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
Lithium and helium aren’t right next to each other though. Lithium is on the far left of the second row, and helium is on the far right of the first row. Carbon is a solid at standard temperature and pressure, not a gas.
One of the trends of the periodic table is that it’s laid out in a way that easily shows how many electrons are in its outer shell (electrons are situated in shells, kind of like layers, and each shell can only hold up to a certain number). An element on the far left will only have one electron in its outermost shell, while one on the far right will be completely full.
The number of electrons in the outer shell is one of the determining factors in chemical and physical properties of that element. So while lithium and helium may only be one proton apart, they’re still quite different in regards to their properties. Helium won’t undergo any bonding with other atoms, including other helium atoms, via electrons because its outer shell is already full, while lithium can pretty easily undergo ionic bonding (if you had ionic lithium)
Another big factor is going to be the atomic radius, which is also laid out nicely on the period table. The further down a column you go, the bigger, the further right in a row, the smaller. The bigger the individual atom, the more potential interactions for noncovalent bonding you can get – basically it’s kind of like they get stuck together, and the bigger they are, the more likely they are to bump into each other to get stuck in the first place. If you’re a tiny atom, it’s not as likely that you’ll be bumping into other really tiny moving atoms, and the attractive energy would have to be stronger than the kinetic energy. This helps explain why one element in a column might be a liquid, while one right under it is a solid (like bromine vs iodine).
Many cases are going to be a combination of these factors and other associated details.
I rewrote this a few times to try and keep it ELI5.
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