Learn English – Opposite of “excess” in this context

antonyms

What would be the appropriate word opposite to "excess" in the following sentence?

The excess number of particles in the box is negative.

I want to write about a depletion of particles rather than an excess. Something like:

The depletion number of particles in the box is positive.

"Depletion number" does not sound right to me.

The context is an advanced physics text, where this "excess number" or "depletion number" is represented by a variable (an integer) that appears in mathematical formulas.

Best Answer

At time t0, the container of gas is electrically neutral, but at time tn, there is a surplus of negatively charged gas molecules, which is to say that there is a comparative scarcity of positively charged gas molecules.

I've tried to pick neutral words that simply describe some obtained valency. My first choices were "excess" and "shortage," but these seem to imply a deviation from some "proper" ratio.