Electron Dot Structures for Covalent Molecules with Single Bonds

Fundamental Rules for Drawing Electron Dot (Lewis) Structures

• Total Valence Electron Calculation: To draw the electron dot structure, one must first determine the total number of valence electrons available for bonding. This is done by summing the valence electrons of each individual atom in the molecule. For neutral molecules, this number is used as the inventory for the final structure.
• Atomic Group Classifications:     * Group 1: Atoms like Hydrogen ($H$) have $1$ valence electron.     * Group 14: Atoms like Carbon ($C$) have $4$ valence electrons.     * Group 15: Atoms like Phosphorus ($P$) and Nitrogen ($N$) have $5$ valence electrons.     * Group 16: Atoms like Oxygen ($O$), Sulfur ($S$), and Selenium ($Se$) have $6$ valence electrons.     * Group 17 (Halogens): Atoms like Fluorine ($F$), Chlorine ($Cl$), and Bromine ($Br$) have $7$ valence electrons.
• Octet and Duet Rules: Most atoms seek to be surrounded by $8$ electrons (the octet rule). However, Hydrogen ($H$) is an exception and seeks only $2$ electrons (the duet rule) to achieve the electronic configuration of Helium.
• Bonding and Lone Pairs: A single covalent bond consists of a pair of shared electrons. After forming bonds to connect the atoms, any remaining electrons in the valence inventory are assigned as lone pairs (non-bonding pairs) to complete the octets of the atoms, starting with the most electronegative terminal atoms.

Molecular Analysis: Hydrogen Disulfide ($H_2S_2$)

• Valence Electron Count:     * Hydrogen: $2 \times 1 = 2$     * Sulfur: $2 \times 6 = 12$     * Total Valence Electrons: $2 + 12 = 14$
• Structural Arrangement: The molecule is arranged in a chain-like structure where the two Sulfur atoms are bonded to each other, and each Sulfur atom is also bonded to one Hydrogen atom ($H-S-S-H$).
• Electron Distribution:     * Three single bonds are formed: two $S-H$ bonds and one $S-S$ bond.     * Total electrons used in bonding: $3 \times 2 = 6$     * Remaining electrons: $14 - 6 = 8$     * The remaining $8$ electrons are distributed as lone pairs. Each Sulfur atom receives two lone pairs ($4$ electrons) to complete its octet. Each Hydrogen atom satisfies the duet rule with the shared pair from its bond to Sulfur.

Molecular Analysis: Hydrogen Selenide ($H_2Se$)

• Valence Electron Count:     * Hydrogen: $2 \times 1 = 2$     * Selenium: $1 \times 6 = 6$     * Total Valence Electrons: $2 + 6 = 8$
• Structural Arrangement: Selenium acts as the central atom with two Hydrogen atoms extending from it ($H-Se-H$).
• Electron Distribution:     * Two single bonds ($Se-H$) are formed.     * Total electrons used in bonding: $2 \times 2 = 4$     * Remaining electrons: $8 - 4 = 4$     * The remaining $4$ electrons are placed on the central Selenium atom as two lone pairs. This completes the octet for Selenium ($4$ bonding electrons + $4$ lone pair electrons), while the Hydrogen atoms satisfy the duet rule.

Molecular Analysis: Phosphorus Trifluoride ($PF_3$)

• Valence Electron Count:     * Phosphorus: $1 \times 5 = 5$     * Fluorine: $3 \times 7 = 21$     * Total Valence Electrons: $5 + 21 = 26$
• Structural Arrangement: Phosphorus is the central atom, surrounded by three terminal Fluorine atoms.
• Electron Distribution:     * Three single bonds ($P-F$) are formed.     * Total electrons used in bonding: $3 \times 2 = 6$     * Remaining electrons: $26 - 6 = 20$     * First, complete the octets of the terminal Fluorine atoms. Each Fluorine atom already has $2$ electrons from the single bond, so each requires $6$ more electrons (three lone pairs). Total electrons for Fluorines: $3 \times 6 = 18$.     * Remaining electrons: $20 - 18 = 2$     * The final $2$ electrons are placed on the central Phosphorus atom as one lone pair. This gives Phosphorus a complete octet ($6$ bonding electrons + $2$ lone pair electrons).

