what is the name of the outermost orbital of chlorine?

The number of electrons in the outermost orbit decides the chemical nature of the atom.

Chlorine (Cl) has Z=17

Electronic Configuration

Ionic bonds are formed when atoms physically exchange electrons to fill their outermost orbitals, forming ions in the process. For example, an atom of sodium (atomic number 11) has only one electron in its outermost orbital, and so can’t hang onto it very tightly. By contrast, chlorine (atomic number 17) has 7 electrons in its outermost orbital, and will readily “steal” electrons from other atoms. (This is one reason why chlorine is so dangerous to living things – it can rip apart organic molecules in its eagerness to get electrons.) Bring a sodium atom and a chlorine atom close-enough, and the chlorine will pull the single electron out of the sodium’s outermost orbit and into its own outer orbital. This makes the sodium atom into a positively-charged ion (Na+) and the chlorine atom into a negatively-charged ion (Cl-). (The chlorine ion is known as “chloride.”) Because the positively-charged sodium ion and the negatively-charged chloride attract each other, they tend to stick together, and so they form the compound sodium chloride (NaCl).