Group 1

How do the similarities in the reactions of these elements with water provide evidence for their recognition as a family of elements?

Group 1 metals such as potassium, sodium and lithium, react with water to produce a metal hydroxide and hydrogen. For example:

          lithium   +   water   →   lithium hydroxide   +   hydrogen

          2Li (s)   +   2H₂O (l)   →   2LiOH (aq)   +   H₂ (g)

The observations for the reaction of water with either potassium or sodium or lithium have the following similarities:

1. fizzing (hydrogen is produced)

2. metal floats and moves around on the water

3. metal disappears

In each case a metal hydroxide solution is produced.

These similarities in the reactions provide evidence that the 3 metals are in the same group of the Periodic Table (i.e. have the same number of electrons in their outer shell).

How do the differences between the reactions of these elements with air and water provide evidence for the trend in reactivity in Group 1?

Lithium is the first element in group 1 of the Periodic Table. The observations for the reaction of lithium and water are:

1. fizzing (hydrogen gas is released)

2. lithium floats and moves around on the water

3. lithium disappears

Sodium is the second alkali metal in the group. The reaction of sodium and water is more vigorous than lithium’s:

1. fizzing (hydrogen gas is released)

2. sodium floats and moves around on the water

3. sodium melts into a silver-coloured ball

4. sodium disappears

Potassium is the third alkali metal in the group. The reaction of potassium and water is more vigorous than sodium’s:

1. fizzing (hydrogen gas is released)

2. potassium floats and moves around on the water

3. catches fire with a LILAC flame

4. potassium disappears

When the group 1 metals react with air they oxidise, showing a similar trend in reactivity as we go down the group of the Periodic Table.

Therefore, as we go down group 1 (increasing atomic number), the elements become more reactive: Li<Na<K<Rb<Cs<Fr

Use knowledge of trends in Group 1 to predict the properties of other alkali metals.

Alkali metal	Melting point (⁰C)	Density (g/cm³)	Reaction with water	Products
lithium (Li)	181	0.53	fizzing	lithium hydroxide + hydrogen
sodium (Na)	98	0.97	rapid fizzing	sodium hydroxide + hydrogen
potassium (K)	63	0.86	vigorous fizzing and lilac flame	potassium hydroxide + hydrogen
rubidium (Rb)	39	1.53	?	rubidium hydroxide + hydrogen
caesium(Cs)	29	1.88	?	caesium hydroxide + hydrogen
francium (Fr)	?	?	?

From the data in the table, it is possible to deduce the properties of francium from the trends in the other group 1 metals.

For example, we can predict that francium will have a melting point around 20⁰C and a density of just over 2g/cm³.

We can also predict that francium will react violently with water, producing francium hydroxide and hydrogen.

Explain the trend in reactivity in Group 1 in terms of electronic configurations.

As you go down the group the outer electron lost from the group 1 metal is further from the nucleus therefore the electron is less attracted, as the electrostatic forces are weaker, by the nucleus and therefore more easily lost.