Group 1 elements are reactive metals, and Group 7 elements are reactive nonmetals. While both groups are reactive, and precautions must be taken when attempting any reaction with either, the actual reactions and the products formed by the groups are characteristically different.
An observation of the new element’s physical state may serve to determine if it is a metal or a nonmetal: Group 1 metals are all solid, while nonmetals may be solid, liquid, or gas. If it is a liquid or gas, and our only choices are Group 1 and Group 7, then it must be in Group 7.
The “chemistry” of an element usually means the various reactions it undergoes. Group 1 metals will react vigorously with nonmetals in general, including oxygen, sulfur, and the Group 7 elements. They react with water to yield hydrogen gas and a solution of a metal hydroxide. If bubbles are seen, a gas is being formed, and it can be collected and tested with a flaming wood splint. If the characteristic “squeaky pop” of burning hydrogen is heard, then the gas produced is hydrogen. Metal hydroxide solutions are basic, so a test of the remaining solution with red litmus paper should detect formation of a base, indicating that the unknown is in Group 1.
Nonmetals react with metal, of course, and the known Group 7 elements all react dramatically with aluminum. They also dissolve in water, reacting to give an acid solution. A test with blue litmus paper will detect the formation of an acid.
A solid that reacts vigorously with known nonmetals and with water (when it forms a basic solution) tells us we have a Group 1 metal. A vigorous reaction with aluminum, and reaction with water to form an acid, indicates a Group 7 nonmetal.
I have attached links to short videos showing some of the characteristic reactions of Group 1 and Group 7 elements.