Question: How is the atomic number of an element related to the number of protons in its nucleus?
About the article "Subatomic particle", section "Dividing an atom".

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The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.
Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion.
The mass number is different for each different isotope of a chemical element.
This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element.
Hence, the difference between the mass number and the atomic number gives the number of neutrons ("N") in a given nucleus: "N=A−Z".
The mass number is written either after the element name or as a superscript to the left of an element's symbol.
For example, the most common isotope of carbon is carbon-12, or , which has 6 protons and 6 neutrons.
The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: .
This is technically redundant, as each element is defined by its atomic number, so it is often omitted. 
The atomic number of an element is equal to the number of protons in each atom, and defines the element.
For example, all carbon atoms contain 6 protons in their atomic nucleus; so the atomic number of carbon is 6.
Carbon atoms may have different numbers of neutrons; atoms of the same element having different numbers of neutrons are known as isotopes of the element.
The number of protons in the atomic nucleus also determines its electric charge, which in turn determines the number of electrons of the atom in its non-ionized state.
The electrons are placed into atomic orbitals that determine the atom's various chemical properties.
The number of neutrons in a nucleus usually has very little effect on an element's chemical properties (except in the case of hydrogen and deuterium).
Thus, all carbon isotopes have nearly identical chemical properties because they all have six protons and six electrons, even though carbon atoms may, for example, have 6 or 8 neutrons.
That is why the atomic number, rather than mass number or atomic weight, is considered the identifying characteristic of a chemical element.
The symbol for atomic number is "Z". 
A nuclide (from nucleus) is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons "Z", its number of neutrons "N", and its nuclear energy state.
The word "nuclide" was proposed by Truman P. Kohman in 1947.
Doctor Kohman originally suggested nuclide as referring to a "species of nucleus" defined by containing a certain number of neutrons and protons.
The word thus was originally intended to focus on the nucleus. 
A chemical element is a pure substance which is composed of a single type of atom, characterized by its particular number of protons in the nuclei of its atoms, known as the atomic number and represented by the symbol "Z".
The mass number is the sum of the number of protons and neutrons in a nucleus.
Although all the nuclei of all atoms belonging to one element will have the same atomic number, they may not necessarily have the same mass number; atoms of an element which have different mass numbers are known as isotopes.
For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, but atoms of carbon may have mass numbers of 12 or 13.
The standard presentation of the chemical elements is in the periodic table, which orders elements by atomic number.
The periodic table is arranged in groups, or columns, and periods, or rows.
The periodic table is useful in identifying periodic trends. 
The mass number, also called the nucleon number, is the number of protons and neutrons in an atomic nucleus.
The mass number is unique for each isotope of an element and is written either after the element name or as a superscript to the left of an element's symbol.
For example, carbon-12 (12C) has 6 protons and 6 neutrons.