Which of the following is a characteristic property of the alkaline earth
metals?
O
A. They are more reactive than alkali metals.
O
B. They are all radioactive.
O
C. They are harder than alkali metals.
O
D. They all have 3 valence electrons.

"They are harder than alkali metals" is a characteristic property of the alkaline earth  metals.

Option: C

Explanation:

The strength of metallic bond is stronger in alkaline earth metals than alkali metals because of the existence of 2 electrons in its outermost or valence shell relative to alkali metal which has only 1 electron in valence shell. Thus alkaline earth metals are therefore harder than the alkali metals.

They are considered alkaline since they formulate solutions that have a pH higher than 7, rendering them alkaline or bases. Radium is produced by uranium decay, not all alkaline elements are radioactive. It's very toxic, and the handling is risky. Calcium and magnesium are essential for animals and plants.