Untitled Flashcards Set
The elements in Group 1of the Periodic Table are called the alkalim e t a l s . The group contains the elements shown in Figure 11.3.
Francium (pronounced france-ee-um), at the bottom of the group, is raOdnioeacoftivites.isotopes is produced during the radioactive decayof uranium-235, but is extremely short-lived. At any one time scientists estimate
that there is only about 20-30g of francium present in the whole of the Earth's crust and no one has ever seen a piece of francium. When you know about the rest of Group 1 you canpredict what francium would be like. We will make those predictions later.
EXTENSIONWORK
The reason that the melting points decrease is that the atoms get bigger as we go down the group.
In the metallic lattice, the nuclei of the positive ions are further from the delocalised electrons in caesium than in sodium and therefore there is weaker electrostatic attraction.
The meltingand boiling points of the elements are very low for metals, and get lower as you move down the group.
Their densities tend to increase down the group, although not regularly. Lithium, sodium and potassium are al less dense than water, and so wil float on it.
The metals are also very soft and are easily cut with a knife, becoming softer as you move down the group. They are shiny and silver when freshly cut, but tarnish very quickly on exposure to air.
All these metals are extremely reactive and get more reactive as you go
down the group. They al react quickly with oxygen ni the air to form oxides, and react rapidly with water to form strongly alkaline solutions of the metal
hydroxides. This is why the Group 1metals are commonly known as the alkali metals.
To stop them reacting with oxygen or water vapour in the air, lithium, sodium and potassium are stored under oil. Rubidium and caesium are so reactive that
they have to be stored in sealed glass tubes to stop any possibility of oxygen getting at them.
Great care must be taken not to touch any of these metals with bare fingers. There could be enough sweat on your skin to give a reaction, producing lots of
heat and a very corrosive metal hydroxide
There are two reasons that we put these elements in Group 1:
1 They all have one electron in their outer shell. The electronic configurations are:
lithium
2, 1 sodium
,2 8, 1 potassium 2, 8, 8, 1
2 They have similar chemical properties, for example:
m they all react with water (this will be discussed below) in the same way to form a hydroxide with the formula MOH (e.g. LiOH, NaOH) and hydrogen
m they react with oxygen to form an oxide with the formula MO (Na,O, K20)
m they react with halogens to form compounds with the formula MX (e.g. LiCI, KBr)
m they form ionic compounds which contain an M* ion (e.g. Na*, *K ).
The chemical properties depend on the number of electrons in the outer shell. The Group 1elements react in very similar ways because they all have the
same number of electrons in the outer shell (one), so reason 2 is really just a consequence of reason .1
LITHIUM
SODIUM
POTASSIUM
•
Figure 11.4 Lithium, sodium and potassium have ot be kept ni oil to stop them reacting with oxygen in the air.
K E YP O I N T
'M' simply represents any one of the alkali metals. 'X' represents any one of the halogens.
We will discuss the chemical properties in more detail below.
Al these metals react in the same way with water to produce a metal hydroxide and hydrogen:
alkali metal + water
- alkali metal hydroxide + hydrogen
2M
+2H,0 -
2МОН
The main difference between the reactions is how quickly they happen.
As you go down the group, the metals become more reactive and the reactions occur more rapidly.
The reaction between sodium and water is typical.
W I T HSODIUM
2Na(s) +2H20(1)- 2NaOH(aq) +H2(g)
The main observations you can make when this reaction occurs are: The sodium floats because it is less dense than water.
The sodium melts into a ball because its melting point is low and a lot of heat is produced by the reaction.
The sodium moves around on the surface of the water. Because the
hydrogen isn't given off symmetrically around the ball, the sodium is pushed around the surface of the water, like a hovercraft.
The piece of sodium gets smaller and eventually disappears. The sodium is used up in the reaction.
If you test the solution that is formed with universal indicator solution, you will see that the universal indicator goes blue, indicating an alkaline solution
has been formed. The metal hydroxide is alkaline (the solution contains the OH- ion)
When you are asked about this in the exam, you are often asked to compare the reactions of sodium and lithium so you should explain how you can see that the reaction of lithium is slower. So, for example, you can say that
ti fizzes more slowly, or the lithium
moves around more slowly, or takes longer to disappear.
HINT
Again, fi you are asked to compare potassium with sodium use phrases such as:
'fizzes more vigorously'
'moves around more quickly' 'disappears more quickly'
The keydifference though is that with potassium the hydrogen bursts into flames but with sodium ti usually does not.
