Carbon has 4 valence electrons, so it means that it can have a total of four (covalent) bonds, correct?
If that's the case, why can't carbon share four bonds, such as with another carbon atom (to form $\ce{C2}$, for example)?
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Sign up to join this communityCarbon has 4 valence electrons, so it means that it can have a total of four (covalent) bonds, correct?
If that's the case, why can't carbon share four bonds, such as with another carbon atom (to form $\ce{C2}$, for example)?
s orbitals have spherical symmetry, and p orbitals have a dumb-bell shape. When these orbitals overlap, then they form the bonding orbitals (and corresponding antibonding orbitals). The p-orbitals are aligned along Cartesian (xyz) axes. If all three p-orbitals overlap, then the s-orbitals can't get 'close enough' to overlap and form a fourth bond. If we start mixing s and p orbitals, the geometry always puts electron density away from the other bonding atom.
Concisely: there are no arrangements which allow overlap of atomic orbitals that create four bonding molecular orbitals with the proper orientation.
C2 as a molecule can exist, it will simply only have 3 bonds plus two unpaired electrons confined to each carbon.
If you want a more elaborate answer we have to unpack the heavy quantum mechanics, and you won't be seeing that until your junior year.