Losing Electrons Does Not Mean Significant Mass Loss

What students typically write

“When sodium loses an electron it gets lighter, so Na+ has a smaller mass than Na.” Or: “Chlorine gains an electron so Cl- is heavier than Cl.”

Students reason that if you add or remove something, the mass must change noticeably. This logic works for macroscopic objects but misleads at the subatomic scale.

Why it is wrong

An electron has a mass of approximately 1/1836 of a proton. For a sodium atom with a relative atomic mass of 23 (11 protons + 12 neutrons), losing one electron changes the mass by about 0.03%. That change is far too small to affect any calculation at IGCSE level.

Relative atomic mass (Ar) and relative formula mass (Mr) are defined using protons and neutrons only. The mass of electrons is ignored. This means Na (Ar = 23) and Na+ (also effectively 23) are treated as having the same mass in every stoichiometry calculation.

The important change when an atom becomes an ion is in charge, not mass. Na loses one electron and becomes Na+ (charge changes from 0 to +1). The number of protons stays at 11, the number of neutrons stays at 12, and the number of electrons drops from 11 to 10. That is what examiners test.

What the mark scheme actually wants

A typical question: “A sodium atom forms a sodium ion. State what happens to the number of protons, neutrons and electrons.” [3]

Marking points:

1. Number of protons stays the same / 11 (1 mark)
2. Number of neutrons stays the same / 12 (1 mark)
3. Number of electrons decreases by 1 / from 11 to 10 (1 mark)

No mark is available for mass change because the mark scheme treats the mass as unchanged.

Worked example: wrong vs right

Question: Explain the difference between a magnesium atom and a magnesium ion, Mg2+. [2]

Wrong answer: “The magnesium ion is lighter because it has lost two electrons, so it has less mass.” This scores 0/2. The answer focuses on mass (which the mark scheme ignores at this level) and fails to state the key facts about electron number and charge.

Right answer: “A magnesium atom has 12 electrons, but a Mg2+ ion has only 10 electrons (1). The ion has a charge of 2+ because it has two more protons than electrons (1).” This scores 2/2.

How to avoid this mistake

When comparing atoms and ions, focus on three quantities: protons (unchanged), neutrons (unchanged), and electrons (changed). Never discuss mass change when describing ionisation at IGCSE level.

If a question asks about mass in the context of ions, it is asking about relative formula mass of an ionic compound (calculated from Ar values of the elements), not about the mass difference caused by electron transfer.

The simple rule: ionisation changes charge, not mass.

For the full treatment of atomic structure and ion formation, see atomic structure and ions and ionic bonds.