Question

Consider the molecules \(\mathrm{C}_{2} \mathrm{H}_{4}\) and \(\mathrm{N}_{2} \mathrm{H}_{4} .\) Draw a Lewis dot structure for each of the molecules.

Short answer

Draw the Lewis structures using the steps: C2H4 with a double bond between carbons, N2H4 with a single bond between nitrogens and lone pairs on nitrogens.

Step-by-step solution

Count the Valence Electrons for C2H4

For the ethylene molecule (\(\mathrm{C}_{2} \mathrm{H}_{4}\)), calculate the total number of valence electrons. Carbon has 4 valence electrons, and hydrogen has 1. Therefore, we have:\[2(4) + 4(1) = 12 \text{ valence electrons.}\]

Draw the Basic Structure for C2H4

Connect the two carbon atoms with a double bond, and attach each of the hydrogen atoms to the carbon atoms. This accounts for the structure \(\text{H}_2\text{C} = \text{CH}_2\).

Distribute Electrons in C2H4

Place the remaining valence electrons around the carbon atoms to satisfy the octet rule. Ensure that the carbons and hydrogens have their required valence electrons, with the carbons completing their octet with the help of a double bond.

Count the Valence Electrons for N2H4

For the hydrazine molecule (\(\mathrm{N}_{2} \mathrm{H}_{4}\)), calculate the total number of valence electrons. Nitrogen has 5 valence electrons and hydrogen has 1. Thus, we have:\[2(5) + 4(1) = 16 \text{ valence electrons.}\]

Draw the Basic Structure for N2H4

Connect the two nitrogen atoms with a single bond, and attach two hydrogen atoms to each nitrogen atom. The basic structure is \(\text{H}_2\text{N} - \text{NH}_2\).

Distribute Electrons in N2H4

Use the remaining valence electrons to complete the octet for each nitrogen atom. The hydrogens are already satisfied with 2 electrons each; the remaining electrons go as lone pairs on the nitrogen atoms.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. These electrons are involved in forming bonds between atoms because they can be shared, lost, or gained. Understanding how to count valence electrons is essential when drawing Lewis structures.

• Carbon has 4 valence electrons.
• Nitrogen has 5 valence electrons.
• Hydrogen, being a simple atom, has just 1 valence electron.

When you calculate the total number of valence electrons for a molecule, you just add up the valence electrons for each atom in the molecule. For example in ethylene ( C₂ H₄), we have 2( 4) + 4( 1) = 12 valence electrons. Similarly, for hydrazine ( N₂ H₄), it's 2( 5) + 4( 1) = 16 valence electrons.

Chemical Bonding

Chemical bonding involves the interaction between atoms that leads to the formation of molecules. Atoms can combine in various ways, but the main types of bonding are covalent and ionic.

• Covalent Bonds: This is where atoms share electrons. In our exercises, the molecules, C₂ H₄ and N₂ H₄, both feature covalent bonding. In ethylene, carbons share electrons with each other and with hydrogens forming stable molecules.
• Ionic Bonds: These occur when electrons are completely transferred from one atom to another, forming ions, but this wasn't encountered in our examples as they focused on covalent bonding.

Bonding serves the purpose of achieving a stable electron configuration, often seen as achieving full outer electron shells.

Octet Rule

The octet rule is a principle that helps to predict how atoms bond together. According to this rule, atoms are most stable when they have eight electrons in their valence shell, similar to the electron configuration of a noble gas. This idealized configuration is often achieved through bonding.

• Carbons in C₂ H₄: The two carbon atoms share a double bond, which counts as four shared electrons, and have enough bonds with hydrogen atoms to satisfy the rule.
• Nitrogens in N₂ H₄: Each nitrogen forms bonds to share electrons and satisfy their octet.
• Hydrogens: While not following the octet rule, hydrogen is stable with just two electrons, occupying its first and only shell.

Following the octet rule allows chemists to predict the arrangement of atoms in a molecule.

Molecular Geometry

Molecular geometry describes the three-dimensional arrangement of atoms within a molecule, which is shaped by the distribution of electrons and bonds around a central atom or atoms.

• Ethylene ( C₂ H₄): It has a planar geometry due to the double bond between the carbon atoms pushing the hydrogens away into a flat arrangement. The carbons and all hydrogens lie in the same plane.
• Hydrazine ( N₂ H₄): This molecule is more spatially complex due to single bonds, allowing for rotations and different angles, typically forming a warped shape.

An understanding of molecular geometry is vital in predicting molecular behavior and interactions, including polarity, reactivity, and biological activity of molecules.