A Level Chemistry OCR Practice Exam

What does the principal quantum number, n, represent?

1. The total number of electrons in an atom

2. The number of protons in the nucleus

3. The relative overall energy of each orbital

4. The shape of the electron cloud around the nucleus

The correct answer is: The relative overall energy of each orbital

The principal quantum number, represented by the symbol n, indicates the relative overall energy of each orbital and the distance of an electron from the nucleus in an atom. As the value of n increases, the energy of the electron also increases, and the electron is located further from the nucleus. The principal quantum number can take on positive integer values (1, 2, 3, etc.), each corresponding to a different energy level; thus, greater values of n correspond to higher energy levels. In contrast, the other choices do not accurately describe what the principal quantum number represents. The total number of electrons in an atom is related to atomic structure but is not defined by n. The number of protons in the nucleus is a property specific to each element and is represented by the atomic number, not by the principal quantum number. The shape of the electron cloud is denoted by angular momentum quantum number (l), which dictates the types of orbitals (s, p, d, f) present in a given energy level, rather than the principal quantum number itself.