Atoms are the smallest known substance to man, the building blocks of everything, the atoms size usually depends on the electrons, the electrons are what really takes up space.
And my source ( Google ) says :
Because electrons are what take up space in atoms, the result is that the size of the biggest filled orbital determines the size of the atom or ion. Sizes of orbitals depend on the quantum numbers (n = 1, n = 3, etc.) and also on the effective nuclear charge.
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