Easier … more seductive …
It’s based on the count of ions (OH¯, H₃O⁺) and 7 happens to be the neutral spot. It was once believed that one can’t go beyond 0 .. 14, but I heard that some chemicals can be even more aggressive.
Usually we talk about acids containing water and then the value has a direct meaning – it’s useful for people dealing with it and good enough for normal people to not need a different scale.
It’s not arbitrary, it’s the value of pH that we measure in pure water.
pH describes the concentration of hydrogen ions (H+) dissolved in a water solution. A solution with a pH of 1 will have about 0.1 gram of H+ per liter (note that number is written with 1 zero in it), a solution with a pH of 4 will have 0.0001 grams of H+ per liter (4 zeros), and pH of 11 corresponds to 0.00000000001 grams of H+ (11 zeros).
Water molecules (H2O) have two hydrogen atoms and one oxygen atom, and they sometimes spontaneously break up into H+ and OH-. In pure water, it so happens that at any given moment about 0.0000001 grams of hydrogen are in the form of loose H+ ions, so pure water has a pH of 7.
* A note for pedants: I’m sweeping a bunch of minor technical details involving activity, concentration, and atomic weight under the rug in the name of ELI5.
pH is literally the negative log of the concentration of hydrogen (H+) in water at that condition. The thing is, even in basic conditions, you have a lot of it. 7 is only neutural because at that value (in water) it’s equally balanced by the amount of (OH-).
There’s a lesser used scale of pOH which uses an “inverted” value. And, pH can go outside of 0 and 14, since it’s a logarithmic expression of concentration, not an arbitrary scale.
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