Ion having highest hydration enthalpy among the given alkaline earth metal ions is:
Consider an A.P. $a_1,a_2,\ldots,a_n$; $a_1>0$. If $a_2-a_1=-\dfrac{3}{4}$, $a_n=\dfrac{1}{4}a_1$, and \[ \sum_{i=1}^{n} a_i=\frac{525}{2}, \] then $\sum_{i=1}^{17} a_i$ is equal to

Group one of alkali metals is s-block elements with just one electron in their s-orbital. They are are alkali metals. They are named so because of the alkaline nature of the hydroxides and oxides.
Alkali metals are characterized by one s-electron in the valence shell of their atoms.
Alkali metals have a corresponding [Noble gas] ns1 electronic configuration. They occupy the first column of the periodic table. Alkali elements are:
They have occupied successive periods from first to seven. Francium is a radioactive element with very low half-life.
This makes them the most electropositive elements and due to the same reason, they are not found in the pure state.