The equilibrium-constant expression for a reaction written in one direction is the of the one for the reaction written for the reverse direction.

Explanation:

The equilibrium constant for an equilibrium reaction is the ratio of the equilibrium constant for the forward reaction, Kf, to the equilibrium constant for the reverse reaction, Kr:

When you write the reaction in the other reaction, the forward and the reverse reaction are exchanged:

As you see:

Thus, the equilibrium-constant expression for a reaction written in one direction is the    reciprocal___ of the one for the reaction written for the reverse direction.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.  

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