Question

Which of the following statement is false? (1) Among alkali metals, lithium is the softest metal. (2) Ionisation potential of lithium will be greater than that of sodium. (3) In an aqueous solution, lithium is the strongest reducing agent. (4) Alkali metals show abrupt increase in second IP.

Short answer

Statement 1 is false.

Step-by-step solution

- Understanding the Question

Identify which of the given statements about alkali metals is false. The statements cover properties such as softness, ionisation potential, reducing agent strength, and second ionisation potential (IP).

- Review Statement 1

Statement 1: 'Among alkali metals, lithium is the softest metal.' We know that among alkali metals, cesium is the softest. Hence, statement 1 is false.

- Review Statement 2

Statement 2: 'Ionisation potential of lithium will be greater than that of sodium.' Since lithium has a smaller atomic radius compared to sodium, it is more difficult to remove an electron from lithium than from sodium. Thus, lithium has a higher ionisation potential than sodium, making statement 2 true.

- Review Statement 3

Statement 3: 'In an aqueous solution, lithium is the strongest reducing agent.' Due to its high hydration energy, lithium indeed behaves as the strongest reducing agent in aqueous solutions. Therefore, statement 3 is true.

- Review Statement 4

Statement 4: 'Alkali metals show abrupt increase in second IP.' Alkali metals have a single electron in their outermost shell. Removing a second electron (the second ionisation potential) requires removing an electron from a fully stable noble gas configuration, leading to a significant increase in energy. Hence, statement 4 is true.

Key concepts

Softness of Alkali Metals

Alkali metals are known for their soft texture, allowing them to be cut with a knife. This property arises from their metallic bonding, which is relatively weak due to the presence of a single valence electron. As we move down the group in the periodic table, the metals get softer.

Lithium, despite its reactivity, is one of the hardest alkali metals. In contrast, cesium is the softest among the alkali metals. It can be easily squished even under a mild pressure. This softness increases due to the larger atomic size and weaker metallic bonds as you descend the group.

Ionisation Potential

The ionisation potential (IP) measures the energy required to remove an electron from an atom. For alkali metals, this first ionisation energy is relatively low, due to their single valence electron. As we go down the group, the ionisation potential decreases. This is because the valence electron is further from the nucleus in larger atoms, making it easier to remove.

Lithium, having a smaller atomic radius, holds its electrons more tightly when compared to sodium. Therefore, it has a higher ionisation potential than sodium. Essentially, more energy is needed to strip an electron from lithium than from sodium.

Reducing Agents

Reducing agents are substances that donate electrons to another substance in a chemical reaction. Alkali metals are potent reducing agents because they readily lose their single valence electron to achieve a stable noble gas configuration. In aqueous solutions, lithium stands out as the strongest reducing agent among the alkali metals.

This is due to lithium's high hydration energy, which makes it energetically favorable for lithium ions to stabilize in water. As a result, lithium is more effective at losing electrons, making it an exceptionally strong reducing agent in aqueous environments.

Second Ionisation Potential

The second ionisation potential is the energy needed to remove a second electron from a singly charged ion. For alkali metals, the first electron is easily lost, but the second ionisation energy is drastically higher.

This abrupt increase is because the removal of a second electron involves breaking into a noble gas configuration, which is extremely stable. Thus, alkali metals exhibit a significant jump in energy required for the second ionisation, making them resistant to losing more than one electron under normal conditions.