Question 31

The atomic number of an element is defined by the number of protons.

The atomic number is a fundamental characteristic of an element, representing the total number of protons found in the nucleus of its atoms. This number uniquely identifies the element and determines its position on the periodic table.

A) Number of neutrons

This option is incorrect because the number of neutrons does not define the atomic number. Instead, neutrons contribute to the atomic mass of an element but not to its atomic number, which is solely dependent on the number of protons.

B) Number of protons

This option is correct because the atomic number is defined explicitly by the number of protons in an atom's nucleus. Each element has a unique atomic number that distinguishes it from all other elements.

C) Number of electrons

This option is incorrect because the number of electrons may vary in ions of the same element and does not determine the atomic number. The atomic number is specifically linked to protons, not electrons.

D) Number of isotopes

This option is incorrect as well. The number of isotopes refers to the different forms of an element that have the same number of protons but different numbers of neutrons. Isotopes do not influence the atomic number itself.

Conclusion

The correct answer is option B, as the atomic number is fundamentally defined by the number of protons in an atom's nucleus. Other options relate to different properties of atoms but do not determine the atomic number, which is a key identifier for each element in the periodic table.