EPFO EO/AO 2023 General Ability Test · Question 54 of 120

Correct — A, (a) Lithium. All four options are alkali metals — the first group of the periodic table — and they are listed in the order in which they occur down that group, so the question is really asking which way the boiling point trend runs. It runs downward. The measured boiling points are about 1342 degrees Celsius for lithium, 883 for sodium, 759 for potassium and 688 for rubidium, with caesium lower still at about 671. Lithium, at the top of the group, boils at roughly twice the temperature of the metal three places below it. The reason is the strength of the metallic bond. Each alkali metal atom contributes just one electron to the shared sea of electrons that holds the lattice together — the fewest of any group — which is why these metals are soft and low-melting to begin with. Going down the group, each element has one more filled shell than the last, so the atom is larger and its single valence electron is further from the nucleus and better screened from it. The positive cores in the lattice are therefore bigger and further apart, and the electrostatic attraction binding them to the electron sea is weaker. Weaker cohesion means less energy is needed to pull the atoms free of one another, and both melting and boiling points fall steadily from lithium to caesium. The same trend accounts for facts a candidate is likelier to have met than the numbers themselves: caesium and rubidium melt at or barely above room temperature, and a block of caesium will liquefy in a warm hand, while lithium remains the hardest and least fusible of the family. Note that this trend must be kept apart from the group's chemical trend, which runs the other way — reactivity INCREASES down the group, because the loosely held outer electron that makes the bonding weak also makes the atom easier to ionise.

• (b)Sodium — Sodium boils at about 883 degrees Celsius, well below lithium's 1342. It is the alkali metal a candidate has handled or seen demonstrated most often, stored under kerosene and cut with a knife, and familiarity is the only thing recommending it here. Its melting point of about 98 degrees is also far below lithium's 181, so it loses on both counts.
• (c)Potassium — Potassium boils at about 759 degrees Celsius, lower again. It is the option chosen by a candidate who reasons from REACTIVITY rather than from bonding: potassium reacts with water far more violently than sodium or lithium, and it is tempting to read vigour as strength. The two trends are opposite. The single valence electron that is held weakly enough to make the metal melt and boil easily is the same electron that is lost readily in reaction, so high reactivity goes with LOW melting and boiling points in this group.
• (d)Rubidium — Rubidium boils at about 688 degrees Celsius and melts at about 39, which is barely above a warm room. It is the lowest of the four offered, so it is the exact opposite of the answer — and it is the option a candidate lands on by inverting the trend, reasoning that a heavier atom must need more energy to boil. Atomic mass is not what holds a metallic lattice together; the strength of the metallic bond is, and that weakens as the atoms grow.

The alkali metals — lithium, sodium, potassium, rubidium, caesium and francium — occupy the first group of the periodic table and each has a single electron in its outermost shell. Almost everything about them follows from that one electron and from the growth in atomic size down the group. Atomic and ionic radii increase downward; ionisation enthalpy and electronegativity decrease downward; melting and boiling points decrease downward, because only one electron per atom is available for metallic bonding and it is progressively further from the nucleus; and chemical reactivity increases downward, since the outer electron is more easily surrendered. Density increases down the group with one famous exception — potassium is lighter than sodium — and lithium is the least dense of all metals. Lithium is anomalous in several other ways as well: it is the hardest of the group, it alone forms a nitride directly with nitrogen, it forms the normal oxide when burnt in air where sodium forms a peroxide and the heavier members form superoxides, and it shows a diagonal relationship with magnesium in the group beyond. All of them are stored out of contact with air and moisture, sodium and potassium under kerosene, because they react vigorously with water to give the hydroxide and hydrogen.

Periodic trends are the most economical thing a candidate can learn for a general science block, because a single remembered direction answers questions about melting point, boiling point, atomic radius, ionisation energy, electronegativity and reactivity in turn. This item is set so that the four options are consecutive members of one group listed in order, which is itself the clue: when the options are a group in sequence, the question is about a trend, and the only decision is which way it points. The habit rewarded is checking whether the physical trend and the chemical trend run the same way, because in this group they do not.

• Approximate boiling points: lithium 1342 degrees Celsius, sodium 883, potassium 759, rubidium 688, caesium 671.
• Approximate melting points: lithium 181 degrees Celsius, sodium 98, potassium 63, rubidium 39, caesium 28.
• Melting and boiling points decrease down group one because the metallic bond weakens as atomic size grows.
• Each alkali metal contributes only one electron to the metallic bond, which is why the group is soft and low-melting overall.
• Chemical reactivity increases down the group, the opposite direction to the melting and boiling point trend.
• Atomic radius increases and ionisation enthalpy decreases down the group.
• Density increases down the group except that potassium is less dense than sodium; lithium is the least dense of all metals.
• Lithium is anomalous — hardest of the group, the only one forming a nitride directly, and diagonally related to magnesium.

• Reasoning from reactivity to melting point, which inverts the answer in this group.
• Assuming the heavier element must have the higher boiling point.
• Confusing the melting point trend of group one with that of the transition metals, which are held by many more electrons and melt far higher.
• Forgetting the density anomaly at potassium when a question moves from boiling point to density.

Periodic table items in EO/AO papers ask for the element at one end of a trend, for the odd one out of a group, or for the element behind a familiar use. Prepare the trends as directions rather than as numbers, keep one anomaly per group in mind, and remember that when the four options are consecutive members of a single group the question can only be about a trend.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which one of the following alkali metals has the lowest melting point ?

  • (a)Lithium
  • (b)Sodium
  • (c)Potassium
  • (d)Caesium

  Answer(d) Caesium

• practice — not a real PYQ

  Sodium metal is stored under kerosene because it :

  • (a)Dissolves in kerosene to form a stable solution
  • (b)Reacts vigorously with air and moisture
  • (c)Would otherwise sublime at room temperature
  • (d)Is radioactive in the presence of light

  Answer(b) Reacts vigorously with air and moisture