Answer
2K(s) +$Cl_{2}$(g) $\longrightarrow$ 2KCl (s)
Work Step by Step
First we find the elements that enter in this chemical reaction by utilizing the information provided and the periodic table of the elements.
Strategy to find these elements: We know that number of protons in nucleus is Atomic Number Z, and this number is provided on the top of the symbol, in the periodic table of elements.
The element that has 17 protons in the nucleus ( or Atomic Number Z = 17) is chlorine. As it is written that it is diatomic gaseous molecule, the chemical formula with the state symbol would be $Cl_{2}$(g).
The element with Atomic Number Z = 19 is K, potassium. The chemical formula with the state symbol would be K(s).
Now we can write the chemical equation, with the state symbols:
K(s) +$Cl_{2}$(g) $\longrightarrow$ KCl (s)
To balance this chemical reaction add 2 before KCl on the right, and 2 before K on the left.
2K(s) +$Cl_{2}$(g) $\longrightarrow$ 2KCl (s)
