Soft, low-density alkali metals react with water
Definition
Alkali metal
An element in Group 1 of the Periodic Table, which reacts with water to form hydrogen and an alkaline solution.
- Lithium, sodium and potassium are soft enough to cut with a knife, and each freshly cut surface is shiny.
- The shiny surface tarnishes quickly as the metal reacts with oxygen in the air, so the metals are stored under oil.
- The alkali metals have low densities, so lithium, sodium and potassium all float on water.
- They have low melting points for metals, and the melting points decrease down the group.
- Every alkali metal reacts with water to form hydrogen gas and a metal hydroxide, for example:
2Na(s)+2H2O(l)→2NaOH(aq)+H2(g)2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)}2Na(s)+2H2O(l)→2NaOH(aq)+H2(g)
- The metal hydroxide dissolves to give an alkaline solution, which turns universal indicator purple.
Example
- Lithium melts at 181 ∘C181\ ^{\circ}\text{C}181 ∘C, sodium at 98 ∘C98\ ^{\circ}\text{C}98 ∘C and potassium at 63 ∘C63\ ^{\circ}\text{C}63 ∘C; predict the melting point of rubidium, the next element down.
- Describe the trend: the melting point falls down the group, first by 83 ∘C83\ ^{\circ}\text{C}83 ∘C and then by 35 ∘C35\ ^{\circ}\text{C}35 ∘C.
- The falls are getting smaller, so the next fall is likely to be less than 35 ∘C35\ ^{\circ}\text{C}35 ∘C.
- A sensible prediction for rubidium is about 40 ∘C40\ ^{\circ}\text{C}40 ∘C, and its real melting point is 39 ∘C39\ ^{\circ}\text{C}39 ∘C.
Group 1 reactivity increases down the group
Definition
Electron shielding
The reduction in attraction between the nucleus and the outer electrons caused by the inner shells of electrons.
- Every alkali metal atom has one outer electron, which it loses to form an ion with a 1+1+1+ charge, such as Na+\text{Na}^{+}Na+.
- Going down the group, each atom has one more shell of electrons, so the outer electron is further from the nucleus.
- The extra inner shells also shield the outer electron from the attraction of the positive nucleus.
- The weaker attraction means the outer electron is lost more easily, so reactivity increases down the group.
- This is why potassium reacts with water more vigorously than sodium, and sodium more vigorously than lithium.
- The trend predicts that rubidium and caesium, below potassium, react with water even more violently.
Example
Lithium floats and fizzes steadily, sodium melts into a ball that races across the surface, and potassium melts and sets the hydrogen alight with a lilac flame.
Halogens darken and boil higher down Group 7
Definition
Halogen
An element in Group 7 of the Periodic Table, whose atoms have seven outer electrons and which exists as diatomic molecules.
- Each halogen exists as diatomic molecules, such as Cl2\text{Cl}_2Cl2, Br2\text{Br}_2Br2 and I2\text{I}_2I2, in which two atoms share a pair of electrons.
- At room temperature, chlorine is a pale green gas, bromine is a red-brown liquid and iodine is a grey-black solid.
- Bromine gives off an orange-brown vapour, and iodine gives off a purple vapour when it is warmed.
- The colour gets darker and the melting and boiling points increase down the group.
- The molecules get larger down the group, so the intermolecular forces between them get stronger and more energy is needed to separate them.
- Halogens react with metals to form metal halides, which are ionic salts, for example: 2Na(s)+Cl2(g)→2NaCl(s)2\text{Na(s)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{NaCl(s)}2Na(s)+Cl2(g)→2NaCl(s)
Example
Chlorine boils at −34 ∘C-34\ ^{\circ}\text{C}−34 ∘C, bromine at 59 ∘C59\ ^{\circ}\text{C}59 ∘C and iodine at 184 ∘C184\ ^{\circ}\text{C}184 ∘C, so astatine, below iodine, is predicted to be a dark solid with an even higher boiling point.
Falling Group 7 reactivity explains halogen displacement
Definition
Displacement reaction
A reaction in which a more reactive element takes the place of a less reactive element in a compound.
- Every halogen atom has seven outer electrons and gains one electron to form a halide ion with a 1−1-1− charge, such as Cl−\text{Cl}^{-}Cl−.
- Going down the group, the outer shell is further from the nucleus and more shielded by inner shells.
- An incoming electron is attracted less strongly, so it is gained less easily and reactivity decreases down the group.
- Fluorine is therefore the most reactive halogen, and iodine is the least reactive of the common halogens.
- A halogen displaces a less reactive halogen from a solution of its halide salt, for example:
Cl2(aq)+2KBr(aq)→2KCl(aq)+Br2(aq)\text{Cl}_2\text{(aq)} + 2\text{KBr(aq)} \rightarrow 2\text{KCl(aq)} + \text{Br}_2\text{(aq)}Cl2(aq)+2KBr(aq)→2KCl(aq)+Br2(aq)
- The colourless potassium bromide solution turns orange as bromine forms.
- A less reactive halogen cannot displace a more reactive one, so iodine does not react with potassium bromide solution.
Example
- Chlorine water turns potassium iodide solution brown, because it displaces iodine: Cl2(aq)+2KI(aq)→2KCl(aq)+I2(aq)\text{Cl}_2\text{(aq)} + 2\text{KI(aq)} \rightarrow 2\text{KCl(aq)} + \text{I}_2\text{(aq)}Cl2(aq)+2KI(aq)→2KCl(aq)+I2(aq).
- Bromine water also turns potassium iodide solution brown, because bromine is more reactive than iodine.
- Iodine solution causes no change with potassium chloride or potassium bromide solution, because iodine is the least reactive of the three.
Full outer shells make the noble gases unreactive
Definition
Noble gas
An element in Group 0 of the Periodic Table, whose atoms have a full outer shell of electrons, which makes it very unreactive.
- Helium has a full first shell of two electrons, and the other noble gases have eight outer electrons.
- A full outer shell gives the atoms no tendency to lose, gain or share electrons, so the noble gases are very unreactive.
- The noble gases are monatomic, existing as single atoms rather than molecules.
- They are colourless gases at room temperature with very low boiling points.
- Boiling points increase down the group, from −269 ∘C-269\ ^{\circ}\text{C}−269 ∘C for helium to −108 ∘C-108\ ^{\circ}\text{C}−108 ∘C for xenon.
- The atoms get larger down the group, so the forces between the atoms get stronger.
- Density also increases down the group, because the atoms get heavier.
Exam technique
- When a question gives data for some elements in a group, describe the trend in the values and continue it to estimate the value for the missing element.
- When it asks you to explain a reactivity trend, link the extra shells to the distance and shielding of the outer electrons, and then to how easily an electron is lost or gained.
- Keep intermolecular forces out of any explanation of Group 1 or Group 7 reactivity, because they explain boiling points, not reactions.
Self review
- What forms when sodium reacts with water?
- Why does reactivity increase down Group 1?
- What are the state and colour of bromine at room temperature?
- Why does chlorine displace bromine from potassium bromide solution?
- Why are the noble gases very unreactive?
