Which of the following choices define atomic number?

The atomic number is defined as the number of protons in the nucleus of an atom. This fundamental property uniquely identifies an element, as each element has a different number of protons. For example, carbon has an atomic number of 6 because it has six protons, while oxygen has an atomic number of 8 because it has eight protons.

The atomic number is essential for understanding the chemical behavior of an element since it determines the number of electrons surrounding the nucleus in a neutral atom, influencing how the element bonds with others. By knowing the atomic number, one can also determine the element's position in the periodic table, which is organized based on atomic number.

In contrast, other options refer to different aspects of atomic structure. The number of orbital shell electrons relates to electron configuration, neutrons are involved in calculating atomic mass but do not affect atomic number, and the number of protons plus electrons provides a different concept that includes electrical neutrality of an atom but does not define atomic number directly.