The alkaline earth metal with the lowest value of density is:
a.) Mg
b.) Be
c.) Sr
d.) Ca
e.) Ba

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Hint: The alkali earth metal group exhibits a rather strange trend in the order of their densities, with the element with the lowest density being a very ductile silvery metal that is essential to the production of Plaster of Paris. Bear this in mind before trying to answer this question.
Complete answer:
Let us first try to understand what the alkali earth metal group elements really are before trying to point out their element with the lowest value of density.
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The elements have very similar properties: they are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure.
Structurally, they have in common an outer s-orbital which is full; that is, this orbital contains its full complement of two electrons, which the alkaline earth metals readily lose to form cations with charge +2, and an oxidation state of +2.
Now, let us try to find out the element with the lowest value of its density
As with the alkali metals, the atomic and ionic radii of the alkaline earth metals increase smoothly from Be to Ba, and the ionization energies decrease. As we would expect, the first ionization energy of an alkaline earth metal, with an ns2 valence electron configuration, is always significantly greater than that of the alkali metal immediately preceding it.
The group 2 elements do exhibit some anomalies, however. For example, the density of Ca is less than that of Be and Mg, the two lightest members of the group, and Mg has the lowest melting and boiling points. Due to this anomaly in density, we observe that Ca can also float in water.
Thus, we can safely conclude that the answer to this question is d) Ca.
Calcium is a mineral that is necessary for life. In addition to building bones and keeping them healthy, calcium enables our blood to clot, our muscles to contract, and our heart to beat. About 99% of the calcium in our bodies is in our bones and teeth.

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