Dot and cross diagrams for ionic compounds

A dot and cross diagram shows what happens to electrons when atoms form ions. It uses different symbols for electrons from different atoms, so a transferred electron can be identified after it has moved from a metal atom to a non-metal atom.

The resulting ions are shown in square brackets, with their charges written outside. Electrons are drawn around each ion to show its completed outer shell.

Sodium chloride dot and cross diagram

A sodium atom has one electron in its outer shell. It transfers this electron to a chlorine atom, which has seven outer-shell electrons. Sodium becomes a positive ion, $\ce{Na+}$ , and chlorine becomes a negative ion, $\ce{Cl-}$ .

In the diagram below, crosses represent electrons originally from sodium and dots represent electrons originally from chlorine.

\left[\begin{array}{c} \times\ \times \\ \times\!\times\ \ce{Na}\ \times\!\times \\ \times\ \times \end{array}\right]^{+} \qquad \left[\begin{array}{c} \bullet\ \bullet \\ \bullet\!\bullet\ \ce{Cl}\ \bullet\!\times \\ \bullet\ \bullet \end{array}\right]^{-}

The cross among the chlorine ion’s seven dots is the electron transferred from sodium. The chlorine ion therefore has eight outer-shell electrons.

After sodium loses its outer electron, its previous inner shell becomes its outer shell. That shell also contains eight electrons, shown as crosses. The brackets and charges identify the two particles as ions; the diagram shows only their outer-shell electrons.

Ionic bond formation

Ionic bonding begins when electrons are transferred from a metal atom to a non-metal atom. This transfer forms ions: charged particles that attract each other.

The metal atom loses one or more electrons from its outer shell. It then has more protons than electrons, so it becomes a positive ion. The non-metal atom gains electrons. It then has more electrons than protons, so it becomes a negative ion.

An ionic bond is the strong electrostatic attraction between oppositely charged ions. The electron transfer forms the ions; the attraction between their opposite charges is the bond.

Electron transfer and noble-gas structure

At this level, the common pattern is:

Metals in Group 1 lose one electron; metals in Group 2 lose two.
Non-metals in Group 6 gain two electrons; non-metals in Group 7 gain one.

The resulting ions have the electronic structure of a noble gas: their outer electron shell is full.

For example, a sodium atom transfers its one outer-shell electron to a chlorine atom. Sodium becomes a positive ion, $\ce{Na+}$ , and chlorine becomes a negative ion, $\ce{Cl-}$ . Both ions have noble-gas electronic structures, and the electrostatic attraction between their opposite charges forms an ionic bond.

Predicting ion charges from group numbers

For these main-group elements, the group number tells you how many electrons an atom has in its outer shell. An atom can lose or gain electrons to form an ion with the electronic structure of a noble gas: a full outer electron shell.

Electrons have a negative charge. Losing electrons therefore leaves an atom with a positive charge, while gaining electrons gives it a negative charge. The number of electrons lost or gained determines the size of the ion’s charge.

Predicting charges of metal and non-metal ions

Group 1 metals have one outer-shell electron. Each atom loses one electron to form an ion with a $$$+1$$$ charge.
Group 2 metals have two outer-shell electrons. Each atom loses two electrons to form an ion with a $$$+2$$$ charge.
Group 6 non-metals have six outer-shell electrons. Each atom gains two electrons to form an ion with a $$$-2$$$ charge.
Group 7 non-metals have seven outer-shell electrons. Each atom gains one electron to form an ion with a $$$-1$$$ charge.

Matching electrons lost and gained

When metal and non-metal ions form together, the numbers of atoms must allow the total number of electrons lost to equal the total number gained.

For example, a magnesium atom is in Group 2 and loses two electrons to form $\ce{Mg^{2+}}$ . A chlorine atom is in Group 7 and gains one electron to form $\ce{Cl^-}$ . Two chlorine atoms are therefore needed to gain the two electrons lost by one magnesium atom. The ions combine in a ratio of one $\ce{Mg^{2+}}$ ion to two $\ce{Cl^-}$ ions, giving magnesium chloride the formula $\ce{MgCl2}$ .