Safety Note: The reactions of rubidium and caesium would be too hazardous to attempt in school as they would explode.
LITHIUM
2Li(s) +2H20()
- 2LiOH(aq)
+Hz(g)
The reaction is very similar to sodium's reaction, except that it is slower. Lithium's melting point is higher andt h e heat isn't produced soquickly, so the lithium doesn't melt.
POTASSIUM
2K(s) +2H20(1) -
2KOH(aq)
+H2(g)
Potassium's reaction is faster than sodium's. Enough heat is produced to ignite the hydrogen, which burns with a lilac flame. The reaction often ends with the potassium spitting around and exploding.
RUBIDIUMANDCAESIUM
These react even more violently than potassium, and the reaction can be explosive. Rubidium hydroxide and caesium hydroxide are formed.
E X P L A I N I N G T H E I N C R E A S E I N REACTIVITY
As you go down the group, the metals become more reactive.
In all these reactions, the metal atoms are losing electrons and forming metal ions in solution. For example:
Na(s) -Na*(aq) +e-
The differences between the reactions depend on how easily the outer electron of the metal is lost in each case. That depends on how strongly it is attracted to the nucleus in the original atom. Remember that the nucleus o fan atom is positive because it containsprotons, and so attracts the negative electrons.
outer electron further from nucleus
Li Na
nucleus n u c l e u s
inner
inner electrons electrons
A Figure 11.7 Electrons of lithium and sodium
As we move down the group, the atoms have more shells of electrons and get bigger: a sodium atom is bigger than a lithium atom and a potassium atom is bigger than a sodium atom. As the atoms get bigger, the outer electron, which is the one lost in the reaction, is further from the nucleus. Because it is further from the nucleus it is less strongly attracted by the nucleus and therefore more easily lost.
Lithium, sodium and potassium are all stored in oil because they react with the air. fI we look at a piece of sodium which has been taken out of the oil, it usually has a crust on the outside. tI is not shiny unless ti has been freshly cut.
When the piece of sodium is cut, the fresh surface is shiny but ti tarnishes rapidly as the freshly exposed sodium reacts with oxygen in the air. If we
do the same with a piece of lithium it tarnishes more slowly because lithium reacts more slowly than sodium. Afreshly cut piece of potassium tarnishes extremely rapidly, more quickly than sodium. In this way we can see again that potassium is more reactive than sodium, which is more reactive than lithium.
In each case the metal reacts with oxygen in the air to form an oxide with the formula M.O.
If we heat each of the metals in the air using a Bunsen burner, we get a much more vigorous reaction and it is more difficult to see which metal is most reactive because all the reactions are s o rapid.
Lithium burns with a red flame to form lithium oxide. Sodium burns with a yellow flame to form sodium oxide. Potassium burns with a lilac flame to form potassium oxide. The equation for all these reactions is:
4M(s) +02(g) - 2MzO(s)
In each case the product formed is a white powder - the alkali metal oxide.
Al Group 1metal ions are colourless. That means that their compounds will be colourless or white unless they are combined with a coloured negative ion. Potassium dichromate(VI) is orange, for example, because the dichromate(VI)
ion is orange, and potassium manganate(VIl) is purple because the manganate(VII) ion is purple. Group 1compounds are typical ionic solids and are mostly soluble in water.
Group 1elements:
• are metals
• are soft with melting points and densities which are very low for metals
• have to b e stored out of contact with air or water
• react rapidly with air to form coatings of the metal oxide
• react with water to produce an alkaline solution of the metal hvdroxide and hydrogen gas
• increase in reactivity as you go down the group
• form compounds in which the metal has a÷1 ion
• have mainlv white/colourless compounds which dissolve to produce colourless solutions.
• is very soft
• will have a melting point around room temperature
• has density which is probably just over 2 a/cm°
• will be a silvery metal, but will tarnish almost instantly in air
• will react violently with water to give francium hydroxide and hydrogen
• will be more reactive than caesium
• wil have a hydroxide, francium hydroxide, with the formula FrOH, which wil be soluble in water and form a strongly alkaline solution
• will form compounds that are white/colourless and dissolve in water to give colourless solutions.
We could use a graphical method to predict the melting point of francium. If we plot the melting point of the alkali metals against atomic number then draw a line of best fit we get:
If we carry on the line to atomic number 87 we can predict a melting point of about 22°C.