Radiation Safety Practice Exam

What does the atomic number (z) represent in an atom?

1. Number of neutrons

2. Number of electrons

3. Number of protons

4. Total mass of the atom

The correct answer is: Number of protons

The atomic number (Z) is a fundamental characteristic of an atom that specifically represents the number of protons found in the nucleus. This number is crucial because it determines the element's identity; for example, an atom with an atomic number of 1 is hydrogen, while an atomic number of 6 corresponds to carbon. The atomic number also has implications for the atom's electronic structure, as it typically equals the number of electrons in a neutral atom, balancing the positive charge of protons with the negative charge of electrons. Understanding the atomic number is essential for studying the properties of elements, their placement on the periodic table, and their behavior in chemical reactions. It does not represent the number of neutrons or the total mass of the atom, which are defined separately, and it is not the same as the number of electrons in all circumstances, especially in charged ions.