International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What is the definition of atomic number?

1. The total number of protons and neutrons in an atom

2. The number of neutrons in an atom

3. The number of electrons in an atom

4. The number of protons in an atom

The correct answer is: The number of protons in an atom

The atomic number is defined as the number of protons in the nucleus of an atom. This fundamental property of an element determines its position in the periodic table and is essential for identifying the element itself. For instance, all atoms of carbon have an atomic number of 6 because they contain six protons. The atomic number not only helps distinguish one element from another but also indicates the number of electrons in a neutral atom, as the number of protons equals the number of electrons in such cases. Other options refer to different atomic properties. The total number of protons and neutrons in an atom corresponds to the atomic mass, while the number of neutrons is part of the atom's isotopic identity, and the number of electrons only applies to neutral atoms, not accounting for ions or charged species. Therefore, recognizing that the atomic number specifically refers to protons is crucial for understanding basic atomic structure and the classification of elements.