Why does the amount of protons in an element wildly change its properties? For example: why is two protons a gas, but suddenly three protons is a metallic solid?
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(Note: for this explanation I’m going to personify atoms and molecules, which is obviously inaccurate, but makes for an easier to understand explanation)
For non-ELI5 reasons, there are certain numbers of electrons that atoms like to have: 0, 2, 10, 18, 36, 54, 86, 118…. Also, opposite charges attract and same charges repel. These two facts are very powerful for understanding some of the “why”s of chemistry.
Helium with its 2 protons and 2 electrons has both a happy number of electrons, and is electrically neutral, so it really feels no need to chemically interact with anything. In fact, it would rather not interact with anything. All it wants to do is be free and bounce around. This libertarian attitude makes it a gas.
Lithium has 3 protons and 3 electrons. It’s really close to having a nice happy 2 electrons, so it really wants to get rid of its third electron. If one, lone lithium atom tried to throw its electron off, it would just come back because the atom would become positively charged so it would attract the electron back to it. But in a whole hunk of lithium it can instead toss the electron to one of its neighbors. But that lithium doesn’t want the electron either so you get this game of hot-potato on a massive scale with all these extra electrons. If one of the lithium atoms tries to get clever and leave the game when it has a happy number of electrons, its positive charge will make its neighbors hold on to their electrons more, and their negative charge will pull the positively charged cheater back in. But if something else gets close, the lithium will try to react with it to break this unhappy game.
Another good example of all of this is table salt! Sodium has one more than a happy number of electrons and chlorine has one too few. On their own, they’re both really dangerous because they really want to react with something to get to a happy number, but if that “something” is *you* they’ll be wreaking havoc on your molecules. But, if you combine them with each other, the sodium tosses its spare electron to the chlorine so they can both have a happy number. Then, those positive sodium atoms and negative chlorine atoms nestle up nice and close in an alternating 3-D grid since they’re attracting each other, but not themselves. This makes a solid with a regular pattern (AKA, a crystal) where each of the atoms is happy, so it’s safe for humans to touch. But that 3-D grid isn’t like, TIED together, so lots of things – for example water – can pull the sodiums and chlorines apart without changing their number of electrons. This is why salt dissolves in water.
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