Ionic( cation and anion) size difference from neutral parent atom?

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Q: Why cation is smaller than its parent atom ?

An atom has equal number of positive protons and negative electrons. This atom becomes cation after losing one or more electrons.Due to this imbalance between negative charge electrons and positive charge protons. Protons in Nucleus exerts more force of attraction on electronic shells. As a result cation becomes smaller than its parent atom. for example if we take Na atom it has 11 electrons in shells and 11 protons in nucleus. To become cation, Na loses its one electron from outermost shell. So one electron become less than proton , so more force of attraction exerts on electrons and size of cation becomes small.

Q: Why anion is greater than its parent atom?

An atom has equal number of positive protons and negative electrons. Atom becomes anion after gaining one or more  electrons. Due to this imbalance between negative charge electrons and positive charge protons.Nuclear charge ( charge due to protons) exerts less attraction on electronic shell. As a result anion becomes larger than its parent atom. For example if we take Chlorine "Cl" atom it has 17 protons in nucleus and 17 electrons in  electronic shells. To become an anion it gains 1 electron in outer most shell. so number of electron increase by 1 than proton. so  nuclear charge attraction on electron becomes weak and anion becomes larger.

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