Question: The number of protons in the nucleus of an atom is referred to as what?
About the article "Proton", section "Abstract".

Candidate passages:
The effective atomic number Zeff, (sometimes referred to as the effective nuclear charge) of an atom is the number of protons an electron in the element effectively 'sees' due to screening by inner-shell electrons.
It is a measure of the electrostatic interaction between the negatively charged electrons and positively charged protons in the atom.
One can view the electrons in an atom as being 'stacked' by energy outside the nucleus; the lowest energy electrons (such as the 1s and 2s electrons) occupy the space closest to the nucleus, and electrons of higher energy are located further from the nucleus.
The binding energy of an electron, or the energy needed to remove the electron from the atom, is a function of the electrostatic interaction between the negatively charged electrons and the positively charged nucleus.
In Iron, atomic number 26, for instance, the nucleus contains 26 protons.
The electrons that are closest to the nucleus will 'see' nearly all of them.
However, electrons further away are screened from the nucleus by other electrons in between, and feel less electrostatic interaction as a result.
The 1s electron of Iron (the closest one to the nucleus) sees an effective atomic number (number of protons) of 25.
The reason why it is not 26 is because some of the electrons in the atom end up repelling the others, giving a net lower electrostatic interaction with the nucleus.
One way of envisioning this effect is to imagine the 1s electron sitting on one side of the 26 protons in the nucleus, with another electron sitting on the other side; each electron will feel less than the attractive force of 26 protons because the other electron contributes a repelling force.
The 4s electrons in Iron, which are furthest from the nucleus, feel an effective atomic number of only 5.43 because of the 25 electrons in between it and the nucleus screening the charge.
Effective atomic numbers are useful not only in understanding why electrons further from the nucleus are so much more weakly bound than those closer to the nucleus, but also because they can tell us when to use simplified methods of calculating other properties and interactions.
For instance, Lithium, atomic number 3, has two electrons in the 1s shell and one in the 2s shell.
Because the two 1s electrons screen the protons to give an effective atomic number for the 2s electron close to 1, we can treat this 2s valence electron with a hydrogenic model.
Mathematically, the effective atomic number Zeff can be calculated using methods known as "self-consistent field" calculations, but in simplified situations is just taken as the atomic number minus the number of electrons between the nucleus and the electron being considered. 
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons.
The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutrons).
The electrons of an atom are bound to the nucleus by the electromagnetic force.
Likewise, a group of atoms can remain bound to each other by chemical bonds based on the same force, forming a molecule.
An atom containing an equal number of protons and electrons is electrically neutral, otherwise it is positively or negatively charged and is known as an ion.
An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.
Chemical atoms, which in science now carry the simple name of "atom," are minuscule objects with diameters of a few tenths of a nanometer and tiny masses proportional to the volume implied by these dimensions.
Atoms can only be observed individually using special instruments such as the scanning tunneling microscope.
Over 99.94% of an atom's mass is concentrated in the nucleus, with protons and neutrons having roughly equal mass.
Each element has at least one isotope with an unstable nucleus that can undergo radioactive decay.
This can result in a transmutation that changes the number of protons or neutrons in a nucleus.
Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels.
The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.
The principles of quantum mechanics have been successfully used to model the observed properties of the atom. 
The atom is the basic unit of chemistry.
It consists of a dense core called the atomic nucleus surrounded by a space called the electron cloud.
The nucleus is made up of positively charged protons and uncharged neutrons (together called nucleons), while the electron cloud consists of negatively-charged electrons which orbit the nucleus.
In a neutral atom, the negatively-charged electrons balance out the positive charge of the protons.
The nucleus is dense; the mass of a nucleon is 1,836 times that of an electron, yet the radius of an atom is about 10,000 times that of its nucleus.
The atom is also the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity, ionization potential, preferred oxidation state(s), coordination number, and preferred types of bonds to form (e.g., metallic, ionic, covalent). 
Another typical example is that of the particles in a nucleus of an atom.
The radius of the nucleus is roughly: :formula_38 where "A" is the number of nucleons.
The number density of nucleons in a nucleus is therefore: :formula_39 Now since the Fermi energy only applies to fermions of the same type, one must divide this density in two.
This is because the presence of neutrons does not affect the Fermi energy of the protons in the nucleus, and vice versa.
So the Fermi energy of a nucleus is about: :formula_40 The radius of the nucleus admits deviations around the value mentioned above, so a typical value for the Fermi energy is usually given as 38 MeV. 
 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.