Draw the skeletal structure of 2-methylprop-1-ene and confirm the valency of each carbon atom.

Let's analyze the bonds for each carbon in your drawing:

• Carbon 1 ($\text{C}_1$): It has a double bond to $\text{C}_2$ and two single bonds to hydrogen atoms. This gives $\text{C}_1$ a total of $2 + 1 + 1 = 4$ bonds, which is correct.
• Carbon 2 ($\text{C}_2$): In your drawing, $\text{C}_2$ has:
  • A double bond to $\text{C}_1$ (2 bonds)
  • A single bond to $\text{C}_3$ (1 bond)
  • A single bond to the $\text{CH}_3$ group (1 bond)
  • A single bond to a hydrogen atom (1 bond) This totals $2 + 1 + 1 + 1 = 5$ bonds for $\text{C}_2$. However, carbon can only form a maximum of 4 bonds.
• Carbon 3 ($\text{C}_3$): It has a single bond to $\text{C}_2$ and three single bonds to hydrogen atoms. This gives $\text{C}_3$ a total of $1 + 1 + 1 + 1 = 4$ bonds, which is correct.

The error is that $\text{C}_2$ in 2-methylprop-1-ene should not have a hydrogen atom directly attached to it. It already forms 4 bonds with the double bond to $\text{C}_1$, the single bond to $\text{C}_3$, and the single bond to the methyl group.

The correct structure for 2-methylprop-1-ene is:

$$\text{H}_2\text{C}=\text{C} \begin{pmatrix} \text{CH}_3 \\ | \end{pmatrix} -\text{CH}_3$$

Or, showing all bonds:

$$\begin{matrix} \text{H} \\ | \\ \text{H}-\text{C}_1 \\ || \\ \text{H}_3\text{C}-\text{C}_2-\text{C}_3-\text{H} \\ | \\ \text{H} \\ | \\ \text{H} \end{matrix}$$

In this correct structure:

• $\text{C}_1$ has 2 bonds to H and 2 bonds to $\text{C}_2$ (total 4 bonds).
• $\text{C}_2$ has 2 bonds to $\text{C}_1$, 1 bond to $\text{C}_3$, and 1 bond to the $\text{CH}_3$ group (total 4 bonds).
• $\text{C}_3$ has 1 bond to $\text{C}_2$ and 3 bonds to H (total 4 bonds).