There's a detailed explanation here: https://chemistry.stackexchange.com/questions/115363/is-a-hy...
Edit: Oops. Still relevant since it's H2 in play here, but yes, not hyrdrogen vs helium.
Edit: Oops. Still relevant since it's H2 in play here, but yes, not hyrdrogen vs helium.
There's a free textbook available here [1] that explains that atomic sizes decrease across the periodic table and increase down. In short, it decreases across a horizontal period because more electrons are in the same orbital shell (really a cloud), increasing the negative nuclear charge of the cloud, which is more attractive to the positive atomic nucleus, so the whole cloud is squeezed together, making the whole atom smaller.
[1]: https://chem.libretexts.org/Bookshelves/General_Chemistry/Bo...
Your definition would make sense to me if you can think of electrons adding to an over all single electrical charge instead of individual electrons, but in that case, I have to ask, what is the original reason a single electron is repulsed by the atom. I.E. why does it stay so far away until more electrons are added. What is the force keeping electrons from falling into the nucleus in the first place and have an "orbital" or "cloud" at its set distance (like one Angstrom or Bohr)?