The modern periodic table is not only constructed on the increasing atomic number but also using the work of Newlands, elements are divided into groups of similar physical properties and reactivities. The groups are the vertical columns of the periodic table.
When referring to reactivity of an element this is its relative ease of creating a compound when reacting with another element to form a compound, this often relates to how easy that element will lose or gain electrons. Scientists constructing groups of the periodic table were able to study how elements reacted with other elements and spot common similarities and differences. There are 18 groups in the periodic table spanning from left to right.
An example of this is the reaction of Group 1 metals with water. All group 1 metals will react with water to produce a hydroxide solution and hydrogen gas but progressing down the group 1 the reaction gets more violent as the outer electron (valence electron) is lost a lot more easily. this can be seen in the observations of the reaction. Lithium will react with water giving a small fizz and bubbling but when reacting caesium with water it reacts violently exploding and creating a lot of heat. It is these similarities in reactivity that have allowed scientists to group chemicals together into groups based on their chemical and physical similarities. This is in contrast the elements in Group 18 (Noble Gases) are relatively unreactive and are all gases. All the gases in group 18 are similar in properties being heavy, unreactive and having full outer shells. The unreactive nature of the group 18 elements is due to their inability to gain or lose electrons due to their full outer shell.
Through the work of Newlands and Mendeleev, the modern periodic table has very interesting patterns. Not just on reactivity and chemical reactions but also the structure of the electron shells, the presence of metals and non-metals and also their appearances. Before most modern techniques elements were often grouped together by their appearances only. The first example of this was the grouping of metals and non metals, the two largest categories of the periodic table. Most metals in their elemental state are shiny, silver and strong compared to non metals that are brittle, coloured and powdery substances. For example, the Group 18 gases whilst being unreactive and colourless all exhibit the property of being able to conduct an electric current which produces different colours based on this appearance they can be classified into the same group.
