Ionic bonding only happens between a non metal and a metal.
Metallic bonding occurs between two metals.
Covalent bonding occurs between two non metals.
All of those atoms are non metals hence its covalent bonding.
I'll explain the reasons.
Metals are in lower groups, like group one and two. In order to obtain a full outer shell of electrons, it must loose electrons.
Non metals are in higher groups, like group 6, or 7. So it must gain electrons to reach a full outer shell of 8.
Electrons are negatively charged, so when a metal looses an electron, it becomes positively charged ion..
When a non metal gains an electron, it becomes a negatively charged ion.
The two forces attract each other and ionic bonding occurs.
Two substances need to gain electrons. All non metals want to gain electrons.
So they share them.