Question

bromomethane, $ch_3br$, is a very useful fumigant against a wide range of agricultural pests, from insects to the verticillium wilt fungus. draw the lewis structure of $ch_3br$.

Explanation:

Step1: Count valence electrons

Carbon ($C$) has 4 valence electrons, hydrogen ($H$) has 1 valence electron (and there are 3 $H$ atoms), bromine ($Br$) has 7 valence electrons. So total valence electrons = $4+(3\times1)+7 = 14$.

Step2: Determine the central atom

Carbon is the central atom as it is less electronegative than bromine and can form more bonds.

Step3: Connect atoms with single bonds

Connect $C$ to 3 $H$ atoms and 1 $Br$ atom with single bonds. Each single bond uses 2 electrons. So far, $(3 + 1)\times2=8$ electrons are used.

Step4: Distribute remaining electrons

We have $14 - 8=6$ electrons left. These are placed as lone pairs on $Br$ (since $H$ can't have lone pairs). $Br$ gets 3 lone pairs (6 electrons).

Answer:

The Lewis structure has $C$ as the central atom. $C$ is bonded to 3 $H$ atoms and 1 $Br$ atom. $Br$ has 3 lone pairs of electrons. So the structure is $H - C - H$ (with $C$ also bonded to $Br$ which has 3 lone pairs) and the other two $H$ atoms are also bonded to $C$.