Chemistry · Periodic Classification Of Properties · NEET
In a metal, positive ions sit in a 'sea' of free (delocalised) electrons. This metallic bond is strong, so most metals need a lot of heat to melt - that is why they are usually high-melting solids. In Hg, Ga and Cs this metallic bonding is unusually WEAK, so very little heat separates the atoms. That is why these three are the famous low-melting exceptions.
LOW. Gallium melts at only about 303 K, which is roughly 30°C - it will melt in the warmth of your hand. A very common NEET trap says 'gallium has a very HIGH melting point' - that statement is FALSE. Remember: Ga is low-melting, right next to Cs (302 K).
Mercury's outer electrons are held tightly and do not join the free electron 'sea' well, so the bonding between mercury atoms is weak. Because the metallic bond is weak, mercury cannot stay solid at room temperature and stays liquid. It is the only common metal that is liquid at 25°C.
NCERT gives gallium = 303 K and caesium = 302 K. Both are just above room temperature (about 29-30°C), so both are near-body-heat melting. Mercury is already liquid, so no melting point above room temperature is listed for it in this context.
They are exceptions to the general statement 'metals have high melting points,' not a break in a smooth group/period trend. NCERT lists them as special cases: 'metals are usually solids... mercury is an exception; gallium and caesium also have very low melting points.' For NEET, just memorise the three exceptions and their reason (weak metallic bonding).
Which of the following statements are true? A. Unlike Ga, that has a very high melting point, Cs has a very low melting point. B. On the Pauling scale, the electronegativity values of N and Cl are not the same. C. Ar, K+, Cl-, Ca2+ and S2- are all isoelectronic species. D. The correct order of the first ionization enthalpies of Na, Mg, Al and Si is Si > Al > Mg > Na. E. The atomic radius of Cs is greater than that of Li and Rb.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Mercury (Hg), gallium (Ga) and caesium (Cs). Hg is liquid at room temperature; Ga melts at 303 K and Cs at 302 K.
Mercury is a liquid at room temperature (about 25°C). It is the only common metal that is liquid under normal conditions, because its metallic bonding is weak.
Yes. Gallium melts at about 303 K (~30°C), which is close to body temperature, so holding a piece of gallium can melt it.
Caesium is a large Group 1 atom with only one loosely held outer electron, so its metallic bond is weak. It melts at about 302 K (~29°C).
Weak metallic bonding. The bond between their atoms is weaker than in typical metals, so much less heat is needed to melt them.
