To answer this, one needs to understand the difference between ionic bonding and covalent bonding. An ionic bond is a bond between an electronegative atom and an electropositive atom in which one atom supplies the valence electrons for the bond and the other uses those electrons to become stable. A covalent bond is a bond between two atoms of similar electrical charge in which both atoms supply a valence electron to form the bond and the electron travels between each atom in the bond.
An easy way to remember these is that an ionic bond is a bond between an atom on the upper right side of the periodic table with an atom on the left side of the periodic table. A covalent bond is a bond between an atom on the upper right side of the periodic table with another atom from the upper right side of the table.
Atoms on the left side don’t really bond with other left-side atoms because they don’t have the negative pull that right-side atoms do. Atoms want to become stable. In order to become stable, they need to have a full valence shell of electrons (that’s 8 electrons). An atom like chlorine has 7 valence electrons, so it REALLY want to bond with something more positive that wouldn’t mind giving up an electron (hence why we have ionic compounds like salt (NaCl)). Because the elements on the left side of the table are electropositive, they don’t have that drive to fill their valence shell, so they won’t bond with other electropositive atoms.
So these molecules that you listed above are covalent because each atom donates an electron to create a bond between atoms. You could also say that they are all elements of the right side of the table, so they can only be covalent.
