Can some one to understand this problem? ?

mercury, magnesium, water and gasoline.

Explanation:

Density:

Density is equal to the mass of substance divided by its volume.

Units:

SI unit of density is Kg/m3.

Other units are given below,

g/cm3, g/mL , kg/L

The liquid with higher density will sink in bottom while the liquid with lower density will float.

In given samples,

density of water = 1 g/mL

density of mercury = 13.6 g/mL

density of gasoline = 0.70 g/mL

density of magnesium = 1.7 g/mL

So on the basis of given density values mercury will sink in bottom then above it magnesium will present than water and above on these three gasoline will present.

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.