Humber Admissions Practice Test

How is the number of electron shells for an element determined?

1. By atomic mass

2. By period number

3. By group number

4. By number of protons

The correct answer is: By period number

The number of electron shells for an element is determined by its period number on the periodic table. Each period corresponds to the highest energy level that is occupied by electrons in the atom. As you move down the periodic table, each new period indicates the addition of an electron shell. For example, elements in the first period have one electron shell, elements in the second period have two, and so forth. This relationship is fundamental in understanding how electrons are arranged around the nucleus and is essential for predicting the chemical behavior of the elements. The other options pertain to different characteristics of elements; atomic mass relates to the average mass of an element's isotopes, group number indicates the number of valence electrons, and the number of protons defines the element itself but does not directly indicate the number of electron shells. Thus, the correct identification of the number of electron shells is clearly associated with the period of the element on the periodic table.