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
I don’t have all the answers, but [ptable](https://www.ptable.com/#Property/BoilingPoint) lets you select properties to view on the periodic table. In particular, helium behaves very differently from lithium because it’s a noble gas, and you can see this in the far right column.
Their chemical behaviour isn’t very similar due to changing nucleus sizes, numbers of electron shells etc though, that makes the strength of the forces/bonds between atoms/molecules of each element different and therefore they’re in different states at the same temperature.
It also has to with how strong the attractive forces between the molecules are. Weaker forces means gas and you can go from there. Why the forces are the strength they are has to with certain factors like size and the number of electrons in their valence shell. However, very elements exist in their pure state naturally because generally it’s a higher energy state and much less stable than forming bonds with other atoms and producing chemical compounds.
Firstly, an elements state depends on its intermolecular bonding, which is the bonding between atoms in a molecule. Not to get into a large amount of details, metals form metallic bonds while non-metals will form covalent bonding. The difference between a gas, solid and liquid is the strength of the intermolecular bonding.
If there is a lack of any significant IMF between atoms, then the element will act as a gas. The amount of energy needed to break these bonds differ on the chemical structure and type of bonding’s present, if the amount of energy needed to break the bonds is very low, for example room temperature, the element will appear as a gas. If the melting/boiling temperature is very high, it will show as a solid etc.
So, Lithium atoms bond via metallic bonding, which is strong and requires relatively large amounts of energy to break the bonds and get it to change state. Helium atoms do not have any attraction between other molecules so do not need a large amount of energy to overcome IMF. Helium is only a solid at below freezing.
That’s the simplest way I can explain without getting into a large amount of details
Carbon and silicon is a different ballpark, and carbon is not a gas, it is a solid, You can have two forms of solid carbon, more known as Graphite and Diamond both with different structures and IMF which change its behaviour completely!
There’s a number of factors that determine the state of an element at room temperature.
By far the most important is the crystalline structure of the given element (or compound). Specifically, the force which keeps the cristal together, which is dictated by the valence electeons of an element, or the structure of a compound. Basically, the stronger the bond, the higher the melting point. Metals for example are bounded by the metallic bond, which is a very strong interaction, and thus, most metals are solid. Water uses Hidrogen bonds, which are much much weaker, so it is only a liquid. Helium is a gas, because only Van der Waals forces act between the atoms, which are very very weak. There are also others I have not mentioned (ionic and covalent bonds, dipole-dipole interactions, etc.)
The molar mass also plays a big role in the state. For example, Bromine is a liquid and not a gas, despite the fact that its crystal is held together by Van der Waals forces, just like Chlorine. That’s because Bromine is heavier, and has a bigger electron cloud, which can moph more than that of Chlorine. If You look just one period lower, You’ll find that Iodine is solid, for the same reason. Heavier atoms with larger radii.
When You are looking at larger molecules (tipically in organic chemistry) the actual shape of the molecule makes a difference too. It’s a bit more technical, but imagine You have two different molecules, made from only carbon and hidrogen both with roughly the same number of each. Only difference is that one is a long line, the other is a ball. Now, the line molecules would have more contact with eachother, thus the interaction would be stronger. This would make something like isobuthane have a lower melting point than butane, even though they are both made out of 4 carbon and 10 hidrogen atoms, just steuctured differently.
I think I listed the most relevant factors, if I left something out, or made a miatake, feel free to correct Me.
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.
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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