9.09.2 Group 1: Alkali Metals

Syllabus

The elements in Group 1 of the Periodic Table are known as the alkali metals and have characteristic properties because of the single electron in their outer shell.
Students should be able to describe the reactions of the first 3 metals with Oxygen, Chlorine & water.
In Group 1, the reactivity of the elements increases going down the group.

Students should be able to:

explain how properties of the elements in Grp 1 depend on the outer shell of electrons
predict properties from given trends down the group.

What does this mean?

Why are Alkali metals in the same group?

All Group 1 metals react with water and air in much the same way - quickly.

Though some much more quickly than others.

Mendeleev knew they would all have to be in the same Group of his Periodic Table because they all behave in the same way - have the same chemical properties.

You should know that their electronic structures all end with 1 ( 2,1 - 2,8,1 - 2,8,8,1 etc)

This is why their Group is now called Group 1

And they all need to lose one electron to gain electronic stability - a full outer shell.

Hence, when they react they do so the same way.

And make similar products.

Reaction with Water

All alkali metals react vigorously with water.

All of them produce Hydrogen gas.

All also produce a Metal Hydroxide (a strong Alkali) that dissolves into the water.

If we look at the word equations and chemical equations for the top and bottom of the group we see that they are almost identical.

Lithium + Water --> Lithium Hydroxide + Hydrogen

2 Li_(s) + 2 H₂O_(l) --> 2 LiOH_(aq) + H_{2 (g)}

Caesium + Water --> Caesium Hydroxide + Hydrogen

2 Cs_(s) + 2 H₂O_(l) --> 2CsOH _(aq) + H_{2 (g)}

Since all the reactions are the same, we might write a general equation:

Alkali Metal+ Water --> Metal Hydroxide + Hydrogen

2 M_(s) + 2 H₂O_(l) --> 2 MOH _(aq) + H_{2 (g)}

We could test the gas given off by collecting it and applying a flame - it should pop.

If we put some Universal Indicator in the water before dropping in the metal we would expect it to leave a blue-purple trail as it is moved about the surface by the bubbling. This is where Alkali metals get their name.

Relative Reactivities

With Water

You will see Lithium, Sodium and Potassium added to water.

Lithium produces Hydrogen quite quickly - but not quickly enough to set fire to easily.
Sodium produces Hydrogen so quickly that it is easy to burn the gas it produces.
Potassium produces Hydrogen so quickly that the reaction generates enough heat to set fire to itself.
You may even see videos of Rubidium and Caesium exploding in water because they react so quickly.
Francium doesn't really exist but, if it did, it should react very quickly and potentially very dangerously

So we could summarise the reactivity of Alkali metals as...

With Air/Oxygen

All Alkali metals burn in air (and a little faster in pure Oxygen).

It can be difficult to say which was burning most quickly but it is usually easier to set fire to the most reactive metals (lower in the group).

Because all the metals share similar electronic structures they have very similar equations that we can generalise (so that you only have to learn one!)

Lithium + Oxygen --> Lithium Oxide

4 Li_(s) + O_2(g) --> 2 Li₂O_(s)

Caesium + Oxygen --> Caesium Oxide

4 Cs_(s) + O_2(g) --> 2 Cs₂O_(s)

_{General Equation}

Alkali Metal + Oxygen --> Alkali Metal Oxide

4 M _(s) + O_2(g) --> 2 M₂O_(s)

_{Explaining Reactivities}

As we go down Group 1 the atoms are bigger due to the extra shells.

The outer electrons are further from the nucleus.

The positive nucleus attracts the negative electron in the outer shell less strongly.

It is easier to lose electrons from large atoms.

So reactions can happen faster.

The bottom graph shows that less energy is needed to remove electrons from elements at the bottom of the Periodic Table.

The easier it is to remove an electron the faster that element will react.

_{So, if we were asked to predict the reactivity of Francium we could reasonably assume that Francium atoms should be bigger than Caesium atoms and that removing an electron should take less energy.}

_{This means we should predict that Francium would be the most reactive alkali Metal.}

_{No one can make measurements of Francium since its atoms have to be made artificially in tiny amounts and then undergo radioactive decay very quickly.}

9.09.2 Group 1: Alkali Metals

Syllabus

What does this mean?

