What does the size of an atom mainly depend on?
Number of protons
An atom’s size is set by how far the outermost electron orbit lies from the nucleus.
The number of protons sets the identity of the element and the atomic number, but as protons rise the stronger nuclear pull also tends to shrink the electron cloud; however among the options, the number of protons is the main factor fixing the atomic radius trend across elements.
Isotopes of the same element differ only in neutrons and have practically the same size, so neutron count is not the main factor.
Within a period, size falls as protons rise (greater pull); within a group, size rises as new electron shells are added. In both trends it is the proton count change that drives the systematic variation.
Hence, Number of protons.
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