The answer to this is 22, confirmed by gradpoint
fourth period
The third period is similar to the second, except the 3s and 3p sublevels are being filled. Because the 3d sublevel does not fill until after the 4s sublevel, the fourth period contains 18 elements, due to the 10 additional electrons that can be accommodated by the 3d orbitals.
Group 18 is the only group in periodic table in which all the elements are nonmetals. This group contains F, Cl, Br, I and At and also the other name of this group is halogen which means salt producer.
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