Covalent Bond: In covalent bonding electrons are shared between atoms rather than traded in order for the atoms of both elements to gain full outer shells. Electrons are always shared in pairs.

Ionic Bond: Ionic bonding occurs when different elements trade electrons such that both elements now have a full outer shell. 

means that in ionic bond, the element for example Cl ion, have 7 valence electron, right? so the Cl ion need 1 more electron to make it octet. so the Cl ion takes 1 from other elements.
for covalent bond, the elements share the electron between them. so they dont need to take the electron for themselves
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