Which of the following is not an alkali metal?

Alkali metals belong to Group 1 of the periodic table and include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). These metals are characterized by one valence electron, high reactivity, softness, and the formation of +1 ions.

Radium (Ra), on the other hand, is an alkaline earth metal found in Group 2. It has two valence electrons and forms Ra²⁺ ions, which behave differently chemically from alkali metals. While alkali metals react vigorously with water to form hydroxides and hydrogen gas, alkaline earth metals like radium are less reactive and form different types of compounds, such as Ra(OH)₂.

Other options—francium, caesium, and rubidium—are all alkali metals. Francium is radioactive, caesium is soft and highly reactive, and rubidium is also reactive with water and air. They share Group 1 properties, which radium does not.

Understanding which elements are alkali metals is a key concept in inorganic chemistry. Group trends explain similarity in chemical behavior, while differences in groups result in distinct reactivity and properties.

In summary, radium (Ra) is not an alkali metal, whereas francium, caesium, and rubidium belong to Group 1. This distinction is essential for predicting chemical reactions and understanding the periodic table.