From the following elements: $4 \text{Be}; 9 \text{F}; 19 \text{K}; 20 \text{Ca}$ (i) Select the element having one electron in the outermost shell. (ii) two elements of the same group. Write the formula of and mention the nature of the compound formed by the union of 19K and element X(2,8,7).

The chemical reactivity and bonding behavior of elements are determined by their valence shell configurations. Potassium ($_{19}\text{K}$), an alkali metal, readily loses its single valence electron to achieve a stable noble gas configuration ($2, 8, 8$). Element $\text{X}$ ($2, 8, 7$), which is Chlorine ($_{17}\text{Cl}$), is a highly electronegative halogen that requires one electron to complete its octet. The transfer of an electron from $\text{K}$ to $\text{X}$ forms $\text{K}^+$ and $\text{X}^-$ ions, which are held together by strong electrostatic forces of attraction, forming the ionic compound $\text{KX}$ (Potassium Chloride).