In a reversible reaction, equilibrium is reached when

The concentration of the reactants and products are equal

There is no product; all reactant

The concentration of the reactants and products remain constant

There is not reactant; all product

In a reversible reaction, equilibrium is reached when, the concentration of the reactants and products remain constant.

Explanation:

In a reversible reaction, equilibrium is reached when, the concentration of the reactants and products remain constant.A reversible reaction is a chemical reaction in which the reactants form products which then react together to give the reactants back.Reversible reactions has no ending, instead they just reach equilibrium.In a chemical reaction, chemical equilibrium is the state in which the forward reaction rate and the reverse reaction rate are equal. The result of this equilibrium is that the concentrations of the reactants and the products remain constant.Example of reversible reactionAmmonium chloride ⇌ ammonia + hydrogen chloride.A reversible reaction is a reaction in which the reactants are converted to products and the products are converted to reactants which occur simultaneously.

Explanation:

Atomic number means the sum of total number of protons present in an element.

Atomic number of fluorine is 9 whereas atomic number of sodium is 11.

Therefore, atomic number of F + atomic number of Na

                 = 9 + 11

                 = 20

It is known that calcium is the element that has atomic number 20. Its electronic distribution is 2, 8, 8, 2.

Thus, we can conclude that the atomic number of F plus the atomic number of Na equals the atomic number of element calcium.