Question

How to calculate the number of electrons gained or lost?

Answer 1

The number of electrons which are gained or lost depends on the initial information given.
Essentially, electrons are extremely small negatively-charged particles that orbit the nucleus of an element (or ion). Elements will usually try to stay neutral by having their protons (positively charged) in the same quantity as their electrons (negatively charged). When an electron is lost, the charge shifts and the element (called an ion when it has lost or gained electrons) becomes positive. The exact opposite is true, where the gaining of one or more electrons makes the overall charge negative.
Finding the number of electrons that you have gained or lost first requires knowledge of how many protons the element started with (the number of protons is equal to the atomic number of the element). For example, if we start with neon (ten protons), we can assume that the electron number is also ten. There are a few ways to tell the number of electrons that were gained or lost.
In notation, neon would read as Ne. If this symbol reads as \(Ne^{-1}|), Neon has gained an electron. If this symbol reads as \(Ne^{-2}|), Neon has gained two electrons (and so on).
In notation, if the symbol for Neon reads as \(Ne^{+1}|), Neon has lost an electron. If this symbol reads as \(Ne^{+2}|), Neon has lost two electrons (and so on).