Question

The compound(s) with two lone pair of electrons on the central atom is/ are:

A. $Br{F_5}$

B. $Cl{F_3}$

C. $Xe{F_4}$

D. $S{F_4}$

Answer 1

The lone pair on the central atom can be calculated via knowing the valence electrons left in the outermost shell in the attached elements. The pairing of electrons in the attached groups to the central atom allows bond formation to take place.

Complete answer:

A lone pair refers to a pair of valence electrons that are not shared or involved with another atom in a covalent bond formation. Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding. Thus, the number of lone pair electrons plus the number of bonding electrons equals the total number of valence electrons around an atom. Valence shell electron pair repulsion theory, or VSEPR theory is a model to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms.

In the compound $Br{F_5}$ bromine central atom has 7 valence electrons out of which five are shared with five Fluorine atoms leaving two electrons and one lone pair. Similarly in $Cl{F_3}$ chlorine has 7 valence electrons, out of which three are shared with three Fluorine atoms, leaving four electrons in their outermost shell giving it two lone pairs.

In the compound $Xe{F_4}$, Xenon has 8 valence electrons, out of which four are shared with four Fluorine atoms, leaving four valence electrons and two lone pairs. In the compound, $S{F_4}$ sulphur has 6 valence electrons, out of which four are shared with four F atoms leaving two electrons which is one lone pair of electrons.

Therefore in the above examples there are two compounds with two lone pairs of electrons on the central atoms are $Cl{F_3}$ and $Xe{F_4}$.

Hence, the correct option can be option (B) or option(C).

Note: The molecular geometries of molecules change when the central atom has one or more lone pairs of electrons. The total number of electron pairs, both bonding pairs and lone pairs, leads to electron domain geometry. When one or more of the bonding pairs of electrons is replaced with a lone pair, the molecular geometry (actual shape) of the molecule is altered.