Question 27

The atomic number is the number of protons in an atom.

The atomic number directly corresponds to the number of protons in the nucleus of an atom, which defines the element itself.

A) Number of neutrons

This option is incorrect because the atomic number specifically counts the protons in an atom, not the neutrons. Neutrons contribute to the atomic mass but do not determine the element's identity.

B) Number of protons

This option is correct as the atomic number is defined as the total number of protons in the nucleus of an atom. It is a fundamental property that distinguishes one element from another on the periodic table.

C) Number of electrons

This option is incorrect because while a neutral atom has an equal number of electrons and protons, the atomic number refers specifically to protons. Electrons can vary in number in ions, thus they do not define the atomic number.

D) Number of isotopes

This option is incorrect as it does not relate to the atomic number. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons, which is not what the atomic number represents.

Conclusion

The atomic number is definitively the number of protons in an atom, making option B the correct answer. Options A, C, and D incorrectly define or relate to the atomic number, failing to capture its essence as a defining characteristic of an element. Thus, understanding the atomic number is essential for grasping the structure of atoms and the organization of the periodic table.