MIT OCW Biology
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Covalent bond is the strongest bond formed by the sharing of electrons..
They belong to what is called intramolecular forces, means they act inside a molecule. In contrast, intermolecular forces like van der Waals act beetwen molecules (or in some cases like protein folding beetwen different parts of one molecule). Every atom "wants" a certain number of electrons. By sharing them, they get to this number. Example: H "wants" two electrons, but has only one. Now two H-atoms form H2 and share their electrons. Thus every atom "feels like" is has two electrons and is "happy".
It is a bond formed by sharing electrons between 2 atoms and if the 2 atoms which involved in the formation of bond has an electronegativity difference lesser than 1.7, we can consider those bonds as covalent bonds.