As I mentioned before, one of the two main types of chemical bonding is covalent bonding. It occurs when pairs of electrons are shared by atoms. Atoms do this to gain stability which is gained by forming a full electron shell. The electrons they share are their outermost electrons: their valence electrons. Although they do not always share their atoms equally.
This type of bonding usually occurs with nonmetals, although it can happen between metals and nonmetals. And actually, molecules that have similar electronegativity have a higher chance of forming a covalent bond to acheive octet configuration.
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Covalent Bonds