Molecular Analysis: Carbon Tetrachloride ($CCl_4$)

• Valence Electron Count:     * Carbon: $1 \times 4 = 4$     * Chlorine: $4 \times 7 = 28$     * Total Valence Electrons: $4 + 28 = 32$
• Structural Arrangement: Carbon is positioned as the central atom with four terminal Chlorine atoms located at the corners of a tetrahedron.
• Electron Distribution:     * Four single bonds ($C-Cl$) are formed.     * Total electrons used in bonding: $4 \times 2 = 8$     * Remaining electrons: $32 - 8 = 24$     * The remaining $24$ electrons are distributed among the four terminal Chlorine atoms to complete their octets. Each Chlorine atom receives three lone pairs ($6$ electrons). Since there are four Chlorine atoms, $4 \times 6 = 24$ electrons are perfectly utilized. The Carbon atom achieves its octet through the four single bonds.

Molecular Analysis: Bromine Monofluoride ($BrF$)

• Valence Electron Count:     * Bromine: $1 \times 7 = 7$     * Fluorine: $1 \times 7 = 7$     * Total Valence Electrons: $7 + 7 = 14$
• Structural Arrangement: This is a diatomic molecule consisting of one Bromine atom and one Fluorine atom ($Br-F$).
• Electron Distribution:     * One single bond ($Br-F$) connects the two atoms.     * Total electrons used in bonding: $1 \times 2 = 2$     * Remaining electrons: $14 - 2 = 12$     * The remaining $12$ electrons are distributed as lone pairs to complete the octets of both atoms. Both Bromine and Fluorine receive three lone pairs ($6$ electrons each). Result: $2$ shared electrons + $12$ lone pair electrons = $14$ total valence electrons used.

Molecular Analysis: Chlorine Monofluoride ($ClF$)

• Valence Electron Count:     * Chlorine: $1 \times 7 = 7$     * Fluorine: $1 \times 7 = 7$     * Total Valence Electrons: $7 + 7 = 14$
• Structural Arrangement: A diatomic molecule consisting of Chlorine and Fluorine ($Cl-F$).
• Electron Distribution:     * One single bond ($Cl-F$) is formed.     * Total electrons used in bonding: $1 \times 2 = 2$     * Remaining electrons: $14 - 2 = 12$     * Each atom (Chlorine and Fluorine) is assigned three lone pairs ($2 \times 6 = 12$) to fulfill the octet requirements. This matches the result for Bromine Monofluoride due to both atoms being in the halogen group.

Molecular Analysis: Ammonia ($NH_3$)

• Valence Electron Count:     * Nitrogen: $1 \times 5 = 5$     * Hydrogen: $3 \times 1 = 3$     * Total Valence Electrons: $5 + 3 = 8$
• Structural Arrangement: Nitrogen is the central atom, with three terminal Hydrogen atoms.
• Electron Distribution:     * Three single bonds ($N-H$) are formed.     * Total electrons used in bonding: $3 \times 2 = 6$     * Remaining electrons: $8 - 6 = 2$     * The remaining two electrons are placed on the central Nitrogen atom as a single lone pair. This completes the octet for Nitrogen ($6$ bonding electrons + $2$ lone pair electrons = $8$), and the Hydrogen atoms fulfill their duet requirements via the single bonds.

Molecular Analysis: Hydrogen Peroxide ($H_2O_2$)

• Valence Electron Count:     * Hydrogen: $2 \times 1 = 2$     * Oxygen: $2 \times 6 = 12$     * Total Valence Electrons: $2 + 12 = 14$
• Structural Arrangement: Similar to Hydrogen Disulfide, the molecule forms a chain where Oxygen atoms bond to one another, and each Oxygen is also bonded to one Hydrogen ($H-O-O-H$).
• Electron Distribution:     * Three single bonds are present: one $O-O$ bond and two $O-H$ bonds.     * Total electrons used in bonding: $3 \times 2 = 6$     * Remaining electrons: $14 - 6 = 8$     * The remaining $8$ electrons are assigned as lone pairs to the Oxygen atoms to complete their octets. Each Oxygen atom receives two lone pairs ($4$ electrons). Hydrogen atoms are satisfied with their single shared pair.