How many valence electrons are there in a correctly drawn Lewis structure for formamide, HCONH2
( You should also be able to draw the lewis structure for this. Hint, the C is in the center with an O ,N and 1 H attached. The other H atoms are attached to the N)
In the formate ion, HCO2–, the carbon atom is the central
atom with the other three atoms attached to it. (a) Draw a
Lewis structure for the formate ion. (b) What hybridization
is exhibited by the C atom? (c) Are there multiple equivalent
resonance structures for the ion? (d) How many electrons are
in the π system of the ion?
Draw Lewis structures of all the important resonance forms of (a) HCO2– (H is attached to C); (b) HBrO4 (HOBrO3).
Answer to question 1
Valence electrons are the electrons present in the outermost shell of an atom.
Example: Valence electrons of O is 6.
Lewis structure of an atom or molecule represents the number of valence electrons of an atoms in the molecule.
Number of valence electrons of C=4.
Number of valence electrons of O=6.
Number of valence electrons of H=1.
Number of valence electrons of F=7.
To determine the total number of valence electrons in HCONH2.
Carbon is the central atom in HCONH2 structure.
Answer Total number of valence electrons are 18.
Answer to question 2
The formate ion (HCO2–) is formed by the sharing of electrons between one carbon atom, one hydrogen and 2 oxygen (O) atoms. The electron configuration of the central atom C having atomic number 6 is 1s22s22p2 and that of O with atomic number 8 is 1s22s22p4. The electron configuration of the H having atomic number 1 is 1s1.
The Lewis structure formation for HCO2– involves the following steps.
The valence electrons from all the atoms are added. As evident from their electronic configurations, C has 4 , H has 1 and each O has 6 valence electrons. So, total number of valence electrons in HCO2– is,
HCO2– = [(6 x 2) + (4 x 1) + (1 x 1) + 1] = 18 e–
In HCO2– each O atom needs 2 electrons to complete its octet, C needs 4 electrons and each H needs 1 electron to complete its duplet. It means H and O atom shares two electrons with the C atom forming one C — H and two C — O single bonds represented by solid line in the Lewis structure and from the 12 remaining electrons, carbon forms one C — O
Π- bond. Therefore, the 10 electrons will remain non-bonded and represents as the lone pairs on O atom. Therefore, the Lewis structure of HCO2– is,
The Lewis structure of HCO2– is,
In HCO2– , the carbon atom is surrounded by three atoms, two oxygen and one hydrogen atoms. Thus, the carbon (C) atom has three electron domains which are bonding. Therefore, the geometry of electron domain is trigonal planar having sp2 hybridization. Hence, the hybridization of carbon (C) atom in HCO2– is sp2.
The Lewis structure of HCO2– ion indicates that it has a π- bond between C and O and non bonded pairs on O which can be delocalized between the single bonded C and O to give rise to the equivalent resonating structures. The two resonating structures are equivalent and the actual structure is a resonance hybrid of these two resonating structures. Therefore, the resonating structures of HCO2– ion are,
Answer to question 3
Lewis structure represents the systematic arrangement of atoms around the central atom. Electrons in this structure are indicated by dots. The central atom is the least electronegative element in a molecule. H is always the surrounding atom.
Answer to question 4
The total number of valence electrons:
O=2 x 6=12
The total number of valence electrons= 4 +12 +1 +1 =18