* Group 1: This refers to the alkali metals, which are found in the first column of the periodic table.

* Higher atomic number than chlorine: Chlorine (Cl) has an atomic number of 17. We need an element with a higher number.

* Lower atomic number than bromine: Bromine (Br) has an atomic number of 35. We need an element with a lower number.

The only alkali metal that fits these criteria is Potassium (K), which has an atomic number of 19.