Question

your turn, from structure to sigma and pi counts
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complete the following table:

bond type	single (~0:06)	double (~0:15)	triple (~0:19)
# of π

complete the following table:

molecule	ethene (02:05) <img src=\ethene_molecule_image\>	acetylene (03:17) <img src=\acetylene_structure_image\ alt=\h-c≡c-h with bond lengths 120.3 pm and 106.0 pm\>	formaldehyde, ch₂o (03:57) <img src=\formaldehyde_structure_image\ alt=\h-c(=o)-h\>
# of σ, # of π

Explanation:

To solve for the number of sigma ($\sigma$) and pi ($\pi$) bonds in each molecule, we analyze their structures:

1. Ethene ($\boldsymbol{\ce{C2H4}}$)

Ethene has a double bond ($\ce{C=C}$) and four single bonds ($\ce{C-H}$).

• A double bond has 1 $\sigma$ and 1 $\pi$ bond.
• Each single bond is 1 $\sigma$ bond.

Step 1: Count $\sigma$ bonds

Double bond $\sigma$: $1$
Single bonds ($\ce{C-H}$): $4$
Total $\sigma$: $1 + 4 = 5$

Step 2: Count $\pi$ bonds

Double bond $\pi$: $1$
Total $\pi$: $1$

2. Acetylene ($\boldsymbol{\ce{C2H2}}$)

Acetylene has a triple bond ($\ce{C≡C}$) and two single bonds ($\ce{C-H}$).

• A triple bond has 1 $\sigma$ and 2 $\pi$ bonds.
• Each single bond is 1 $\sigma$ bond.

Step 1: Count $\sigma$ bonds

Triple bond $\sigma$: $1$
Single bonds ($\ce{C-H}$): $2$
Total $\sigma$: $1 + 2 = 3$

Step 2: Count $\pi$ bonds

Triple bond $\pi$: $2$
Total $\pi$: $2$

3. Formaldehyde ($\boldsymbol{\ce{CH2O}}$)

Formaldehyde has a double bond ($\ce{C=O}$) and two single bonds ($\ce{C-H}$).

• A double bond has 1 $\sigma$ and 1 $\pi$ bond.
• Each single bond is 1 $\sigma$ bond.

Step 1: Count $\sigma$ bonds

Double bond $\sigma$: $1$
Single bonds ($\ce{C-H}$): $2$
Total $\sigma$: $1 + 2 = 3$

Step 2: Count $\pi$ bonds

Double bond $\pi$: $1$
Total $\pi$: $1$

Final Table (Filled)

Molecule	$\boldsymbol{\sigma}$ Bonds	$\boldsymbol{\pi}$ Bonds
Acetylene	3	2
Formaldehyde	3	1

Key Takeaway

• Single bonds: $1\ \sigma$, $0\ \pi$
• Double bonds: $1\ \sigma$, $1\ \pi$
• Triple bonds: $1\ \sigma$, $2\ \pi$

This follows the rule: all bonds have one $\sigma$ bond, and additional bonds (beyond the first) in multiple bonds are $\pi$ bonds.

Answer:

To solve for the number of sigma ($\sigma$) and pi ($\pi$) bonds in each molecule, we analyze their structures:

1. Ethene ($\boldsymbol{\ce{C2H4}}$)

Ethene has a double bond ($\ce{C=C}$) and four single bonds ($\ce{C-H}$).

• A double bond has 1 $\sigma$ and 1 $\pi$ bond.
• Each single bond is 1 $\sigma$ bond.

Step 1: Count $\sigma$ bonds

Double bond $\sigma$: $1$
Single bonds ($\ce{C-H}$): $4$
Total $\sigma$: $1 + 4 = 5$

Step 2: Count $\pi$ bonds

Double bond $\pi$: $1$
Total $\pi$: $1$

2. Acetylene ($\boldsymbol{\ce{C2H2}}$)

Acetylene has a triple bond ($\ce{C≡C}$) and two single bonds ($\ce{C-H}$).

• A triple bond has 1 $\sigma$ and 2 $\pi$ bonds.
• Each single bond is 1 $\sigma$ bond.

Step 1: Count $\sigma$ bonds

Triple bond $\sigma$: $1$
Single bonds ($\ce{C-H}$): $2$
Total $\sigma$: $1 + 2 = 3$

Step 2: Count $\pi$ bonds

Triple bond $\pi$: $2$
Total $\pi$: $2$

3. Formaldehyde ($\boldsymbol{\ce{CH2O}}$)

Formaldehyde has a double bond ($\ce{C=O}$) and two single bonds ($\ce{C-H}$).

• A double bond has 1 $\sigma$ and 1 $\pi$ bond.
• Each single bond is 1 $\sigma$ bond.

Step 1: Count $\sigma$ bonds

Double bond $\sigma$: $1$
Single bonds ($\ce{C-H}$): $2$
Total $\sigma$: $1 + 2 = 3$

Step 2: Count $\pi$ bonds

Double bond $\pi$: $1$
Total $\pi$: $1$

Final Table (Filled)

Molecule	$\boldsymbol{\sigma}$ Bonds	$\boldsymbol{\pi}$ Bonds
Acetylene	3	2
Formaldehyde	3	1

Key Takeaway

• Single bonds: $1\ \sigma$, $0\ \pi$
• Double bonds: $1\ \sigma$, $1\ \pi$
• Triple bonds: $1\ \sigma$, $2\ \pi$

This follows the rule: all bonds have one $\sigma$ bond, and additional bonds (beyond the first) in multiple bonds are $\pi$ bonds.