Question 5.2: Draw Lewis structures for the following molecules. a. PF3, a......
Draw Lewis structures for the following molecules.
a. PF_{3}, a molecule in which P is the central atom and all F atoms are bonded to it (see Figure 5.5).
b. HCN, a molecule in which C is the central atom (see Figure 5.6).
a. Step 1: Phosphorus (Group VA) has 5 valence electrons, and each of the fluorine atoms (Group VIIA) has 7 valence electrons. The total electron count is 5 + 3(7) = 26.
Step 2: Drawing the molecular skeleton with single covalent bonds (2 electrons) placed between all bonded atoms gives
\quad\quad\quad\quad \begin{matrix}F:\underset{.\ .}{P}:F\\F\end{matrix}
Step 3: Adding nonbonding electrons to the structure to complete the octets of all atoms bonded to the central atom gives
\quad\quad\quad\quad \begin{matrix}:\overset{.\ .}{\underset{.\ .}{F}}:\underset{.\ .}{P}:\overset{.\ .}{\underset{.\ .}{F}}:\\:\underset{.\ .}{F}:\end{matrix}
At this point, we have used 24 of the 26 available electrons.
Step 4: The central P atom has only 6 electrons; it needs 2 more. The 2 remaining available electrons are placed on the P atom, completing its octet. All atoms now have an octet of electrons.
\quad\quad\quad\quad \begin{matrix}:\overset{.\ .}{\underset{.\ .}{F}}:\overset{.\ .}{\underset{.\ .}{P}}:\overset{.\ .}{\underset{.\ .}{F}}:\\:\underset{.\ .}{F}:\end{matrix}
Step 5: This step is not needed; the central atom already has an octet of electrons.
Step 6: There are 26 electrons in the Lewis structure, the same number of electrons we calculated in Step 1.
b. Step 1: Hydrogen (Group IA) has 1 valence electron, carbon (Group IVA) has 4 valence electrons, and nitrogen (Group VA) has 5 valence electrons. The total number of electrons is 10.
Step 2: Drawing the molecular skeleton with single covalent bonds between bonded atoms gives
\quad\quad\quad\quadH : C : N
Step 3: Adding nonbonding electron pairs to the structure such that the atoms bonded to the central atom have “octets” gives
\quad\quad\quad\quad H:C:\overset{.\ .}{\underset{.\ .}{N}}:
Remember that hydrogen needs only 2 electrons.
Step 4: The structure in Step 3 has 10 valence electrons, the total number available. Thus there are no additional electrons available to place on the carbon atom to give it an octet of electrons.
Step 5: To give the central carbon atom its octet, 2 nonbonding electron pairs on the nitrogen atom are used to form a carbon-nitrogen triple bond.
Step 6: The Lewis structure has 10 electrons, as calculated in Step 1.