Answer – Mercury (Hg) and bromine (Br) are the only two naturally occurring elements that are liquid at room temperature.

Explanation:

Unlike other metals on the periodic table, mercury is the only liquid metal at room temperature. This is largely because of the unique way in which its electrons arrange themselves, causing its atoms to keep from sharing electrons with one another.

To understand this better, let us observe the electron configuration of mercury: [Xe] 4f¹⁴ 5d¹⁰ 6s². 

Here, although the 4f orbital is completely filled, the electrons in it do not protect the valence electrons in the 6s shell from the positively charged nucleus as well as they should. So the 6s electrons are drawn closer to the nucleus, reducing the atomic radius and hence the size of the atom. Also, because the 6s shell of mercury atoms is full, it is highly stable. So the atoms neither gain nor lose electrons easily.

The combined effect of the strong attraction of valence electrons to the nucleus and the highly stable valence shell cause mercury to behave like a noble gas. This prevents the atoms from interacting with each other as much as they should to solidify, causing the element to remain a liquid at room temperature.

Similarly, bromine is the only non-metal that is a liquid at room temperature. Let’s explore why this is.

Bromine (Br) occurs as a diatomic molecule (with 2 atoms in a molecule) similar to other halogens like fluorine (F), chlorine (Cl), and iodine (I).

As we go down Group 17 of the periodic table (from F through Cl and Br to I), the atomic radius increases due to the increase in the number of energy levels (to accommodate the increasing number of electrons). This causes weak intermolecular forces of attraction called the London dispersion forces to also increase between the atoms of each molecule at the same temperature. 

London dispersion forces are responsible for nonpolar molecules like those of the halogens to condense into liquids or freeze into solids. So, as we move down the group, the molecules have a greater tendency to stick to one another due to the increasing dispersion forces, changing the physical state in which they occur. Thus, in the case of Br, the London dispersion forces are stronger than that of F or Cl, causing it to occur as a liquid at room temperature. Similarly, the forces are stronger in I than in Br causing I to occur as a solid at room temperature.

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