These notes explain how atoms become charged particles by losing or gaining electrons, and how to represent those changes clearly using electronic configurations and dot-and-cross diagrams.

Ion formation

Atoms are neutral because they contain equal numbers of protons and electrons. During chemical change, the number of protons in the nucleus stays the same, but electrons can be transferred. If electrons are lost or gained, the atom becomes an ion.

An atom that loses electrons forms a positive ion, also called a cation. This is because it then has more protons than electrons. An atom that gains electrons forms a negative ion, also called an anion, because it then has more electrons than protons.

For the elements usually met at this level, ions form so that the outer shell becomes full. This often gives the ion the same electronic configuration as the nearest noble gas. The important point is that only electrons are transferred; the identity of the element does not change.

Common patterns include:

• Group 1 atoms lose 1 electron to form 1+ ions.

• Group 2 atoms lose 2 electrons to form 2+ ions.

• Group 6 atoms gain 2 electrons to form 2− ions.

• Group 7 atoms gain 1 electron to form 1− ions.

For example:

Electron-transfer diagram for the formation of NaCl, showing a sodium atom losing one electron to become $\mathrm{Na^+}$ and a chlorine atom gaining that electron to become $\mathrm{Cl^-}$. This makes the link between electron transfer and ion charge explicit, supporting the octet/noble-gas-configuration explanation. Source

• sodium: 2,8,1 becomes Na+ with configuration 2,8

• magnesium: 2,8,2 becomes Mg2+ with configuration 2,8

• chlorine: 2,8,7 becomes Cl− with configuration 2,8,8

• oxygen: 2,6 becomes O2− with configuration 2,8

Why the charge appears

The charge on an ion depends on the balance between protons and electrons. Losing electrons leaves an overall positive charge. Gaining electrons produces an overall negative charge. A 2+ ion has lost two electrons, while a 3− ion has gained three electrons.

This means you should always connect the charge to the number of electrons transferred, not to the number of shells or the position of the atom alone.

Electronic configurations of ions

When writing the electronic configuration of an ion, start with the neutral atom and then change the number of outer-shell electrons.

A reliable method is:

• write the electronic configuration of the neutral atom

• decide whether electrons are lost or gained

• remove electrons from, or add electrons to, the outer shell

• write the final electronic configuration of the ion

• add the ion charge to the formula

For cations, the outer shell may disappear completely after electrons are lost. Sodium is a good example: the neutral atom is 2,8,1, but Na+ is 2,8 because the third shell no longer contains any electrons.

For anions, electrons are added to the existing outer shell until it is full. Chlorine changes from 2,8,7 to Cl− with 2,8,8. The nucleus is still chlorine because the proton number has not changed.

What exam questions are testing

Questions on ion formation usually check whether you can do two linked things:

• identify whether electrons are lost or gained

• write the correct electronic configuration after the change

Examiners often include ions with charges greater than 1. You should be comfortable with ions such as Mg2+, O2−, Al3+, and N3−, where more than one electron is transferred.

Dot-and-cross diagrams

A dot-and-cross diagram helps show where electrons came from when ions are formed.

Dots and crosses do not represent different kinds of electrons. They are simply a way of tracking electron transfer. One atom’s electrons may be drawn as dots and the other atom’s as crosses.

When drawing ions in this style:

• put each ion in square brackets

• write the charge outside the bracket, usually at the top right

• show the correct number of electrons in the outer shell

• for anions, include the gained electrons using the symbol from the atom that supplied them

• for cations, show the electron arrangement after electrons have been removed

In a sodium chloride diagram, Na+ is shown in brackets with a + charge and no electron in its former third shell. Cl− is shown in brackets with a − charge and eight outer-shell electrons. Seven belong to chlorine originally, and the eighth comes from sodium, so one symbol is different.

Schematic of the sodium chloride crystal lattice, showing alternating $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ ions arranged in a regular 3D structure. This reinforces that ionic bonding produces an extended lattice of repeating ions rather than discrete NaCl molecules. Source

In a magnesium oxide diagram, Mg2+ has lost two electrons and is shown as 2,8 inside brackets with a 2+ charge. O2− has gained two electrons, so its outer shell contains eight electrons, with two marked as having come from magnesium.

Full diagrams and clear layout

Some questions ask for a full electronic configuration diagram, while others only need the outer-shell electrons. Read the command word carefully. If the question says draw a dot-and-cross diagram of the ions, make sure the electron arrangement and charges are both visible.

A neat layout helps avoid mistakes:

• keep electrons spaced clearly around the ion

• make the different symbols easy to distinguish

• do not forget brackets around both the cation and the anion

• do not place the charge inside the bracket

Avoiding common mistakes

Several errors appear frequently in this topic:

• writing a positive ion as if it had gained electrons

• forgetting that electron loss happens from the outer shell

• drawing an anion with the wrong total number of outer electrons

• missing out square brackets

• writing the charge but not changing the electron arrangement

• using dots and crosses as if they changed the charge by themselves

Another common mistake is confusing atom diagrams with ion diagrams. A neutral atom does not need a charge in brackets, but an ion does. In diagrams containing more than one ion, each ion should be shown separately with its own bracket and charge.