How to Extract Information from the Periodic Table - Example: Chlorine
Chlorine (Cl) is a common disinfectant, which is used for example for the disinfection of drinking water or to keep swimming pools clean and sanitary (fig. 1). Furthermore, it is used to manufacture plastics and solvents.
Figure 1: Chlorine is used as a disinfectant for drinking water and swimming pools [left: CC0 Public Domain; right: ABZ Private School, licensed under the Creative Commons Attribution-Share Alike 4.0 International license, https://commons.wikimedia.org/wiki/File:FreeGreatPicture.com-30424-swimming-pool.jpg]
You can extract the following information on chlorine from the periodic table:
Chlorine (Cl) is a non-metal because it is in the upper right-hand side of the periodic table (fig. 2). More specifically, it is located in the 7th (or 17th, depending on how you count) group (i.e. column) and 3rd period (row). As it is in the 7th group, it belongs to the group of halogens.
Figure 2: Chlorine in the periodic table
The atomic number of chlorine is 17. Consequently, a chlorine atom has 17 protons and the electrically neutral chlorine atom has 17 electrons. The atomic weight of chlorine is 35.45.
The full electron configuration of chlorine is 1s22s22p63s23p5. It can be abbreviated as [Ne] 3s23p5 because all but the last few subshells are identical to the one of the noble gas neon (Ne). In this abbreviated notation we can spot the valence shell configuration more clearly: 3s23p5. Consequently, chlorine has 7 valence electrons. (Remember: The valence electrons are the electrons in the outermost shell. They are important because they are involved in the formation of chemical bonds, which you will learn more about in the following units.)
If we compare the valence shell configuration of chlorine with the valence shell configuration of other halogens, we can see that all halogens have 2 electrons in the s subshell and 5 electrons in the p subshell (table 1). This is the reason why halogens have similar properties.
Table 1: Electron configuration of the halogens
Within its group chlorine has a larger radius than fluorine (F) and a smaller radius than bromine (Br) because the valence electrons are placed in higher shells (i.e. larger orbitals) as we go down the periodic table. Within its period, chlorine has a smaller radius than the elements to its left because the positive charge of chlorine’s nucleus is higher than the positive charge of e.g. sulfur (S) or silicon (Si), which draws the electrons closer to the nucleus.