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

C2H4 has a double bond between carbons and N2H4 has a single bond between nitrogens, each completing their valence with hydrogens.

Step-by-step solution

Count Valence Electrons for C2H4

To draw the Lewis structure for \(\mathrm{C}_2\mathrm{H}_4\), start by counting the total number of valence electrons available. Carbon (C) has 4 valence electrons, and hydrogen (H) has 1 valence electron. Since there are two carbon atoms and four hydrogen atoms, the total number is \(2 \times 4 + 4 \times 1 = 12\) valence electrons.

Draw Skeleton Structure for C2H4

Organize the atoms with carbon atoms connected by a double bond, and each carbon has two hydrogen atoms attached: \(\mathrm{H} - \mathrm{C} = \mathrm{C} - \mathrm{H}\). This leaves all valence electrons accounted for (each bond uses 2 electrons).

Count Valence Electrons for N2H4

For \(\mathrm{N}_2\mathrm{H}_4\), calculate the total number of valence electrons. Nitrogen (N) has 5 valence electrons, and hydrogen has 1 valence electron. Thus, \(2 \times 5 + 4 \times 1 = 14\) valence electrons.

Draw Skeleton Structure for N2H4

Arrange the atoms with nitrogen atoms connected by a single bond and each nitrogen attached to two hydrogen atoms: \(\mathrm{H} - \mathrm{N} - \mathrm{N} - \mathrm{H}\). Distribute the remaining valence electrons to satisfy the octet rule for each nitrogen atom, achieving a final structure: \(\mathrm{H} \left(\mathrm{H} - \mathrm{N} - \mathrm{N} - \mathrm{H}\right) \mathrm{H}.\)

Key concepts

Valence Electrons

Understanding valence electrons is crucial for drawing Lewis structures, which are simple representations showing the connections between atoms in a molecule. Valence electrons are the outermost electrons of an atom, participating in chemical bonding. These electrons influence the chemical properties of the element and are the ones you count when constructing Lewis structures.

For instance, in the molecule \( \mathrm{C}_2\mathrm{H}_4 \) (ethylene), each carbon atom has 4 valence electrons since carbon belongs to group 14 of the periodic table. Hydrogen, with only one electron, has a single valence electron. Summing these up, for \( \mathrm{C}_2\mathrm{H}_4 \), we have a total of \( 2 \times 4 + 4 \times 1 = 12 \) valence electrons. Understanding these totals allows us to arrange atoms correctly to reflect actual molecular structures.

Similarly, for \( \mathrm{N}_2\mathrm{H}_4 \), each nitrogen atom has 5 valence electrons, as it is in group 15. Adding these to hydrogen's 4 electrons, we have \( 2 \times 5 + 4 \times 1 = 14 \) valence electrons for the entire molecule. Recognizing the number of valence electrons is the first step in correctly drawing any Lewis structure.

Molecular Geometry

Molecular geometry refers to the 3-dimensional arrangement of atoms within a molecule. The shape is determined by the number of bonds and lone pairs around a central atom, influencing molecular behavior and reactivity.

In \( \mathrm{C}_2\mathrm{H}_4 \), the carbon atoms form a double bond. This configuration creates a planar geometry, often referred to as trigonal planar around each carbon atom. The shape allows the molecule to exhibit specific reactions due to steric and electronic effects of this arrangement.

• In trigonal planar geometry, each atom lies in a plane, and bond angles are approximately 120 degrees surrounding the central atom.

In the case of \( \mathrm{N}_2\mathrm{H}_4 \), each nitrogen is bonded in a manner that adopts a distorted tetrahedral shape. This occurs due to repulsion among the lone pair electrons and the bonded hydrogen atoms.

• This bent tetrahedral geometry results in smaller bond angles than a perfect tetrahedral (typically less than 109.5 degrees).

Understanding these shapes is essential for predicting how a molecule will interact in chemical reactions and the types of physical properties it might exhibit.

Octet Rule

The octet rule is a key concept in chemistry that refers to the tendency of atoms to prefer having eight electrons in their valence shell. This rule provides stability to chemical compounds by allowing each atom to achieve a noble gas configuration.

In \( \mathrm{C}_2\mathrm{H}_4 \), the carbon atoms achieve an octet by sharing electrons through a double bond. Each carbon shares four electrons with adjacent atoms, fulfilling the octet rule. Hydrogen atoms, however, are an exception; they follow the "duet rule" due to having only one energy level, needing just two electrons for stability.

• By achieving an electron configuration similar to helium, hydrogen attains stability with its single bond.

For \( \mathrm{N}_2\mathrm{H}_4 \), the nitrogen atoms also achieve stability by sharing electrons. Each nitrogen atom forms single bonds with hydrogen atoms and another nitrogen atom to complete its octet. This sharing allows nitrogen to emulate the closest noble gas configuration.

Even though there are exceptions to the octet rule, it remains a vital guideline for predicting and understanding how atoms bond in both simple and complex molecules.