Question

Write the Lewis structure for each molecule or ion. \begin{equation}\mathrm a.{N}_{2} \mathrm{H}_{2} \quad \text { b. } \mathrm{N}_{2} \mathrm{H}_{4} \end{equation}\begin{equation} \mathrm c.{C}_{2} \mathrm{H}_{2} \quad \text { d. } \mathrm{C}_{2} \mathrm{H}_{4}\end{equation}

Short answer

N2H2: H-N=N-H with triple bond between N atoms. N2H4: H2N-NH2 with single bond between N atoms. C2H2: H-C\triple bondC-H with triple bond between C atoms. C2H4: H2C=CH2 with double bond between C atoms.

Step-by-step solution

Determining Valence Electrons for N2H2

Nitrogen has 5 valence electrons and hydrogen has 1 valence electron. For N2H2, there are a total of 2(5) + 2(1) = 14 valence electrons.

Drawing the Lewis Structure for N2H2

Put two N atoms in the center with a triple bond between them and single bonds to each H atom. Assign the remaining electrons to fulfill the octet rule where possible. Hydrogens will have 2 electrons each.

Determining Valence Electrons for N2H4

For N2H4, there are a total of 2(5) + 4(1) = 14 valence electrons.

Drawing the Lewis Structure for N2H4

Put two N atoms in the center with a single bond between them. Arrange four H atoms, two on each N atom. Complete the octets of the N atoms and ensure each H has 2 electrons.

Determining Valence Electrons for C2H2

Carbon has 4 valence electrons. For C2H2, there are a total of 2(4) + 2(1) = 10 valence electrons.

Drawing the Lewis Structure for C2H2

Put two C atoms in the center with a triple bond between them and single bonds to the H atoms. Each C has a total of 4 bonds (3 to another C and 1 to an H).

Determining Valence Electrons for C2H4

For C2H4, there are a total of 2(4) + 4(1) = 12 valence electrons.

Drawing the Lewis Structure for C2H4

Put two C atoms in the center with a double bond between them and two H atoms bonded to each C atom. Each C atom has a total of 4 bonds.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how the atom can bond with others. These are the electrons involved in forming chemical bonds because they have the most interaction with neighboring atoms. For instance, in the case of the molecule N₂H₂, nitrogen (N) has 5 valence electrons and hydrogen (H) has 1 valence electron. To calculate the total number of valence electrons for the molecule, we sum up the valence electrons for each atom, taking into account how many of each atom is present in the molecule.

In a Lewis structure, valence electrons are represented as dots around the atomic symbols, with lines or pairs of dots representing shared electrons in chemical bonds. This visual representation helps us understand the bonding and electron arrangement in molecules. When drawing Lewis structures, it's essential to count the valence electrons correctly to predict the molecular structure accurately.

• Nitrogen's 5 valence electrons plus hydrogen's 1 valence electron equal 14 valence electrons for N₂H₂.
• In carbon's case, with 4 valence electrons, for a molecule like C₂H₂, that would be 2(4) + 2(1) = 10 valence electrons.

Understanding and correctly counting valence electrons is the first step in building accurate Lewis structures for molecules and ions.

Octet Rule

The octet rule is a chemical rule of thumb that reflects observation that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electron configuration as a noble gas. The rule is applicable with exceptions, mainly to the second and third periods of the periodic table.

When drawing a Lewis structure, the octet rule guides us in the distribution of electrons to ensure that most atoms (especially Carbon, Nitrogen, Oxygen, and Halogens) achieve an octet either by sharing, losing, or gaining electrons. Hydrogen is an exception to this rule, as it only requires two electrons to achieve the equivalent of a noble gas configuration. In the given examples, N₂H₂ and C₂H₄, nitrogen and carbon aim to complete their octet while hydrogen aims for two electrons:

• For N₂H₂, each hydrogen atom needs one more electron, and each nitrogen atom needs three more electrons.
• In C₂H₄, the carbon atoms share a double bond, which gives them an octet, while each hydrogen atom is satisfied with a single bond.

The desire to fulfill the octet rule drives the formation of bonds in these examples, showing why understanding this principle is vital for mastering chemical bonding.

Chemical Bonding

Chemical bonding is the process by which atoms combine to form compounds. Atoms bond by either transferring (ionic bonding) or sharing (covalent bonding) valence electrons, as dictated by the octet rule. Covalent bonds form when two atoms share one or more pairs of valence electrons, and this can result in single, double, or triple bonds.

The strength of the bond correlates with the number of shared electron pairs, with a triple bond being stronger and shorter than a double or single bond. In the exercise, N₂H₄ and C₂H₂ demonstrate how atoms share electrons to achieve full valence shells and stable structures:

• N₂H₄ illustrates a molecule where a single bond between nitrogen atoms allows the sharing of a pair of electrons, thus satisfying the octet rule after additional bonding with hydrogen atoms.
• C₂H₂ demonstrates a situation where carbon atoms share three pairs of electrons in a triple bond to meet the octet rule, with additional single bonds to hydrogen atoms.

Via these bonds, molecules acquire the stability of noble gas electron configurations. Recognizing the types of chemical bonds is key to understanding molecular structure, properties, and reactivity.