Atom is derived from?

cations have lesser number of shells and higher effective nuclear charge and so they are smaller than that of its neutral atom.

No, they're made up of protons, neutrons, and electrons.

Atom means indivisible in Greek so they named the atom atom because it is the smallest form (indivisible)

the outermost electrons of an atom. these are called valence electrons. atoms are also grouped in the periodic table based on their valence electrons. to complete a full outermost shell there must be...

it is its electrons, if an atom of an element is not stable which means its outershell is not full of electron is going to react more than an atom which has full shells... e.g.. take the noble gases...