Students should be able to: explain how properties of the elements in…

The idea

Group 1 metals, the alkali metals, each have one electron in their outer shell. That single outer electron is lost relatively easily when the metal reacts, forming a 1⁺ ion and giving the group its characteristic reactivity. Down the group, atomic radius increases and the outer electron is further from the nucleus and shielded by more inner shells, so the attraction holding it is weaker. Consequently reactivity increases down Group 1: lithium reacts steadily, sodium more vigorously, potassium vigorously enough to ignite, and rubidium and caesium are more reactive still. Melting points and boiling points decrease down the group because metallic bonding weakens as ions become larger and charge density falls. To predict properties, identify the trend from the data given, then extend it to the next element, justifying the prediction using outer-shell electron loss and increased atomic size.

What earns the marks

• State that Group 1 atoms each have one electron in their outer shell and lose it to form a 1⁺ ion.
• Explain that down the group the outer electron is in a shell further from the nucleus and is shielded by more inner shells, so less energy is needed to remove it.
• Link weaker attraction for the outer electron to increased reactivity down the group.
• Use given data, such as melting point or reaction observations, to identify a trend and predict a property of another Group 1 element.
• Justify a prediction by referring to atomic size, shielding and the ease of losing the outer electron.
• Describe observations such as fizzing, movement on water and flame colour as evidence of reactivity.

Common mistakes and how to put them right

• Saying reactivity decreases down Group 1 because atoms get bigger.

  Put it right by explaining that the larger distance and extra shielding weaken the attraction for the outer electron, so reactivity increases.

• Confusing Group 1 with Group 7 trends.

  Put it right by stating that Group 1 reactivity increases down the group whereas Group 7 reactivity decreases.

• Writing the ion as a baseline numeral with a postfix sign.

  Put it right by using the superscript form such as Na⁺ and K⁺.

• Treating melting point and reactivity as following the same trend.

  Put it right by noting that melting point decreases down Group 1 while reactivity increases.

Exam tips

• When asked to predict, quote the trend in the data first, then state the predicted value or behaviour for the named element.
• Use the phrase outer electron throughout your explanation, because it keeps the answer focused on electronic structure.
• For six-mark questions, structure your answer as electronic structure, then distance and shielding, then reactivity, then a prediction.
• Check that any ion you write uses a superscript charge, for example Li⁺, not a baseline form.