Forming ions

1. Atoms have no overall charge, but they can lose or gain electrons during reactions to form charged particles called ions.
2. Metal atoms have only a few outer-shell electrons, so they tend to lose electrons and form positive ions, called cations.
3. Non-metal atoms have nearly full outer shells, so they tend to gain electrons and form negative ions, called anions.
4. Losing or gaining electrons in this way leaves the ion with a full outer shell, the same electronic configuration as a noble gas.
5. A Group I atom loses its single outer-shell electron to form an ion with a charge of $1+$.
   1. A sodium atom, $2,8,1$, loses one electron to become a sodium ion, ${Na}^{+}$, with the configuration $2,8$.
6. A Group VII atom gains one electron to fill its outer shell and form an ion with a charge of $1-$.
   1. A chlorine atom, $2,8,7$, gains one electron to become a chloride ion, ${Cl}^{-}$, with the configuration $2,8,8$.
7. A negative ion made from a single non-metal atom has a name ending in -ide, so chlorine forms chloride, bromine forms bromide and fluorine forms fluoride.
8. The nucleus does not change when an ion forms, so a sodium ion still has 11 protons and is still sodium.

Ionic bonds

1. When a Group I metal reacts with a Group VII non-metal, one electron is transferred from each metal atom to a non-metal atom.
2. The transfer produces cations and anions, which have opposite charges.
3. Oppositely charged ions attract each other strongly, and this strong electrostatic attraction between them is the ionic bond.
4. A substance held together by ionic bonds is called an ionic compound.
5. Sodium chloride, $NaCl$, lithium fluoride, $LiF$, and potassium bromide, $KBr$, are all ionic compounds formed from Group I and Group VII elements.
6. Electron transfer creates the ions, but it is the attraction between the ions afterwards that forms the bond.

Dot-and-cross diagrams

1. A dot-and-cross diagram shows how electrons are transferred when an ionic bond forms.
2. The electrons from one atom are drawn as dots and the electrons from the other atom are drawn as crosses, so you can see where each electron came from.
3. To draw the bonding between a Group I and a Group VII element, follow four steps.
   1. Draw the Group I atom with one electron in its outer shell and the Group VII atom with seven electrons in its outer shell.
   2. Show the single outer electron of the Group I atom moving across to the outer shell of the Group VII atom, often with a curved arrow.
   3. Draw the Group VII ion with eight electrons in its outer shell, made up of seven of its own electrons and the one electron it gained.
   4. Put each ion in square brackets and write its charge at the top right outside the brackets.
4. In sodium chloride, the sodium ion is left with the configuration $2,8$ and the chloride ion has the configuration $2,8,8$, so both ions have full outer shells.
5. One electron is transferred from each sodium atom to each chlorine atom, so the ions combine in a $1:1$ ratio and the formula is $NaCl$.
6. The charges in the formula balance, because the $+1$ charge on each sodium ion cancels the $-1$ charge on each chloride ion.

Properties of ionic compounds

1. Ionic compounds have high melting points and high boiling points, so they are solids at room temperature.
   1. Sodium chloride melts at $801{}^{\circ }C$ and boils at $1413{}^{\circ }C$.
2. This is because the electrostatic attractions between the oppositely charged ions are strong, so a large amount of energy is needed to overcome them.
3. Ionic compounds are poor conductors of electricity when solid, because the ions are held in fixed positions and cannot move.
4. Ionic compounds are good conductors of electricity when molten or aqueous, because the ions are then free to move and carry charge.
5. Molten means melted into a liquid, and aqueous means dissolved in water, which is shown by the state symbol $(aq)$.