Question

Calculate the steric numbers for iodine in $\left(I{F}_4^{-}\right)$ and for bromine in $\left(Br{O}_4^{-}\right)$.

• A. $6$ and $4$
• B. $5$ and $6$
• C. $4$ and $6$
• D. $3$ and $4$

Answer 1

Answer: (A)

$6$ and $4$

The central $I^{-}$ has eight valence electrons
Each $F$ atom has seven valence electrons of its own and needs to one of the electrons from $I^{-}$ to attain noble-gas configuration $\left(s^2{p}^6\right)$. Thus, four, of the $I^{-}$ valence electrons take part in covalent bonds, leaving the remaining four to form two lone pairs. Thus,
$S_N=$ 4 (bonded atoms) + 2 (lone pairs) $=6$
The central $B{r}^{-}$ has also eight electrons. In $Br{O}_4^{-}$, each oxygen atom needs to share two of electrons from $B{r}^{-}$ to attain noble-gas configurations thus all the electrons are used in bonding and no lone pair exists.
Thus, $S_N=$ 4 (bonded pair) + 0 (lone pairs) $=4$.