As in: Helium is a gas but Lithium is a metal, even though they’re right next to each other on the Periodic table.
Or: Carbon is a gas but Silicon is a mineral, even though their chemical behavior is exceedingly similar due to their similar position (on different rows) of the table?
In: Chemistry
Well for starters, pure carbon isn’t a gas, it’s most common forms (allotropes) are graphite and diamond which are both crystalline like silicon.
As for why similar atomic numbered elements behave differently, i.e. Lithium and Helium the increasing number of electrons (in line with atomic number) is the biggest determinant of physical structure and state.
For example, the noble gases are grouped vertically because in that position of the periodic table they all share the same property, that their outer shell (valence) electrons are completely full. E.g. Helium has two electrons which fills the first shell, Neon has ten, two to fill the first shell and eight for the second.
As such the periodic table is mostly referred to by groups which form the columns because this (for the most part) is based on how many outer shell electrons they have which determines their bonding behaviour both with other elements and with atoms of their own element. These bonds have varying strengths with weaker strengths tending to produce gases and liquids and solids ever increasing in strength.
Tldr: Elements next to each other have different numbers of electrons in their outer shell whereas columns share this number and are therefore more similar in properties such as being solid, liquid or gas.
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