Elements tend to have attributes that are not unique. Many element share properties with each other and make groups in the periodic table. The groups are to be found in the interactive PTE and the most known group would probably the Carbon group.
Carbon, Silicon and Germanium are the first 3 of the Carbon group and they are more or less similar in chemical behaviour. This is reflected for example in the valence number where all these Carbon group element tend to have a +2/+4 configuration. See "The Carbon group"
Another similarity we can point out is the fact that the further to the right we go in the PTE the smaller elements tend to become. The largest elements are actually on the left side represented by the Alkali metals. So despite the fact that an element grows from Sodium 23 to Argon 36 in the 3rd row the Argon is smaller then the Sodium. When we look for an explanation we can see that the atoms become more and more complete the further we go to the right side of the PTE and the Alkali metals have an active "protruding outward" active nuclet. this results in the fact that the elements become smaller when growing in size (atomic weight)
The similarity in the groups are caused by the similarity of the nuclets, meaning a particular group has mostly similar active nuclets, So the Lithium group has an active Lithium nuclet and the Carbon group an active Carbon nuclet. This similarity and growth pattern means that the periodicity of the PTE is 'created'. See "The cycle of eight"
To complete a new branch on the atomic tree we need to add 15 in number to return to a similar configuration and therefore have the next element that has similar properties. The heavier element is bigger and has more protons and neutrons, but the active nuclet is what dictates many properties and they shine through in the whole. So chemical behavior is similar for similar (group) elements yet they are much heavier.