NDA (I) 2018 General Ability Test · Question 106 of 150

Correct — D, Caesium. Melting point falls steadily down group 1, and caesium is the lowest of the four offered: lithium melts at about 180·5 °C, sodium at 97·7, potassium at 63·4, rubidium at 39·3 and caesium at only 28·4 °C. That is barely above a warm room, which is why a sealed sample of caesium turns to liquid on a hot day. The reason is metallic bonding. Each alkali atom contributes just one electron to the shared electron sea, and as the atom grows larger down the group that single electron is spread ever more thinly over a bigger ion, so the metallic bond weakens and less heat is needed to break the lattice apart.

• (a)Sodium — Sodium is the most familiar alkali metal and so the most tempting guess, but at about 97·7 °C it has the highest melting point of the four listed — it barely melts in boiling water.
• (b)Potassium — Potassium melts at about 63·4 °C, low for a metal but still more than twice the caesium figure. It matters in another question on this very paper: potassium alloyed with sodium gives a coolant that is liquid at room temperature.
• (c)Rubidium — Rubidium at about 39·3 °C is the runner-up and the closest call on the list, but caesium is still eleven degrees lower. Rubidium sits one period above caesium, so it is the one element you have to reason past.

Group 1 metals are soft, light and low-melting because each atom donates only one valence electron to the metallic lattice. Descending the group the atomic radius grows and the nuclear pull on the outer electron weakens, so the metallic bond becomes progressively feebler. Melting point, boiling point and hardness all fall in step, while density mostly rises and reactivity with water increases sharply.

Two habits make items like this reliable. The first is to know the direction of the trend rather than the numbers — melting point decreases down group 1, so the answer is whichever element sits lowest in the group among the options offered. Had the list included francium the reasoning would be the same, though francium is intensely radioactive and its properties are estimated rather than measured, which is why examiners leave it out. The second habit is to keep group 1 apart from group 2: magnesium and calcium melt far higher than their group 1 neighbours because each contributes two electrons to the lattice instead of one. Caesium's low melting point is not a curiosity — together with its very low ionisation energy it is why caesium and rubidium are the metals of choice for photoelectric cells and atomic clocks.

• Melting points: lithium about 180·5 °C, sodium 97·7, potassium 63·4, rubidium 39·3, caesium 28·4.
• Melting point, boiling point and hardness all decrease down group 1 as the metallic bond weakens.
• The only metals that are liquid at ordinary room temperature and pressure are mercury and, in a warm room, caesium and gallium — bromine is the one liquid non-metal.
• Group 2 metals melt much higher than their group 1 neighbours because each atom supplies two bonding electrons.

• Picking sodium because it is the alkali metal one meets most often.
• Reversing the trend and assuming the heaviest element must melt highest, as it would in many other groups.
• Mixing up melting point with density, which does not fall neatly down group 1.

NDA asks for the extreme member of a periodic trend — lowest melting point, largest atom, most reactive — with three plausible neighbours as distractors.

• practice — not a real PYQ

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

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

  Answer(a) Lithium — at about 180·5 °C it is the highest of the group, since melting point falls as one moves down.

• practice — not a real PYQ

  The melting points of the alkali metals decrease down the group mainly because

  • (a)the number of valence electrons increases
  • (b)the atomic size increases and the metallic bond weakens
  • (c)the elements become more electronegative
  • (d)the nuclear charge decreases

  Answer(b) the atomic size increases and the metallic bond weakens — one electron is shared over an ever larger ion.