An element's group predicts its type of reaction
Definition
Group
A vertical column of the Periodic Table.
- Elements in the same group have the same number of outer electrons, so they take part in similar reactions.
- Metals in Groups 1 and 2 react by losing their outer electrons, forming ions with 1+1+1+ and 2+2+2+ charges.
- Sodium reacts with water to form hydrogen and sodium hydroxide, so potassium and lithium, in the same group, are predicted to do the same.
- Non-metals in Groups 6 and 7 react with metals by gaining electrons, forming ions with 2−2-2− and 1−1-1− charges.
- Chlorine reacts with sodium to form sodium chloride, so bromine is predicted to form sodium bromide in the same way.
- Two non-metals, such as carbon and oxygen, react by sharing electrons to form a covalent compound.
- Group 0 elements have full outer shells, so no reaction is predicted for them.
Key Idea
A known reaction of one element in a group predicts the same type of reaction, with the same kind of product, for the other elements in that group.
Using ion charges to predict product formulae
Definition
Ion
An atom or group of atoms that carries an overall electric charge.
- The charges on the ions must cancel out, so the formula of an ionic product follows from the two group numbers.
- A Group 1 metal and a Group 7 non-metal combine in a 1:11:11:1 ratio, as in KBr\text{KBr}KBr and LiCl\text{LiCl}LiCl.
- A Group 2 metal and a Group 7 non-metal combine in a 1:21:21:2 ratio, so calcium and chlorine form calcium chloride, CaCl2\text{CaCl}_2CaCl2.
- A Group 1 metal and a Group 6 non-metal combine in a 2:12:12:1 ratio, so lithium and oxygen form lithium oxide, Li2O\text{Li}_2\text{O}Li2O.
- With the formula known, balance the equation using the diatomic formula of the non-metal element: 4Li(s)+O2(g)→2Li2O(s)4\text{Li(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Li}_2\text{O(s)}4Li(s)+O2(g)→2Li2O(s)
Example
- Predict the product and write the balanced equation for the reaction of rubidium with chlorine.
- Rubidium is in Group 1, so it forms Rb+\text{Rb}^{+}Rb+ ions, and chlorine is in Group 7, so it forms Cl−\text{Cl}^{-}Cl− ions.
- The charges cancel in a 1:11:11:1 ratio, so the product is rubidium chloride, RbCl\text{RbCl}RbCl.
- Chlorine exists as Cl2\text{Cl}_2Cl2, so two rubidium atoms are needed: 2Rb(s)+Cl2(g)→2RbCl(s)2\text{Rb(s)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{RbCl(s)}2Rb(s)+Cl2(g)→2RbCl(s).
Position within a group predicts reactivity
Definition
Reactivity
How readily an element takes part in chemical reactions.
- Metals become more reactive down a group, because the outer electrons are further from the nucleus and are lost more easily.
- Caesium, near the bottom of Group 1, is predicted to react with water even more violently than potassium.
- Non-metals become less reactive down a group, because an extra electron is attracted less strongly by the nucleus.
- Astatine, at the bottom of Group 7, is predicted to be the least reactive halogen, so it should not displace iodine from potassium iodide solution.
- Predicted reactivity also shows whether a displacement will happen, because only a more reactive element can take the place of a less reactive one in its compound.
Common Mistake
Do not assume that the element lower in every group is more reactive, because the trend for non-metals runs the opposite way to the trend for metals.
Reactivity trends across a period
Definition
Period
A horizontal row of the Periodic Table, in which every element has the same number of occupied electron shells.
- A Group 1 metal is more reactive than the Group 2 metal in the same period, so sodium reacts vigorously with cold water while magnesium reacts very slowly.
- A Group 2 atom must lose two electrons, and its nucleus has more protons attracting them, so it forms ions less readily.
- The transition metals, in the centre of the table, are much less reactive than the metals in Groups 1 and 2.
- Among the non-metals, a Group 7 element is usually more reactive than the Group 6 element in the same period, so fluorine is more reactive than oxygen.
- Combining both trends puts the most reactive metals at the bottom left and the most reactive non-metal, fluorine, at the top right, next to Group 0.
- Group 0 breaks the pattern, because full outer shells make its elements almost completely unreactive.
Exam technique
- When a question asks you to predict the reactivity of an unfamiliar element, say whether it is a metal or a non-metal and where it sits in its group, then compare it with a named element you know.
- Use words such as 'likely' or 'predicted', because position gives a probable outcome rather than a certain one.
Self review
- What charge is the ion formed by a Group 2 metal predicted to have?
- What is the formula of the compound formed between calcium and bromine?
- Why is caesium predicted to be more reactive than potassium?
- Would you expect astatine to displace iodine from potassium iodide solution?
- Why is sodium more reactive than magnesium?
