Ions and nuclide notation: explain how positive and negative ions form by losing or gaining electrons, and interpret nuclide symbols to count protons, neutrons and electrons.

What this topic is asking

The Core Curriculum asks you to explain how atoms form ions by losing or gaining electrons, to compare the particle counts of an atom and its ion, and to interpret nuclide notation ($^{A}_{Z}\text{X}$) to count protons, neutrons and electrons. This is heavily tested in Part A and Part B-2 because it ties together everything from the atomic-structure pages.

How ions form

Metals, which have low ionization energies, tend to lose their few valence electrons to form cations: sodium ($2\text{-}8\text{-}1$) loses one electron to become $\text{Na}^{+}$ ($2\text{-}8$). Nonmetals, which are highly electronegative, tend to gain electrons to fill their outer level and form anions: chlorine ($2\text{-}8\text{-}7$) gains one electron to become $\text{Cl}^{-}$ ($2\text{-}8\text{-}8$). In both cases the ion often achieves a stable, noble-gas-like full outer level, which is why the charges are predictable from the group.

What stays the same and what changes

This is the most common Part A comparison: an ion has the same protons and neutrons as the atom it came from, and a different number of electrons. A charge of $2+$ means two electrons fewer than protons; a charge of $2-$ means two electrons more than protons.

Reading nuclide notation

Nuclide notation packs three pieces of information around the element symbol:

where $A$ is the mass number (top left), $Z$ is the atomic number (bottom left), $\text{X}$ is the element symbol, and $c$ is any ionic charge (top right). From this:

• protons $= Z$;
• neutrons $= A - Z$;
• electrons $= Z - c$ (subtract the charge, treating a $2-$ charge as $c = -2$, which adds electrons).

Hyphen notation such as carbon-14 gives the same mass number ($14$) without writing the atomic number, which you then look up on the Periodic Table.

Try this

Q1. Write the number of electrons in a $\text{Mg}^{2+}$ ion (magnesium has $12$ protons). [1 point]

• Cue. $12 - 2 = 10$ electrons.

Q2. Explain why a fluoride ion, $\text{F}^{-}$, has the same number of protons as a fluorine atom. [1 point]

• Cue. Forming an ion only adds or removes electrons; the proton count is fixed, so it stays fluorine.