The periodic table: predict possible reactions and probable reactivity of elements from their positions in the periodic table.

The idea

The periodic table arranges elements by increasing atomic number, so elements in the same group have the same number of outer-shell electrons and therefore similar chemical properties. To predict reactivity, first locate the element's group and period. Group 1 metals become more reactive down the group because the outer electron is lost more easily as atomic radius increases and shielding increases. Group 7 non-metals become less reactive down the group because it becomes harder to gain an electron. Group 0 noble gases are unreactive due to full outer shells. For example, potassium is below sodium in Group 1, so it reacts more vigorously with water, producing hydrogen and potassium hydroxide. You can also predict likely reaction partners: Group 1 metals react with Group 7 non-metals to form ionic salts such as sodium chloride.

What earns the marks

• Identify the element's group and period from its position in the periodic table.
• Relate group number to the number of outer-shell electrons and hence to similarity in chemical properties.
• State and apply the trend in reactivity down Group 1: reactivity increases as the outer electron is lost more easily.
• State and apply the trend in reactivity down Group 7: reactivity decreases as it becomes harder to gain an electron.
• Recognise that Group 0 elements are unreactive because they have full outer electron shells.
• Predict a plausible reaction by combining a reactive metal from Group 1 or 2 with a non-metal from Group 6 or 7, for example sodium with chlorine forming sodium chloride.
• Use a named example, such as potassium reacting more vigorously than sodium with water, to support a prediction.

Common mistakes and how to put them right

• Assuming all Group 1 metals react identically.

  Put it right by stating that reactivity increases down the group, so potassium reacts more vigorously than lithium.

• Confusing the trends in Group 1 and Group 7.

  Put it right by remembering that Group 1 reactivity increases down the group, while Group 7 reactivity decreases down the group.

• Thinking noble gases are reactive because they are in the periodic table.

  Put it right by explaining that Group 0 elements have full outer shells and are chemically unreactive.

• Predicting reactivity from atomic mass alone.

  Put it right by using group position and outer-shell electron configuration as the main basis for prediction.

Exam tips

• Always name the group and period of the element before making a prediction, so your reasoning is clear.
• Link reactivity to electron loss or gain and mention the outer-shell electron arrangement.
• Use a specific example, such as caesium reacting more vigorously than lithium with water, to show understanding of the trend.
• If asked to predict a reaction, name the likely products and the type of reaction, for example a metal plus a non-metal forming an ionic compound.