Final answer:
The alkali metal family contains the most chemically reactive metals and these metals are always found in compounds, not freely in nature. Halogens, while also highly reactive nonmetals, similarly do not exist free in nature. For the listed elements, krypton is a noble gas, selenium is a chalcogen, barium is an alkaline earth metal, and lithium is an alkali metal.
Step-by-step explanation:
The most chemically reactive metals are found in the alkali metal family, which includes elements like lithium (Li), sodium (Na), and potassium (K). These elements belong to Group 1 on the periodic table. The alkali metals are incredibly reactive and therefore do not exist freely in nature; they are always found in compounds. For example, sodium reacts rapidly with water and oxygen in the air. In contrast, the halogens, which include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At), belong to Group 17 and are very reactive nonmetals. These halogens also do not exist as free elements in nature due to their high reactivity, particularly with metals to form salts such as sodium chloride.
Regarding your specific query about the group name for certain elements:
- Krypton falls into the noble gas family, Group 18.
- Selenium is a member of the chalcogen family, Group 16.
- Barium is an alkaline earth metal belonging to Group 2.
- Lithium is an alkali metal from Group 1.