Question: What is the atomic number of chlorine?
About the article "Proton", section "Atomic number".

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Chlorine-37, or , is one of the stable isotopes of chlorine, the other being chlorine-35 ().
Its nucleus contains 17 protons and 20 neutrons for a total of 37 nucleons.
Chlorine-37 accounts for 24.23% of natural chlorine, chlorine-35 accounting for 75.77%, giving chlorine atoms in bulk an apparent atomic weight of .
Remarkably, the Solar neutrinos were discovered by an experiment using a radiochemical method based on Chlorine-37 transmutation. 
The mass number should also not be confused with the relative atomic mass (also called atomic weight) of an element, which is the ratio of the average atomic mass of the different isotopes of that element (weighted by abundance) to the unified atomic mass unit.
This weighted average can be quite different from the near-integer values for individual isotopic masses.
For instance, there are two main isotopes of chlorine: chlorine-35 and chlorine-37.
In any given sample of chlorine that has not been subjected to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37.
This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/mol).
Moreover, the weighted average mass can be near-integer, but at the same time not corresponding to the mass of any natural isotope.
For example, bromine has only two stable isotopes, 79Br and 81Br, naturally present in approximately equal fractions, which leads to the standard atomic mass of bromine close to 80 (79.904 g/mol), even though the isotope 80Br with such mass is unstable. 
  The atomic number of chlorine.
The Brodmann area defining the primary visual processing area of mammalian brains.
Group 17 of the periodic table is called the halogens.
The number of elementary particles in the Standard Model of physics.  
 in chemistry.
Atomic number.
In chemistry, the number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs.
For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms.
The chemical properties of each atom are determined by the number of (negatively charged) electrons, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero.
For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl− anion has 17 protons and 18 electrons for a total charge of −1.
All atoms of a given element are not necessarily identical, however, as the number of neutrons may vary to form different isotopes, and energy levels may differ forming different nuclear isomers.
For example, there are two stable isotopes of chlorine: with 35 − 17 = 18 neutrons and with 37 − 17 = 20 neutrons.
Chlorine and oxygen can bond in many different ways:  chlorine monoxide (ClO), chlorine(II) oxide  chlorine dioxide (ClO2), chlorine(IV) oxide  chlorine trioxide (ClO3), chlorine(VI) oxide  dichlorine monoxide (Cl2O), chlorine(I) oxide  dichlorine dioxide (Cl2O2), chlorine(I) peroxide  dichlorine trioxide (Cl2O3), chlorine(I,V) oxide  dichlorine tetroxide, also known as chlorine perchlorate, (ClOClO3), chlorine(I,VII) oxide  dichlorine hexoxide (Cl2O6), chlorine(V,VII) oxide  dichlorine heptoxide (Cl2O7), chlorine(VII) oxide  chlorine tetroxide, the peroxide ((OClO3)2), chlorine(VII) oxide peroxide In addition, a number of ions are chlorine oxides:  chloryl (ClO2+)  perchloryl (ClO3+)  hypochlorite (ClO-)  chlorite (ClO2-)  chlorate (ClO3-)  perchlorate (ClO4-)