What is a terminal atom and why are hydrogen and the halogen elements considered terminal atoms?

Explanation:

Terminal atoms -

The atoms that are always present at the terminal position , because they have the capability of bonding via one single bond .

hence ,

Hydrogen atom is a terminal atom , because it can only form one single bond , since , hydrogen has only one electron in its shell , so the octet is completed by sharing that electron with another one electron , and forming a bond .

Therefore , hydrogen atom is considered to be a terminal atom .

Similarly , Halogens have 7 electrons , hence , to attain noble gas configuration , it require only one electron .Hence , it can form only one single bond .

Therefore ,  halogen atom is considered to be a terminal atom .

The activity series is shown in the attached picture. if the element is below the other, then you can displace the other. following the activity series as a guide, the reactions are as follows: nickel and aluminum chloride3 ni + 2 alcl₃  → 3 nicl₂ + 2 albarium and copper bromideba + cubr  →  no reaction because cu is less oxidizable than bapotassium nitrate and aluminum3 kno₃ + al  → al(no₃)₃ + 3 ksodium phosphate and zincnaso₄ + zn  → znso₄ + na