The alkali metals

1. The elements in Group I are called the alkali metals, and the three you need to know in detail are lithium, sodium and potassium.
2. Below potassium the group continues with rubidium, caesium and francium.
3. Every alkali metal atom has one electron in its outer shell, so lithium is $2,1$, sodium is $2,8,1$ and potassium is $2,8,8,1$.
4. In reactions, each atom loses this single outer electron to form an ion with a charge of $+1$, such as ${Li}^{+}$, ${Na}^{+}$ and $K^{+}$.
5. Because every member loses one electron in the same way, the alkali metals have similar chemical properties and form compounds with matching formulae, such as $LiCl$, $NaCl$ and $KCl$.
6. The alkali metals react so readily with oxygen and water vapour that they are stored under oil to keep the air away from them.

Soft metals with low densities

1. Lithium, sodium and potassium are relatively soft metals, so a small piece can be cut with a knife or scalpel.
2. Most everyday metals, such as iron and copper, are far too hard to cut in this way.
3. Of the three, lithium is the hardest to cut and potassium is the easiest.
4. A freshly cut surface is shiny, which is typical of a metal.
5. The shiny surface quickly turns dull as the metal reacts with oxygen and water vapour in the air, which is called tarnishing.
6. The alkali metals also have low densities for metals, and lithium, sodium and potassium are all less dense than water.

Trends down Group I

1. Melting point decreases down Group I.
   1. Lithium melts at $181{}^{\circ }C$, sodium at $98{}^{\circ }C$ and potassium at $63{}^{\circ }C$.
   2. The metallic bonds get weaker down the group, because the positive ions get larger and are held less strongly by the sea of delocalised electrons, so less energy is needed to melt the metal.
2. Density increases down Group I.
   1. Lithium has a density of $0.53{g/cm}^3$, rubidium $1.53{g/cm}^3$ and caesium $1.93{g/cm}^3$.
   2. Potassium, at $0.86{g/cm}^3$, is slightly less dense than sodium, at $0.97{g/cm}^3$, which is why this is described as a general trend.
3. Reactivity increases down Group I.
   1. Lithium reacts steadily, sodium reacts more vigorously and potassium reacts very vigorously with the same substance.
   2. Each atom reacts by losing its single outer electron, and further down the group this electron is in a shell further from the nucleus.
   3. The outer electron is therefore attracted less strongly by the nucleus and is lost more easily, so the metal reacts faster.
4. All three trends run in a fixed direction, so an element lower in the group has a lower melting point, a higher density and a higher reactivity than an element above it.

Predicting other Group I elements

1. To predict the properties of an unfamiliar Group I element, first decide whether it is above or below a known element in the group.
2. An element below potassium, such as rubidium or caesium, is predicted to have:
   1. a lower melting point than potassium;
   2. a higher density than potassium;
   3. a higher reactivity than potassium.
3. Every Group I element is predicted to be a relatively soft metal with one outer-shell electron that forms a $+1$ ion.
4. Its compounds are predicted to have the same type of formula as those of sodium and potassium, so rubidium forms $RbCl$ and ${Rb}_{2}O$.
5. When data for the known elements are given, use the size of the steps in the data to estimate a sensible value for the unfamiliar element.