Humber Admissions Practice Test

How can you determine the number of valence electrons for an element?

1. By atomic number

2. By group number

3. By period number

4. By mass number

The correct answer is: By period number

The number of valence electrons for an element can be determined primarily by its group number in the periodic table. Elements are arranged in columns, known as groups, and all elements within the same group typically possess the same number of valence electrons. For example, elements in Group 1 have one valence electron, while those in Group 2 have two. Groups 13 through 18 correspond to three to eight valence electrons, respectively. While the atomic number indicates the total number of protons (and hence the total number of electrons in a neutral atom), it does not directly indicate the number of valence electrons. Similarly, the period number refers to the row in which an element is located and conveys information about the energy levels of the electrons rather than their specific arrangement or count among the outermost shell. The mass number, being the sum of protons and neutrons in the nucleus, holds no relevance concerning valence electrons. Thus, focusing on the group number is the most effective way to accurately assess the number of valence electrons for any given element.