Why do crystals (like snowflakes) form geometric patterns?
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Molecules will make straight patterns. Some naturally occuring elements will also form straight lines, Pyrite(Fe2S) is one of those things, it makes perfect cubes.
The reason being is how molecules from patterns. They are all either positively or negatively charged. And like magnets they are drawn to one another but only in a certain way. As positive only attracts negative and vise versa. Water or H2O has 2 hydrogen atoms (positively charged) and one oxygen atom (negatively charged) that’s why they attract each other but also other water molecules. And when there is a lot of them they’ll form a roster and thus creating such patterns.
Now as this is ELI5 it’s very basic and why atoms are negative or positive isn’t explained. There is a lot more to it at times. Glass for instance doesn’t have a nice molecular structure.
Because of the shapes of [water molecules](https://qph.cf2.quoracdn.net/main-qimg-3304fa98cc368625796eb95809b164c2) (H2O) themselves, and which parts are attracted to which other parts when you put them together, water molecules all [align themselves in a hexagonal geometric pattern](https://upload.wikimedia.org/wikipedia/commons/8/88/Ice_XI_View_along_c_axis.png) on the atomic level when they form a solid (ice). The shape of the whole snowflake is just a scaling-up of this same hexagonal structure as more and more H2Os join the crystal.
Think of it like LEGO bricks, they’re square and connect at 90 degree angles, so when you build with them the resulting shapes have 90 degree corners too (up close). Water molecules are like hexagonal LEGO bricks, they click together to form big crystals with the same 6-sided symmetry their tiny building blocks have.
While water forms a hexagonal scaffold when it solidifies, different substances (think different minerals for example) click together to form different shapes. Some make [cubic structures](https://upload.wikimedia.org/wikipedia/commons/f/fd/Pyrite_crystal_structure.png) (like [pyrite](https://hips.hearstapps.com/hmg-prod.s3.amazonaws.com/images/pyrite-crystals-in-matrix-mineral-royalty-free-image-1596210717.jpg?crop=0.898xw:0.908xh;0.0697xw,0.0612xh&resize=320:*)), some make [octahedrons](https://upload.wikimedia.org/wikipedia/commons/thumb/b/b8/CaF2_extenda.jpg/250px-CaF2_extenda.jpg) (like [fluorite](https://crystal-information.com/wp-content/uploads/2015/08/fluorite-octahedron-example-3.jpg)), etc. The shape of the bulk crystal is a scaled-up version of the atomic arrangement of the tiny building blocks, which is determined by their own size, shape, and attractions/bonding.
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