An ionic bond can be formed when one or more electrons are(1) equally shared by two atoms(2) unequally shared by two atoms(3) transferred from the nucleus of one atom to the nucleus of another atom(4) transferred from the valence shell of one atom to the valence shell of another atom

Correct option 4 i.e. transferred from valence shell of atom atom to the valence shell of another atom.

Reason:
Ionic bonds are preferentially formed when there is large difference in electronegativity of bonding atoms.

Atom that is highy electronegative in nature tends to accept electron from electropositive element. Stability of both of these atoms increases because, during the process to exchange of electrons, they acquire the electronic configuration of nearest inert gas.

For example: In NaCl, Na donates the proton to Cl. In ionized form, Na acquires the electronic configuration of 1s2 2s2 2p6, while electronic configuration of Cl becomes 1s2 2s2 2p6 3s2 3p6. Thus, in present case, electron is transferred from valence shell of Na to valence shell of Cl

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