Question

What is the formula for potassium phosphide?

Answer 1

Hint: Using the formula of potassium cation and phosphide anion we can deduce the chemical formula of potassium phosphide by applying criss-cross method.

Complete step by step answer:
Potassium (\[{\text{K}}\]) has only one electron in its outer shell. Thus, it prefers to donate this electron to attain noble gas configuration. In this process, it becomes \[{{\text{K}}^ + }\]cation.
Phosphorous (\[{\text{P}}\]) has five electrons in its outer shell. When it makes bond with a strong electropositive element like \[{\text{K}}\], it prefers to gain three more electrons to attain noble gas configuration. In this process, it becomes \[{{\text{P}}^{3 - }}\]anion. To gain three electrons, it will make ionic bonds with three potassium atoms.
Criss-cross method to write chemical formula of an ionic compound:
First write the cation and then anion side by side as: \[{{\text{K}}^ + }\]\[{{\text{P}}^{3 - }}\]
Divide their valencies by H.C.F. In this case, it is \[1\], thus, the valencies remain the same.
Now by criss crossing the valencies, we get the formula of compound as: \[
  {{\text{K}}^ + } \searrow {\text{ }} \swarrow {{\text{P}}^{3 - }} \\
  {\text{ }}{{\text{K}}_3}{\text{P}} \\
 \]

Hence, the formula for potassium phosphide is \[{{\text{K}}_3}{\text{P}}\].

Additional Information:
Few important binary (with only two elements) mono-phosphides (absence of \[{\text{P}} - {\text{P}}\]bonds) are formed by metals like sodium (\[{\text{N}}{{\text{a}}_3}{\text{P}}\]), calcium (\[{\text{C}}{{\text{a}}_3}{{\text{P}}_2}\]), magnesium (\[{\text{M}}{{\text{g}}_3}{{\text{P}}_2}\]), aluminium (\[{\text{AlP}}\]), gallium (\[{\text{GaP}}\]), indium (\[{\text{InP}}\]) and copper (\[{{\text{K}}_3}{\text{P}}\]).
Polyphosphides contain \[{\text{P}} - {\text{P}}\]bonds. The simplest one is \[{{\text{P}}_2}^{4 - }\]. Potassium forms nine phosphides out of which few are polyphosphides.

Note: Few important binary (with only two elements) mono-phosphides (absence of \[{\text{P}} - {\text{P}}\]bonds) are formed by metals like sodium (\[{\text{N}}{{\text{a}}_3}{\text{P}}\]), calcium (\[{\text{C}}{{\text{a}}_3}{{\text{P}}_2}\]), magnesium (\[{\text{M}}{{\text{g}}_3}{{\text{P}}_2}\]), aluminium (\[{\text{AlP}}\]), gallium (\[{\text{GaP}}\]), indium (\[{\text{InP}}\]) and copper (\[{{\text{K}}_3}{\text{P}}\]).
Polyphosphides contain \[{\text{P}} - {\text{P}}\]bonds. The simplest one is \[{{\text{P}}_2}^{4 - }\]. Potassium forms nine phosphides out of which few are polyphosphides.