A Level Chemistry OCR Practice Exam

What does the atomic (proton) number indicate?

1. The number of neutrons in an atom

2. The total number of electrons

3. The number of protons in the nucleus

4. The mass of an atom

The correct answer is: The number of protons in the nucleus

The atomic (proton) number is defined as the number of protons present in the nucleus of an atom. It is a fundamental characteristic of an element and is unique to each element on the periodic table. This number determines the identity of the element, as well as its position on the periodic table. For example, hydrogen has an atomic number of 1 because it has one proton, while carbon has an atomic number of 6 because it has six protons. Additionally, in a neutral atom, the number of electrons equals the number of protons, which helps to maintain a balanced electrical charge. However, the atomic number specifically refers to protons, not neutrons or electrons, making it a key identifying factor for each element. The mass of an atom is predominantly determined by its total number of protons and neutrons, but the atomic number itself does not provide a direct indication of mass.