Why are Alkali metals in the same group?

Reaction with Water

Relative Reactivities

With Water

With Air/Oxygen

Explaining Reactivities

As we go down Group 1 the atoms are bigger due to the extra shells.

The outer electrons are further from the nucleus.

The positive nucleus attracts the negative electron in the outer shell less strongly.

It is easier to lose electrons from large atoms.

So reactions can happen faster.

The bottom graph shows that less energy is needed to remove electrons from elements at the bottom of the Periodic Table.

The easier it is to remove an electron the faster that element will react.

So, if we were asked to predict the reactivity of Francium we could reasonably assume that Francium atoms should be bigger than Caesium atoms and that removing an electron should take less energy.

This means we should predict that Francium would be the most reactive alkali Metal.

No one can make measurements of Francium since its atoms have to be made artificially in tiny amounts and then undergo radioactive decay very quickly.

For anyone interested in such things, Physics suggests that things are not quite so straight forward.

But that is Physics and so of no interest to anyone.

Metallic Bonding.

You should know how metallic bonding works.

All Alkali metals form 1+ ions and give up one electron.

So, the only difference is the size of the ions.

Small ions are more attractive than large ions of the same charge.

So this means that bonding is strongest in Lithium and gets weaker down to Caesium.

And the stronger the bonding the harder it will be to melt or boil the element.

Also, strong bonds make it difficult to pull atoms apart.

So Lithium should be stronger and harder than Caesium.

However, bigger atoms weigh a great deal more than smaller ones.

So density increases down the group.

Asked to predict numbers you must use the numbers given to you.

Look how they are changing.

Melting Point:

180 --> 97 --> 63 --> 39 --> 28

The changes are 83, 34, 24, 11

So, it seems unlikely that the melting point would decrease all that much.

So a prediction around 5oC seems sensible

Boiling Point:

1342 --> 883 --> 759 --> 688 --> 671

The changes are 359, 124, 71, 17

So, it again seems unlikely that there would be much decrease.

So a prediction around 665oC seems sensible

Density:

0.53 --> 0.97 --> 0.89 --> 1.53 --> 1.93

The changes are +0.44, -0.08, +0.64, +0.40

Predictions here would be unreliable since the trend is unclear

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Francium

_{Explaining Reactivities}

_{So, if we were asked to predict the reactivity of Francium we could reasonably assume that Francium atoms should be bigger than Caesium atoms and that removing an electron should take less energy.}

_{This means we should predict that Francium would be the most reactive alkali Metal.}

_{No one can make measurements of Francium since its atoms have to be made artificially in tiny amounts and then undergo radioactive decay very quickly.}

_{For anyone interested in such things, Physics suggests that things are not quite so straight forward.}

_{But that is Physics and so of no interest to anyone.}

_{You should know how metallic bonding works.}

_{All Alkali metals form 1+ ions and give up one electron.}

_{So, the only difference is the size of the ions.}

_{Small ions are more attractive than large ions of the same charge.}

_{So this means that bonding is strongest in Lithium and gets weaker down to Caesium.}

_{And the stronger the bonding the harder it will be to melt or boil the element.}

_{Also, strong bonds make it difficult to pull atoms apart.}

_{So Lithium should be stronger and harder than Caesium.}

_{However, bigger atoms weigh a great deal more than smaller ones.}

_{So density increases down the group.}

_{Asked to predict numbers you must use the numbers given to you.}

_{Look how they are changing.}

_{Melting Point:}

_{180 --> 97 --> 63 --> 39 --> 28}

_{The changes are 83, 34, 24, 11}

_{So, it seems unlikely that the melting point would decrease all that much.}

_{So a prediction around 5}^o_{C seems sensible}

_{Boiling Point:}

_{1342 --> 883 --> 759 --> 688 --> 671}

_{The changes are 359, 124, 71, 17}

_{So, it again seems unlikely that there would be much decrease.}

_{So a prediction around 665}^o_{C seems sensible}

_Density:

_{0.53 --> 0.97 --> 0.89 --> 1.53 --> 1.93}

_{The changes are +0.44, -0.08, +0.64, +0.40}

_{Predictions here would be unreliable since the trend is unclear}