Group 1 and Group 7 Chemistry.

Reactions of group 1 and 7 elements

The Group 1 elements

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The group 1 elements in the periodic table are known as the alkali metals. They include lithium, sodium and potassium, which all react vigorously with air and water.

The reactivity of the alkali metals increases down the group. Flame tests are used to identify alkali metal ions in compounds.

The alkali metals

The group 1 elements are placed in the vertical column on the left hand side of the periodic table.

Lithium, sodium and potassium are the three group 1 elements you are likely to see at school. Like all the group 1 elements, they are very reactive. They must be stored under oil to keep air and water away from them. group 1 elements form alkaline solutions when they react with water, which is why they are called alkali metals.

Reactions with water

The reaction of potassium with water gives a lilac flame

Group 1 elements react vigorously with water to produce an alkaline metal hydroxide and hydrogen gas. In general:

Metal + water → metal hydroxide + hydrogen

For example, here are the equations for the reaction of sodium with water:

sodium + water → sodium hydroxide + hydrogen 2Na + 2H₂O → 2NaOH + H₂ (the 2s in front of Na, H₂O and NaOH are for balancing)

The reactivity of the alkali metals increases down the group. Lithium is the least reactive and potassium is the most reactive of the three. The hydrogen ignites immediately during the reaction between potassium and water with the potassium producing a lilac coloured flame.

Making predictions

Because there are patterns in the way the elements are arranged in the periodic table, it can be used to predict their properties and interpret data.

Reactivity

The group 1 elements become more reactive as you go down the group. At the top, lithium is the least reactive and francium at the bottom is the most reactive. Francium is rare and radioactive, so it would be difficult to confirm predictions made about it. However, it is possible to predict the properties of rubidium and caesium and to see if the predictions were accurate.

 

Group 1 element	Features of reaction with water
Lithium	Fizzes steadily, gradually disappears
Sodium	Fizzes rapidly, melts into a ball and disappears quickly
Potassium	Ignites with sparks and a lilac flame, disappears very quickly
Rubidium	Explodes with sparks
Caesium	Violent explosion due to rapid production of heat and hydrogen

Physical properties - Higher tier

The table shows the melting points of Group 1 elements, with one value missing.

 

Group 1 element	Melting point/ °C
Lithium	181
Sodium	98
Potassium	63
Rubidium	?
Caesium	29

The melting points show a pattern, or trend, down the group. It is therefore possible to predict that the melting point of rubidium is between 29°C and 63°C (it is actually 39°C). The same can be done with other physical properties, such as the densities of rubidium and caesium, for example.

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Science

The Group 1 elements

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Flame tests

It's possible to test a compound to detect the presence of an alkali metal ion.

A cleaned, moistened flame test wire is dipped into a solid sample of the compound and then put into a blue Bunsen flame. The flame colour indicates which alkali metal ion is present in the compound.

Flame colours and the alkali metal ion they represent

Flame colour	Ion present
Red	Lithium
Orange	Sodium
Lilac	Potassium

Explaining trends

The Group 1 elements have similar properties because of the electronic structure of their atoms - they all have one electron in their outer shell.

 

Group 1 element	Electronic structure	Diagram of atom
Lithium	2.1
Sodium	2.8.1
Potassium	2.8.8.1

The group 7 elements

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The group 7 elements are also known as the halogens. They include fluorine, chlorine, bromine and iodine, which all have seven electrons in their outer shell.

In a displacement reaction, a less reactive element is displaced by a more reactive element.

The halogens

The group 7 elements are placed in the vertical column second from right in the periodic table.

The periodic table

Chlorine, bromine and iodine are the three Group 7 elements you are likely to see at school. Fluorine is too reactive to keep or produce safely at school. Group 1 elements form salts when they react with metals, which is why they are called the halogens ('salt formers').

Properties and uses of the halogens

The table summarises some of the properties and uses of three halogens.

 

Group 7 element	Properties	Typical use
Chlorine	Green gas	Sterilising water
Bromine	Orange liquid	Making pesticides and plastics
Iodine	Grey solid	Sterilising wounds

Iodine forms a purple vapour when it is warmed.

Reactions with group 1 elements

The group 7 elements react vigorously with group 1 elements such as sodium and potassium. In each case, a metal halide is formed (fluoride, chloride, bromide or iodide).

The table summarises the names and formulae for the metal halides formed by the reaction of group 1 elements with group 7 elements.

 

Group 1\7	Chlorine	Bromine	Iodine
Lithium	Lithium chloride - LiCl	Lithium bromide - LiBr	Lithium iodide - LiI
Sodium	Sodium chloride - NaCl	Sodium bromide - NaBr	Sodium iodide - NaI
Potassium	Potassium chloride - KCl	Potassium bromide - KBr	Potassium iodide - KI

For example, sodium reacts with chlorine to produce sodium chloride

Sodium + chlorine → sodium chloride

2Na + Cl₂ → 2NaCl (the 2s in front of Na and NaCl are for balancing)

Notice that chlorine is Cl₂ in the symbol equation, not Cl. The halogens all exist asdiatomic molecules, two halogen atoms joined together by a covalent bond:

• Fluorine, F₂
• Chlorine, Cl₂
• Bromine, Br₂
• Iodine, I₂

Displacement and reactivity

If a reactive element comes into contact with the compound of a less reactive element a chemical reaction may take place. The less reactive element isremoved from the compound and replaced by the more reactive element.

For example, if chlorine is added to a solution of potassium bromide, the bromine is replaced by the chlorine forming potassium chloride. Bromine is formed at the same time and can be detected by its colour.

Reactions of this type are called displacement reactions

Examples of displacement reactions

Halogen	Metal halide	The most reactive halogen	Reaction
Chlorine	Sodium bromide	Chlorine	Chlorine + sodium bromide → sodium chloride + bromine
Chlorine	Sodium iodide	Chlorine	Chlorine + sodium iodide → sodium chloride + iodine
Bromine	Sodium chloride	Chlorine	No reaction
Bromine	Sodium iodide	Bromine	Bromine + sodium iodide → sodium bromide + iodine
Iodine	Sodium chloride	Chlorine	No reaction
Iodine	Sodium bromide	Bromine	No reaction

Explaining properties

The group 7 elements have have similar properties because of the electronic structure of their atoms - they all have seven electrons in their outer shell.

 

Group 1 element	Electronic structure	Diagram of atom
Fluorine	2.7
Chlorine	2.8.7

An extra outer electron is gained during chemical reactions.

Now try a Test Bite.

Higher tier

In a reaction, an atom of a group 7 element will form an ion with a single negative charge (a halide ion). For example, for a chlorine molecule forming two chloride ions

Cl₂ + 2e^– → 2Cl^–

A change like this, where an electron is gained, is an example of reduction.

The ions formed have a stable electronic structure, like a noble gas from group 0.

 

Group 1 element	Electronic structure of ion	Diagram of ion
Fluorine	[2.8]-
Chlorine	[2.8.8]-

The reactivity of group 7 elements decreases down the group because, as you go down the group:

• The atoms get larger
• The outer shell gets further from the nucleus
• The attraction between the nucleus and electrons gets weaker, so an electron is less easily gained