Here's why:

* Cs represents the cesium ion (Cs⁺) which is a Group 1 alkali metal and always has a +1 charge.

* HCO₃ represents the hydrogen carbonate ion (HCO₃⁻), also known as the bicarbonate ion.

To form a neutral ionic compound, the charges must balance. Since cesium has a +1 charge and hydrogen carbonate has a -1 charge, they combine in a 1:1 ratio.