are each moving at half the speed around their common center of mass, and each has only one fourth the kinetic energy.
The total kinetic energy is half what it would be for a single electron moving around a heavy nucleus.
formula_19 (positronium)  
1.2% of the population has a tertiary education, and another 4.4% has completed secondary education.
The overall literacy rate is 26.1% (the lowest for any commune in Adrar Province), and is 33.3% among males (the lowest in the province) and 17.8% among females (the second lowest in the province). 
The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.
Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e.
non-quantum) mechanics.
These properties are special because they are visible from outside a black hole.
For example, a charged black hole repels other like charges just like any other charged object.
Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole.
Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.
When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers.
The behavior of the horizon in this situation is a dissipative system that is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance—the membrane paradigm.
This is different from other field theories like electromagnetism, which do not have any friction or resistivity at the microscopic level, because they are time-reversible.
Because a black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of a black hole give very little information about what went in.
The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including approximately conserved quantum numbers such as the total baryon number and lepton number.
This behavior is so puzzling that it has been called the black hole information loss paradox.