dimitri mendeleev organized his periodic table of the elements according to their recurring properties and eventually this was revised into an observation called periodic law we came to understand that these recurring properties occur according to the atomic numbers of the elements rather than the atomic mass so the modern periodic table is a slight revision of mendeleev's version that shows the elements arranged by increasing atomic number this is what is meant by periodic law that these properties are periodic functions of the atomic numbers of the elements let's take a look at the periodic table as it is today on this table each box represents an element and these elements are arranged into rows which are called periods this is how we display the periodicity of properties because with the start of each new row we start again with a new repetition of these shared properties that the elements tend to exhibit that means that elements in the same column or group are the ones that will display similar behavior there are seven periods and eighteen groups it used to be the case that the groups were numbered one through eight using roman numerals and skipping over the section in the middle but now it is more typical that we use the numerical symbols we see here in order to save space a section of the table is removed and placed below the rest these are the lanthanides and actinides and they would technically belong in this area because of the way the elements are arranged it was realized that the elements could be described as being either metals metalloids or non-metals and these different types of elements have a tendency to display particular properties that will be discussed later so these seven periods and 18 groups summarize the way that we organize the elements today and the periodic table that displays these patterns is something that we will refer to frequently as we study chemistry in order to describe atoms and predict chemical behavior