Periodic Table Basics

Although we have already encountered the periodic table in the last few lessons, it would be unwise to jump into a discussion of periodic table trends without first taking a closer look at the periodic table.

The periodic table of today is very different than the first periodic table published. Not only are there far more elements they are organized very differently. With the discovery of quantum mechanics, it became apparent that organizing elements by their quantum electronic structure most accurately grouped similar elements.

Today’s periodic table reflects this and we will focus on viewing the table through this lens.


Big picture the periodic table is described in terms of groups, periods, and blocks. Each is based on a different similarity in electronic structure and helps us explain the behavior certain elements exhibit.

Groups = Columns

Groups refer to the columns in the periodic table and each element in a period shares the same number of valence electrons. This is important because an element’s valence electrons define to a great degree what types of reactions and properties an element has. For example: Be (Beryllium), Mg (Magnesium), Ca (Calcium), and Sr (Strontium) all have two valence electrons. They all also tend to form ionic bonds, cations with 2+ charge, and when combined with OH form bases.

Valence electrons aren’t the whole story though. Even though elements in the same period have similar features they aren’t the same. Elements near the bottom of the periodic table have larger nuclei, which are more unstable. So while Ra (Radium) is radioactive Mg (Magnesium) isn’t. Additionally, as we go down the periodic table we add more and more electron shells. Each shell adds additional space so not only is the nucleus of Ra (Radium) significantly bigger than Mg (Magnesium) is its overall size is too.

These factors help to explain the trends we be exploring throughout the remainder of this lesson and help us predict the properties of molecules as we will see later.

Periods = Row

Periods by contrast refer to the rows in a periodic table. Their properties are wildly different from one another as with each step from left to right or vise versa changes the number of valence electrons. What they do have in common is the number of shells surrounding their nucleus. For instance, both O (Oxygen) and F (Fluorine) have 2 shells (n=1 and n=2).

Since they have the same number of shells they differ most significantly in how many protons they contain in their nucleus as well as how many electrons occupy those shells. This will become important later as we discuss the periodic trends.


Lastly, the periodic table is organized by blocks. Blocks refer to elements that have the same terminal subshell or the last type of subshell that contains electrons. There are four blocks and each one refers to the four subshells (s, p, d, and f).

The elements in the same block also tend to have similar properties due to having similar terminal electron configurations. Ultimately, groups do a much better job of rounding up elements with like properties.